Design Guide for Improving Critical Facility Safety 
from Flooding and High Winds
FEMA 543 / January 2007 

MAKING CRITICAL FACILITIES SAFE FROM High Wind

3.1 GENERAL DESIGN CONSIDERATIONS

Wind with sufficient speed to cause damage to weak critical 
facilities can occur anywhere in the United States 
and its territories. Even a well-designed, constructed, 
and maintained critical facility may be damaged in a wind event 
much stronger than one the building was designed for. However, 
except for tornado damage, this scenario is a rare occurrence. 
Rather, most damage occurs because various building elements 
have limited wind resistance due to inadequate design, poor installation, 
or material deterioration. Although the magnitude and 
frequency of strong windstorms vary by locale, all critical facilities 
should be designed, constructed, and maintained to minimize 
wind damage (other than that associated with tornadoes—see 
Section 3.5).

This chapter discusses structural, building envelope, and 
nonstructural building systems, and illustrates various types of 
wind-induced damage that affect them. Numerous examples of 
best practices pertaining to new and existing critical facilities are 
presented as recommended design guidelines. Incorporating 
those practices applicable to specific projects will result in greater 
wind resistance reliability and will, therefore, decrease expenditures 
for repair of wind-damaged facilities, provide enhanced 
protection for occupants, and avoid disruption of critical services.

1. The U.S. territories include American Samoa, Guam, Northern Mariana Islands, 
Puerto Rico, and the U.S. Virgin Islands. ASCE provides basic 
wind speed criteria for all but Northern Mariana Islands.

3.1.1 Nature of High Winds

A variety of windstorm types occur in different areas of the United 
States. The characteristics of the types of storms that can affect the 
site should be considered by the design team. The primary storm 
types are listed below.

Straight-line wind: This type of wind generally blows in a straight 
line and is the most common. Straight-line wind speeds range 
from very low to very high. High winds associated with intense 
low pressure can last for approximately a day at a given location. 
Straight-line winds occur throughout the United States and its 
territories.

Down-slope wind: Wind blowing down the slope of mountains is referred 
to as down-slope wind. Down-slope winds with very high 
speeds frequently occur in Alaska and Colorado. In the continental 
United States, mountainous areas are referred to as “special wind regions” 
(see Figure 3-1). Neither ASCE 7 nor model building codes provide specific wind 
speeds 
in special wind regions. ASCE 7 does provide 
guidance on how to determine design wind 
speeds in these regions. If the local building 
department has not established the basic 
speed, use of regional climatic data and consultation 
with a wind engineer or meteorologist is advised. 

Thunderstorm: This type of storm can form rapidly and produce 
high wind speeds. Approximately 10,000 severe thunderstorms 
occur in the United States each year, typically in the spring and 
summer. They are most common in the Southeast and Midwest. 
Besides producing high winds, they often create heavy rain and 
sometimes spawn tornadoes and hail storms. Thunderstorms commonly 
move through an area quite rapidly, causing high winds for 
only a few minutes at a given location. However, thunderstorms 
can also stall and become virtually stationary.

Downburst: Also known as a microburst, this is a powerful downdraft 
associated with a thunderstorm. When the downdraft 
reaches the ground, it spreads out horizontally, and may form one 
or more horizontal vortex rings around the downdraft. The out flow is typically 
6,000 to 12,000 feet across, and the vortex ring 
may rise 2,000 feet above the ground. The lifecycle of a downburst 
is usually 15 to 20 minutes. Observations suggest that approximately 
5 percent of all thunderstorms produce a downburst, 
which can result in significant damage in a localized area.

ASCE 7, Minimum Design Loads for 
Buildings and Other Structures, provides 
guidance for determining wind loads on 
buildings. The IBC and NFPA 5000 refer 
to ASCE 7 for wind load determination.

Northeaster (nor’easter): A northeaster is a cyclonic storm occurring 
off the east coast of North America. These winter weather 
events are notorious for producing heavy snow, rain, and high 
waves and wind. A nor’easter gets its name from the continuously 
strong northeasterly winds blowing in from the ocean ahead of 
the storm and over the coastal areas. These storms may last for several 
days. 

Figure 3-1: Hurricane-prone regions and special wind regions
SOURCE: adapted from asce 7-05

Hurricane: This is a system of spiraling winds converging with 
increasing speed toward the storm’s center (the eye of the hurricane). 
Hurricanes form over warm ocean waters. The diameter 
of the storm varies and can be between 50 and 600 miles. A hurricane’s 
forward movement (translational speed) can vary between 
approximately 5 miles per hour (mph) to 
more than 25 mph. Besides being capable 
of delivering extremely strong winds for several 
hours and moderately strong winds for a 
day or more, many hurricanes also bring very 
heavy rainfall. Hurricanes also occasionally 
spawn tornadoes. The Saffir-Simpson Hurricane 
Scale (see Table 3-1) categorizes the 
intensity of hurricanes. The five-step scale 
ranges from Category 1 (the weakest) to Category 
5 (the strongest). Hurricane-prone 
regions are defined in Section 3.1.3.

Of all the storm types, hurricanes have the 
greatest potential for devastating a large geographical area and, 
hence, affect the greatest number of people. The terms “hurricane,” 
“cyclone,” and “typhoon” describe the same type of storm. 
The term used depends on the region of the world where the 
storm occurs. See Figure 3-1 for hurricane-prone regions.

The Saffir-Simpson Hurricane Scale is based 
on measurements of sustained wind speeds 
in hurricanes; these measurements are 
taken over open water. The wind speeds 
described in the Saffir-Simpson Hurricane 
Scale are used to prepare storm response 
actions and are not intended to be used 
for building design. Design wind loads on 
buildings should be determined using the 
basic wind speeds given in ASCE 7.

Table 3-1: Saffir-Simpson Hurricane Scale

Tornado: This is a violently rotating column 
of air extending from the base of a 
thunderstorm to the ground. The Fujita 
scale categorizes tornado severity based on 
observed damage. The six-step scale ranges 
from F0 (light damage) to F5 (incredible 
damage). Weak tornadoes (F0 and F1) are 
most common, but strong tornadoes (F2 and F3) frequently occur. 
Violent tornadoes (F4 and F5) are rare. Tornado path widths are 
typically less than 1,000 feet; however, widths of approximately 
2.5 miles have been reported. Wind speed rapidly decreases with 
increased distance from the center of a tornado. A critical facility 
on the periphery of a strong or violent tornado could be subjected 
to moderate to high wind speeds, depending upon the distance 
from the core of the tornado. However, even 
though the wind speed at a given facility 
might not be great, a facility on the periphery 
could still be impacted by many large 
pieces of wind-borne debris. Tornadoes are 
responsible for the greatest number of wind-
related deaths each year in the United States. 
Figure 3-2 shows the frequency of tornado 
occurrence for a period between 1950 and 
1998, and Figure 3-3 shows the design wind 
speeds recommended by FEMA for designing 
community shelters.

3.1.2 Probability of Occurrence

Via the importance factor,2 ASCE 7 requires Category III and IV 
buildings to be designed for higher wind loads than Category I 
and II buildings (see Section 1.1.1). Hence, critical facilities designed 
in accordance with ASCE 7 have greater resistance to 
stronger, rarer storms. When designing a critical facility, design 
professionals should consider the following types of winds.

Routine winds: In many locations, winds with low to moderate 
speeds occur daily. Damage is not expected to occur during 
these events.

The majority of the tornadoes spawned 
during hurricanes are classified as F2 or 
weaker. However, a few F3 and at least 
two F4 tornadoes have been reported. 

Beginning in February 2007, the National 
Weather Service will use the Enhanced 
Fujita Scale (EF-Scale) to categorize 
tornado severity. This new scale has 
six steps, ranging from EF0 to EF5. The 
new scale was developed by Texas Tech 
University’s Wind Science and Engineering 
Center. See www.spc.noaa.gov/efscale/ 
for further information on the EF-Scale.

2. The importance factor accounts for the degree of hazard to human life and 
damage to property. Importance factors are given in ASCE 7.

Stronger winds: At a given site, stronger winds (i.e., winds with a 
speed in the range of 70 to 80 mph peak gust, measured at 33 
feet in Exposure C --- refer to Section 3.1.3) may occur from several 
times a year to only once a year or even less frequently. This 
is the threshold at which damage normally begins to occur to 
building elements that have limited wind resistance due to problems 
associated with inadequate design, insufficient strength, poor 
installation, or material deterioration.

Design level winds: Critical facilities exposed to design level events 
and events that are somewhat in excess of design level should experience 
little, if any damage. Actual storm history, however, 
has shown that design level storms frequently cause extensive 
building envelope damage. Structural damage also occurs, but less 
frequently. Damage incurred in design level events is typically associated 
with inadequate design, poor installation, or material 
deterioration. The exceptions are wind-driven water infiltration 
and wind-borne debris (missiles) damage. Water infiltration is discussed 
in Sections 3.3.3.1, 3.3.3.2, and 3.3.3.3. 

Figure 3-2: Frequency of recorded F3, F4, and F5 tornadoes (1950-1998)

Tornadoes: Although more than 1,200 tornadoes typically occur 
each year in the United States, the probability of a tornado occurring 
at any given location is quite small. The probability of 
occurrence is a function of location. As shown in Figure 3-2, only 
a few areas of the country frequently experience tornadoes, and 
tornadoes are very rare in the west. The Oklahoma City area is the 
most active location, with 112 recorded tornadoes between 1890 
and 2003 (www.spc.noaa.gov/faq/tornado/#History). 

Figure 3-3: Design wind speeds for community shelters 

Well designed, constructed, and maintained 
critical facilities should experience little if 
any damage from weak tornadoes, except 
for window breakage. However, weak tornadoes 
often cause building envelope damage 
because many critical facilities have wind resistance 
deficiencies. Most critical facilities 
experience significant damage if they are in 
the path of a strong or violent tornado because 
they typically are not designed for this 
type of storm. 

3.1.3 Wind/Building Interactions

When wind interacts with a building, both 
positive and negative (i.e., suction) pressures 
occur simultaneously (see Figure 3-4). Critical facilities must 
have sufficient strength to resist the applied loads from these 
pressures to prevent wind-induced building failure. Loads exerted 
on the building envelope are transferred to the structural 
system, where in turn they must be transferred through the 
foundation into the ground. The magnitude of the pressures is 
a function of the following primary factors.

Missile damage is very common during 
hurricanes and tornadoes. Missiles can 
puncture roof coverings, many types of 
exterior walls, and glazing. The IBC does 
not address missile-induced damage 
except for glazing in wind-borne debris 
regions. (Wind-borne debris regions are 
limited to portions of hurricane-prone 
regions.) In hurricane-prone regions, 
significant missile-induced building 
damage should be expected, even during 
design level hurricane events, unless 
special enhancements are incorporated 
into the building’s design (discussed in 
Section 3.4).

Figure 3-4: Schematic of wind-induced pressures on a building

Exposure: The characteristics of the terrain 
(i.e., ground roughness and surface irregularities 
in the vicinity of a building) influence 
the wind loading. ASCE 7 defines three exposure 
categories, Exposures B, C, and D. 
Exposure B is the roughest terrain category 
and Exposure D is the smoothest. Exposure 
B includes urban, suburban, and wooded 
areas. Exposure C includes flat open terrain with scattered obstructions 
and areas adjacent to water surfaces in hurricane-prone 
regions (which are defined below under “basic wind speed”). 
Exposure D includes areas adjacent to water surfaces outside hurricane-
prone regions, mud flats, salt flats, and unbroken ice. 
Because of the wave conditions generated by hurricanes, areas 
adjacent to water surfaces in hurricane-prone regions are considered 
to be Exposure C rather than the smoother Exposure D. The 
smoother the terrain, the greater the wind pressure; therefore, 
critical buildings located in Exposure C would receive higher wind 
loads than those located in Exposure B, even at the same basic 
wind speed. 

Basic wind speed: ASCE 7 specifies the basic wind speed for determining 
design wind loads. The basic wind speed is measured at 33 
feet above grade in Exposure C (flat open terrain). If the building 
is located in Exposure B or D, rather than C, an adjustment for 
the actual exposure is made in the ASCE 7 calculation procedure.

Since the 1995 edition of ASCE 7, the basic wind speed measurement 
has been a 3-second peak gust speed. Prior to that time, 
the basic wind speed was a fastest-mile speed (i.e., the speed averaged 
over the time required for a mile-long column of air to pass 
a fixed point).3 Most of the United States has a basic wind speed 
(peak gust) of 90 mph, but much higher speeds occur in Alaska 
and in hurricane-prone regions. The highest speed, 170 mph, occurs 
in Guam. 

Hurricane-prone regions include Atlantic and Gulf coastal areas 
(where the basic wind speed is greater than 90 mph), Hawaii, and the 
U.S. territories in the Caribbean and South Pacific (see Figure 3-1).

For additional exposure information, 
see the Commentary of ASCE 7, which 
includes several aerial photographs that 
illustrate the different terrain conditions 
associated with Exposures B, C, and D. 

3. Peak gust speeds are about 15 to 20 mph higher than fastest-mile speeds 
(e.g., a 90-mph peak basic wind speed is equivalent to a 76-mph 
fastest-mile wind speed). IBC Chapter 16 provides a table of equivalent basic 
wind speeds.

In the formula for determining wind pressures, 
the basic wind speed is squared. 
Therefore, as the wind speed increases, the 
pressures are exponentially increased, as 
illustrated in Figure 3-5. This figure also illustrates 
the relative difference in pressures 
exerted on the main wind-force resisting 
system (MWFRS) and the components and 
cladding (C&C) elements.

Topography: Abrupt changes in topography, such as isolated hills, 
ridges, and escarpments, cause wind to speed up. Therefore, a 
building located near a ridge would receive higher wind pressures 
than a building located on relatively flat land. ASCE 7 provides a 
procedure to account for topographic influences.

Building height: Wind speed increases with height above the 
ground. Taller buildings are exposed to higher wind speeds and 
greater wind pressures. ASCE 7 provides a procedure to account 
for building height.

The MWFRS is an assemblage of structural 
elements assigned to provide support and 
stability for the overall structure. The system 
generally receives wind loading from more 
than one surface. The C&C are elements of 
the building envelope that do not qualify as 
part of the main wind-force resisting system. 

Figure 3-5: Wind pressure as a function of wind speed

Internal pressure (building pressurization/depressurization): Openings 
through the building envelope, in combination with wind interacting 
with a building, can cause either an increase in the 
pressure within the building (i.e., positive internal pressure), 
or it can cause a decrease in the pressure (i.e., negative internal 
pressure). Building envelope openings occur around doors and 
window frames, and by air infiltration through walls that are not 
absolutely airtight. A door or window left open, or glazing that is 
broken during a storm, can greatly influence the magnitude of 
the internal pressure. 

Wind striking an exterior wall exerts a positive pressure on the 
wall, which forces air through openings and into the interior of 
the building (this is analogous to blowing up a balloon). At the 
same time that the windward wall is receiving positive pressure, 
the side and rear walls are experiencing negative (suction) pressure 
from winds going around the building. As this occurs, air 
within the building is pulled out at openings in these walls. As 
a result, if the porosity of the windward wall is greater than the 
combined porosity of the side and rear walls, the interior of the 
building is pressurized. But if the porosity of the windward wall is 
lower than the combined porosity of the side and rear walls, the 
interior of the building is depressurized (this is analogous to letting 
air out of a balloon). 

When a building is pressurized, the internal pressure pushes up 
on the roof. This push from below the roof is combined with suction 
on the roof above, resulting in an increased upward wind 
pressure on the roof. The internal pressure also pushes on the 
side and rear walls. This outward push is combined with the suction 
on the exterior side of these walls (see Figure 3-6). When a 
building becomes fully pressurized (e.g., due to window breakage 
or soffit failure), the loads applied to the exterior walls and roof 
are significantly increased. The rapid build-up of internal pressure 
can also blow down interior partitions and blow suspended 
ceiling boards out of their support grid. The breaching of a small 
window can be sufficient to cause full pressurization of the facility’s 
interior.

When a building is depressurized, the internal pressure pulls the 
roof down, which reduces the amount of uplift exerted on the 
roof. The decreased internal pressure also pulls inward on the 
windward wall, which increases the wind load on that wall (see 
Figure 3-7).

The ASCE 7 wind pressure design procedure accounts for the influence 
of internal pressure on the wall and roof loads, and it 
provides positive and negative internal pressure coefficients for 
use in load calculations. Buildings that are designed to accommodate 
full pressurization are referred to as partially enclosed 
buildings. Buildings that are only intended to experience limited 
internal pressurization are referred to as enclosed buildings. 
Buildings that do not experience internal pressurization are referred 
to as open buildings (such as covered walkways and most 
parking garages).

Figure 3-6: Schematic of internal pressure condition when the 
dominant opening is in the windward wall

Figure 3-7: Schematic of internal pressure condition when the 
dominant opening is in the leeward wall

Building shape: The highest uplift pressures occur at roof corners 
because of building aerodynamics (i.e., the interaction 
between the wind and the building). The roof perimeter has a 
somewhat lower load compared to the corners, and the field of 
the roof has still lower loads. Exterior walls typically have lower 
loads than the roof. The ends (edges) of walls have higher suction 
loads than the portion of wall between the ends. However, 
when the wall is loaded with positive pressure, the entire wall is 
uniformly loaded. Figure 3-8 illustrates these aerodynamic influences. 
The negative values shown in Figure 3-8 indicate suction 
pressure acting upward from the roof surface and outward from 
the wall surface. Positive values indicate positive pressure acting 
inward on the wall surface.

Aerodynamic influences are accounted for by using external pressure 
coefficients in load calculations. The value of the coefficient 
is a function of the location on the building (e.g., roof corner or 
field of roof) and building shape as discussed below. Positive coefficients 
represent a positive (inward-acting) pressure, and negative 
coefficients represent negative (outward-acting [suction]) pressure. 
External pressure coefficients for MWFRS and C&C are 
listed in ASCE 7.

Building shape affects the value of pressure coefficients and, 
therefore, the loads applied to the various building surfaces. For 
example, the uplift loads on a low-slope roof are larger than the 
loads on a gable or hip roof. The steeper the slope, the lower the 
uplift load. Pressure coefficients for monoslope (shed) roofs, sawtooth 
roofs, and domes are all different from those for low-slope 
and gable/hip roofs.

Building irregularities, such as re-entrant corners, bay window projections, 
a stair tower projecting out from the main wall, dormers, 
and chimneys can cause localized turbulence. Turbulence causes 
wind speed-up, which increases the wind loads in the vicinity of 
the building irregularity, as shown in Figures 3-9 and 3-10. Figure 
3-9 shows the aggregate ballast on a hospital’s single-ply membrane 
roof blown away at the re-entrant corner and in the vicinity 
of the corners of the wall projections at the window bays. The 
irregular wall surface created turbulence, which led to wind speed-
up and loss of aggregate in the turbulent flow areas. 

Figure 3-8: Relative roof uplift pressures as a function of roof geometry, roof 
slope, and location on roof, and relative positive and negative wall pressures 
as a function of location along the wall

Figure 3-10 shows a stair tower at a hospital 
that caused turbulence resulting in wind 
speed-up. The speed-up increased the suction 
pressure on the base flashing along the 
parapet behind the stair tower. The built-
up roof’s base flashing was pulled out from 
underneath the coping because its attachment 
was insufficient to resist the suction 
pressure. The base flashing failure propagated 
and caused a large area of the roof 
membrane to lift and peel. Some of the wall covering on the 
stair tower was also blown away. Had the stair tower not existed, 
the built-up roof would likely not have been damaged. To avoid 
damage in the vicinity of building irregularities, attention needs 
to be given to the attachment of building elements located in 
turbulent flow areas. 

To avoid the roof membrane damage shown in Figure 3-10, it 
would be prudent to use corner uplift loads in lieu of perimeter 
uplift loads in the vicinity of the stair tower, as illustrated in 
Figure 3-11. Wind load increases due to building irregularities 
can be identified by wind tunnel studies; however, wind tunnel 
studies are rarely performed for critical facilities. Therefore, 
identification of wind load increases due to building irregularities will 
normally be based on the designer’s professional 
judgment. Usually load increases will only need to be applied to 
the building envelope, and not to the MWFRS. 

Figure 3-9: Aggregate blow-off associated with building 
irregularities. Hurricane Hugo (South Carolina, 1989)

Information pertaining to load calculations 
is presented in Section 3.3.1.2. For further 
general information on the nature of 
wind and wind-building interactions, see 
Buildings at Risk: Wind Design Basics for 
Practicing Architects, American Institute of 
Architects, 1997.

Figure 3-10: The irregularity created by the stair tower (covered with a metal 
roof) caused turbulence resulting in wind speed-up and roof 
damage. Hurricane Andrew (Florida, 1992) 

Figure 3-11: Plan view of a portion of 
the building in Figure 310 showing the use of a 
corner uplift zone in lieu of a perimeter uplift zone 
on the low-slope roof in the vicinity of the stair tower 

3.1.4 Building Codes

The IBC is the most extensively used model code. However, in 
some jurisdictions NFPA 5000 may be used. In other jurisdictions, 
one of the earlier model building codes, or a specially written 
State or local building code, may be used. The specific scope and/
or effectiveness and limitations of these other building codes will 
be somewhat different than those of the IBC. It is incumbent 
upon the design professionals to be aware of the specific code (including 
the edition of the code and local amendments) that has 
been adopted by the authority having jurisdiction over the location 
of the critical facility.

3.1.4.1 Scope of Building Codes

With respect to wind performance, the scope of the model 
building codes has greatly expanded since the mid-1980s. Some of 
the most significant improvements are discussed below.

Recognition of increased uplift loads at the roof perimeter and corners: 
Prior to the 1982 edition of the Standard Building Code (SBC), 
Uniform Building Code (UBC), and the 1987 edition of the National 
Building Code (NBC), these model codes did not account 
for the increased uplift at the roof perimeter and corners. Therefore, 
critical facilities designed in accordance with earlier editions 
of these codes are very susceptible to blow-off of the roof deck 
and/or roof covering.

Adoption of ASCE 7 for design wind loads: Although the SBC, UBC, 
and NBC permitted use of ASCE 7, the 2000 edition of the IBC 
was the first model code to require ASCE 7 for determining 
wind design loads. ASCE 7 has been more reflective of the current 
state of the knowledge than the earlier model codes, and 
use of this procedure typically has resulted in higher design 
loads. 

Roof coverings: Several performance and prescriptive requirements 
pertaining to wind resistance of roof coverings have been 
incorporated into the model codes. The majority of these additional 
provisions were added after Hurricanes Hugo (1989) and 
Andrew (1992). Poor performance of roof coverings was wide
spread in both of those storms. Prior to the 
1991 edition of the SBC and UBC, and the 
1990 edition of the NBC, these model codes 
were essentially silent on roof covering wind 
loads and test methods for determining 
uplift resistance. Code improvements continued 
to be made through the 2006 edition 
of the IBC, which added a provision that 
prohibits aggregate roof surfaces in hurricane-
prone regions.

Glazing protection: The 2000 edition of the 
IBC was the first model code to address 
wind-borne debris requirements for glazing 
in buildings located in hurricane-prone regions 
(via reference to the 1998 edition of 
ASCE 7). The 1995 edition of ASCE 7 was 
the first edition to address wind-borne debris 
requirements.

Parapets and rooftop equipment: The 2003 edition of the IBC was 
the first model code to address wind loads on parapets and 
rooftop equipment (via reference to the 2002 edition of ASCE 7, 
which was the first edition of ASCE 7 to address these elements).

3.1.4.2 Effectiveness and Limitations of Building 
Codes

A key element of an effective building code is for a community to 
have an effective building department. Building safety depends on 
more than the codes and the standards they reference. Building 
safety results when trained professionals have the resources and 
ongoing support they need to stay on top of the latest advancements 
in building safety. An effective building safety system 
provides uniform code interpretations, product evaluations, and 
professional development and certification for inspectors and 
plan reviewers. Local building departments play an important role 
in helping to ensure buildings are designed and constructed in 
accordance with the applicable building codes. Meaningful plan 
review and inspection by the building department are particularly 
important for critical facilities.

ASCE 7 requires impact-resistant glazing in 
wind-borne debris regions within hurricane-
prone regions. Impact-resistant glazing can 
either be laminated glass, polycarbonate, 
or shutters tested in accordance with 
standards specified in ASCE 7. The 
wind-borne debris load criteria were 
developed to minimize property damage 
and to improve building performance. The 
criteria were not developed for occupant 
protection. Where occupant protection is 
a specific criterion, the more conservative 
wind-borne debris criteria given in FEMA 
361, Design and Construction Guidance 
for Community Shelters is recommended.

General limitations to building codes include the following:

 Because codes are adopted and enforced on the local or 
State level, the authority having jurisdiction has the power to 
eliminate or modify wind-related provisions of a model code, 
or write its own code instead. In places where important 
wind-related provisions of the current model code are not 
adopted and enforced, critical facilities are more susceptible 
to wind damage. Additionally, a significant time lag often exists 
between the time a model code is updated and the time 
it is implemented by the authority having jurisdiction. Buildings 
designed to the minimum requirements of an outdated 
code are, therefore, not taking advantage of the current 
state of the knowledge. These buildings are prone to poorer 
wind performance compared to buildings designed according 
to the current model code.

 Adopting the current model code alone does not ensure good 
wind performance. The code is a minimum tool that should 
be used by knowledgeable design professionals in conjunction 
with their training, skills, professional judgment, and the best 
practices presented in this manual. To achieve good wind 
performance, in addition to good design, the construction 
work must be effectively executed, and the building must be 
adequately maintained and repaired.

 Critical facilities need to perform at a higher level than 
required by codes and standards. See Section 1.3 on 
performance based design.

IBC 2006: The 2006 edition of the IBC is believed to be a relatively 
effective code, provided that it is properly followed and 
enforced. Some limitations of the 2006 IBC have, however, been 
identified:

 With respect to hurricanes, the IBC provisions pertaining 
to building envelopes and rooftop equipment do not 
adequately address the special needs of critical facilities. 
For example: (1) they do not account for water infiltration 
due to puncture of the roof membrane by missiles; (2) 
they do not adequately address the vulnerabilities of brittle 
roof coverings (such as tile) to missile-induced damage 
and subsequent progressive failure; 
(3) they do not adequately address 
occupant protection with respect to 
missiles; (4) they do not adequately 
address protection of equipment in 
elevator penthouses; (5) they do not 
account for interruption of water service 
or prolonged interruption of electrical 
power; and (6) the current requirements 
for shelters are limited. All of these 
elements are of extreme importance for 
critical facilities, which need to remain 
operational before, during, and after a 
disaster. Guidance to overcome these 
shortcomings is given in Section 3.4.
m The 2006 IBC does not account for 
tornadoes; therefore, except for weak 
tornadoes, it is ineffective for this type 
of storm.4 Guidance to overcome this 
shortcoming is given in Section 3.5.
The 2000, 2003, and 2006 IBC rely on several 
referenced standards and test methods 
developed or updated in the 1990s. Prior 
to adoption, most of these standards and 
test methods had not been validated by actual building performance 
during design level wind events. The hurricanes of 2004 
and 2005 provided an opportunity to evaluate the actual performance 
of buildings designed and constructed to the minimum 
provisions of the IBC. Building performance evaluations conducted 
by FEMA revealed the need for further enhancements:

 A limitation of the 2006 IBC pertains to some of the test 
methods used to assess wind and wind-driven rain resistance 
of building envelope components. However, before this code 
limitation can be overcome, research needs to be conducted 
and new test methods need to be developed. 
The International Code Council (ICC) 
is developing a consensus standard, 
ICC/NSSA Standard for the Design and 
Construction of Storm Shelters, to provide 
basic requirements for the design and 
construction of emergency shelters in areas 
affected by hurricanes and tornadoes. If 
it is adopted by a community when it 
becomes available in early 2007, it will 
provide design and construction standards 
for buildings intended to resist the impact 
of high winds and wind-borne debris. This 
stand-alone standard will be linked to the 
IBC and IRC. It is the intent of the ICC 
that the shelter standard be incorporated 
by reference into the IBC and IRC in 
2009. FEMA should be consulted prior to 
designing or constructing shelters to the 
ICC/NSSA standard with FEMA funds 
to ensure all program requirements are 
met, as some components may need to 
be designed to stricter criteria than those 
included in the consensus standard. 

4. Except for glass breakage, code-compliant buildings should not experience 
significant damage during weak tornadoes.

 Except to the extent covered by reference to ASCE 7, the 2006 
IBC does not address the need for continuity, redundancy, 
or energy-dissipating capability (ductility) to limit the effects 
of local collapse, and to prevent or minimize progressive 
collapse after the loss of one or two primary structural 
members, such as a column. Chapter 1 of ASCE 7 addresses 
general structural integrity, and the Chapter 1 Commentary 
provides some guidance on this issue. 

3.2 CRITICAL FACILITIES EXPOSED TO HIGH WINDS

3.2.1 Vulnerability: What High Winds Can Do to Critical Facilities

This section provides an overview of the common types of wind 
damage and their ramifications. 

3.2.1.1 Types of Building Damage

When damaged by wind, critical facilities typically experience 
a variety of building component damage. The most common 
types of damage are discussed below in descending order of 
frequency. 

Roof: Roof covering damage (including rooftop mechanical, 
electrical, and communications equipment) is the most 
common type of wind damage. The school, illustrated in Figure 
3-12, was being used as a hurricane shelter at the time a portion 
of the roof membrane blew off. In addition to the membrane 
damage, several pieces of rooftop equipment were damaged, 
and virtually all of the loose aggregate blew off and broke many 
windows in nearby houses. The cast-in-place concrete deck kept 
most of the water from entering the building. 

Glazing: Exterior glazing damage is very common during hurricanes 
and tornadoes, but is less common during other storms. 
The glass shown in Figure 3-13 was broken by the aggregate 
from a built-up roof. The inner panes had several impact craters. 
In several of the adjacent windows, both the outer and 
inner panes were broken. The aggregate flew more than 245 
feet (the estimated wind speed was 104 mph, measured at 33 
feet in Exposure C).

Figure 3-12: A portion of the built-up 
membrane lifted and peeled after the metal 
edge flashing lifted. Hurricane Andrew (Florida, 1992)

Figure 3-13: The outer window panes were broken by aggregate 
from a built-up roof. Hurricane Hugo (South Carolina, 1989)

Wall coverings, soffits, and large doors: Exterior wall covering, soffit, 
and large door damage is common during hurricanes and tornadoes, 
but is less common during other storms. At the building 
shown in Figure 3-14, metal wall covering was attached to plywood 
over metal studs. The CMU wall behind the studs did not appear 
to be damaged. The building was located on the periphery of a violent 
tornado. 

Wall collapse: Collapse of non-load-bearing exterior walls is 
common during hurricanes and tornadoes, but is less common 
during other storms (see Figure 3-15).

Figure 3-14: Collapsed metal stud wall—the wall was 
blown completely away in another part of the 
building. (Oklahoma, 1999)

Figure 3-15: The unreinforced CMU wall collapsed at a school 
during Hurricane Marilyn. (U.S. Virgin Islands, 1995)

Structural system: Structural damage (e.g., roof deck blow-off, blow-
off or collapse of the roof structure, collapse of exterior bearing 
walls, or collapse of the entire building or major portions thereof) 
is the principal type of damage that occurs during strong and violent 
tornadoes (see Figure 3-16).

3.2.1.2 Ramifications of Damage

The ramifications of building component damage on critical facilities 
are described below.

Property damage: Property damage requires repairing/replacing 
the damaged components (or replacing the entire facility), and 
may require repairing/replacing interior building components, 
furniture, and other equipment, and mold remediation. Even 
when damage to the building envelope is 
limited, such as blow-off of a portion of the 
roof covering or broken glazing, substantial 
water damage frequently occurs because 
heavy rains often accompany strong winds 
(particularly in the case of thunderstorms, 
tropical storms, hurricanes, and tornadoes). 
At the newly constructed gymnasium 
shown in Figure 3-17, the structural metal 
roof panels were applied over metal purlins. The panels with 3-
inch-high trapezoidal ribs at 24 inches on center detached from 
their concealed clips. A massive quantity of water entered the 
building and buckled the wood floor. 

Figure 3-16: The school wing was destroyed by a violent 
tornado. (Oklahoma, 1999)

Modest wind speeds can drive rain into 
exterior walls. Unless adequate provisions 
are taken to account for water infiltration 
(see Sections 3.3.3.1 to 3.3.3.3), 
damaging corrosion, dry rot, and mold 
can occur within walls.

Wind-borne debris such as roof aggregate, gutters, rooftop 
equipment, and siding blown from buildings can damage vehicles 
and other buildings in the vicinity. Debris can travel well 
over 300 feet in high-wind events.

Portable classrooms on school campuses are often particularly 
vulnerable to significant damage because they are seldom designed 
to the same wind pressures as permanent buildings. 
Portable classrooms are frequently blown over during high-wind 
events, because the anchoring techniques typically used are inadequate 
to secure the units to the ground. Wind-borne debris 
from portable classrooms, or an entire portable classroom, may 
strike the permanent school building and cause serious damage.

Ancillary buildings (such as storage buildings) adjacent to critical 
facilities are also vulnerable to damage. Although loss of 
these buildings may not be detrimental to the operation of the 
critical facility, debris from ancillary buildings may strike and 
damage the critical facility. The damaged building shown in 


Figure 3-17: A massive quantity of water entered the building 
after the roof blew off. Typhoon Paka (Guam, 1997) 

Figure 3-18 contained the hospital’s supplies and maintenance 
shop. With the loss of this building, tents had to be set up to provide 
supply storage. Almost all of the tools and stock materials for 
repairs were lost.

Injury or death: Although infrequent, critical 
facility occupants or people outside the 
facility have been injured and killed when 
struck by collapsed building components 
(such as exterior masonry walls or the roof 
structure) or wind-borne debris. The greatest 
risk of injury or death is during strong hurricanes 
and strong/violent tornadoes.

Interrupted use: Depending on the magnitude 
of wind and water damage, it can 
take days, months, or more than a year to repair the damage 
or replace a facility. In addition to the costs associated with repairing/
replacing the damage, other social and financial costs 
can be even more significant. The repercussions related to interrupted 
use of the critical facility can include the loss of emergency 
and first-responder services, lack of medical care, and the costs 
to rent temporary facilities. These additional costs can be quite 
substantial.

Figure 3-18: A hurricane-damaged, pre-engineered storage 
building adjacent to a hospital. Hurricane Charley (Florida, 2004)

Although people are not usually outside 
during hurricanes, it is not uncommon for 
people to seek shelter or assistance in 
critical facilities during a storm. Missiles, 
such as roof aggregate or tile shedding 
from a critical facility, could injure or kill 
late arrivals before they have a chance to 
enter the building. 

3.2.2 Evaluating Critical Facilities for Risk from High Winds

This section describes the process of hazard risk assessment. Although 
no formal methodology for risk assessment has been 
adopted, prior experience provides a sufficient knowledge base 
upon which a set of guidelines can be structured into a recommended 
procedure for risk assessment of critical facilities. The 
procedures presented below establish guidelines for evaluating 
the risk to new and existing buildings from wind storms other 
than tornadoes. These evaluations will allow development of a 
vulnerability assessment that can be used along with the site’s 
wind regime to assess the risk to critical facilities.

In the case of tornadoes, neither the IBC nor ASCE 7 requires 
buildings (including critical facilities) to be designed to resist 
tornado forces, nor are occupant shelters required in buildings 
located in tornado-prone regions. Constructing tornado-resistant 
critical facilities is extremely expensive because of the 
extremely high pressures and missile impact loads that tornadoes 
can generate. Therefore, when consideration is voluntarily 
given to tornado design, the emphasis is 
typically on occupant protection, which is 
achieved by “hardening” portions of a critical 
facility for use as safe havens. FEMA 361 
includes a comprehensive risk assessment 
procedure that designers can use to assist 
building owners in determining whether a 
tornado shelter should be included as part 
of a new critical facility. See Section 3.5 for 
the design of tornado shelters and other 
recommendations pertaining to critical facilities 
in tornado-prone regions.

3.2.2.1 New Buildings

When designing new critical facilities, a 
two-step procedure is recommended for 
evaluating the risk from wind storms (other 
than tornadoes).

In this manual, the term “tornado-prone 
regions” refers to those areas of the United 
States where the number of recorded 
F3, F4, and F5 tornadoes per 3,700 
square miles is 6 or greater per year 
(see Figure 3-2). However, an owner of 
a critical facility may decide to use other 
frequency values (e.g., 1 or greater, 16 
or greater, or greater than 25) in defining 
whether the building is in a tornado-prone 
area. In this manual, tornado shelters are 
recommended for all critical facilities in 
tornado-prone regions.

Where the frequency value is 1 or greater, 
and the facility does not have a tornado 
shelter, the best available refuge areas 
should be identified, as discussed in 
Section 3.5.

Step 1: Determine the basic wind speed from ASCE 7. As the 
basic wind speed increases beyond 90 mph, the risk of damage increases. 
Design, construction, and maintenance enhancements 
are recommended to compensate for the increased risk of damage 
(see Section 3.3).

Step 2: For critical facilities in hurricane-prone regions, refer to 
the design, construction, and maintenance enhancements recommended 
in Section 3.4. 

For particularly important critical facilities (such as hospitals) 
in remote areas outside of hurricane-prone regions, it is recommended 
that robust design measures be considered to minimize 
the potential for disruption resulting from wind damage. Because 
of their remote location, disruption of such facilities could 
severely affect the occupants or community. Some of the recommendations 
in Section 3.4 may therefore be prudent.

3.2.2.2 Existing Buildings

The resistance of existing buildings is a function of their original 
design and construction, various additions or modifications, and 
the condition of building components (which may have weakened 
due to deterioration or fatigue). For existing buildings, a two-step 
procedure is also recommended.

Step 1: Calculate the wind loads on the building using the current 
edition of ASCE 7, and compare these loads with the 
loads for which the building was originally designed. The original 
design loads may be noted on the contract drawings. If 
not, determine what building code or standard was used to develop 
the original design loads, and calculate the loads using 
that code or standard. If the original design loads are significantly 
lower than current loads, upgrading the load resistance 
of the building envelope and/or structure should be considered. 
An alternative to comparing current loads with original 
design loads is to evaluate the resistance of the existing facility 
as a function of the current loads to determine what elements 
are highly overstressed.

Step 2: Perform a field investigation to evaluate the primary 
building envelope elements, rooftop equipment, and structural 
system elements, to determine if the facility was generally 
constructed as indicated on the original contract drawings. 
As part of the investigation, the primary elements should be 
checked for deterioration. Load path continuity should also be 
checked.

If the results of either step indicate the need for remedial work, 
see Section 3.6.

3.3 CRITICAL FACILITY DESIGN CONSIDERATIONS

3.3.1 General Design Considerations

The performance of critical facilities in past wind storms indicates 
that the most frequent and the most significant factor in 
the disruption of the operations of these facilities has been the 
failure of nonstructural building components. While acknowledging 
the importance of the structural systems, Chapter 3 emphasizes 
the building envelope components and the nonstructural systems. 
According to the National Institute of Building Science (NIBS), 
the building envelope includes the below-grade basement walls 
and foundation and floor slab (although these are generally considered 
part of the building’s structural system). The envelope includes 
everything that separates the interior of a building from the outdoor 
environment, including the connection of all the nonstructural 
elements to the building structure. The nonstructural systems include 
all mechanical, electrical, electronic, communications, and 
lightning protection systems. Historically, damage to roof coverings 
and rooftop equipment has been the leading cause of 
building performance problems during wind storms. Special consideration 
should be given to the problem of water infiltration 
through failed building envelope components, which can cause 
severe disruptions in the functioning of critical facilities. 
The key to enhanced wind performance is paying sufficient attention 
to all phases of the construction process (including site 
selection, design, and construction) and to post-occupancy maintenance 
and repair. 

3.3.1.1 Site 

When selecting land for a critical facility, sites located in Exposure 
D (see Section 3.1.3 for exposure definitions) should be avoided if 
possible. Selecting a site in Exposure C or preferably in Exposure 
B would decrease the wind loads. Also, where possible, avoid selecting 
sites located on an escarpment or the upper half of a hill, 
where the abrupt change in the topography would result in increased 
wind loads.6 

Trees with trunks larger than 6 inches in diameter, poles (e.g., 
light fixture poles, flag poles, and power poles), or towers (e.g., 
electrical transmission and large communication towers) should 
not be placed near the building. Falling trees, poles, and towers 
can severely damage a critical facility and injure the occupants 
(see Figure 3-19). Large trees can crash through pre-engineered 
metal buildings (which often house fire stations) and 
wood frame construction (which is commonly used for nursing 
homes). Falling trees can also rupture roof membranes and 
break windows.

Figure 3-19: Had this tree fallen in the opposite direction, it 
would have landed on the school. Hurricane Ivan (Florida, 2004)

6. When selecting a site on an escarpment or the upper half of a hill is 
necessary, the ASCE 7 design procedure accounts for wind speed-up 
associated with this abrupt change in topography.

Providing at least two means of site egress is 
prudent for all critical facilities, but is particularly 
important for facilities in hurricane-prone 
regions. If one route becomes blocked by trees 
or other debris, or by floodwaters, the other access 
route may still be available.

3.3.1.2 Building Design 

Good wind performance depends on good 
design (including details and specifications), 
materials, installation, maintenance, 
and repair. A significant shortcoming in 
any of these five elements could jeopardize 
the performance of a critical facility against 
wind. Design, however, is the key element to 
achieving good performance of a building against wind damage. 
Design inadequacies frequently cannot be compensated for with 
other elements. Good design, however, can compensate for other 
inadequacies to some extent. The following steps should be included 
in the design process for critical facilities.

Step 1: Calculate Loads 

Calculate loads on the MWFRS, the building envelope, and 
rooftop equipment in accordance with ASCE 7 or the local 
building code, whichever procedure results in the highest loads. 
In calculating wind loads, design professionals 
should consider the following items.

Importance factor: The effect of using a 1.15 
importance factor versus 1 is that the design 
loads for the MWFRS and C&C are increased 
by 15 percent. The importance factor for 
most critical facilities is required to be 1.15. 
However, ASCE 7 permits a factor of 1 for 
some critical facilities. For example, schools 
with an occupant load of less than 250 
people are permitted to be designed with an 
importance factor of only 1 (provided they 
are not used as a shelter—if used as a shelter, 
schools are required to be designed with an importance factor of 
1.15, regardless of occupant load). Other facilities where occupant 
load controls the importance factor include certain health 
care facilities, such as nursing homes with 50 or more residents, 
for which a factor of 1.15 is required. For nursing homes with less 
than 50 residents, a factor of 1 can be used. Some critical facilities 
are not specifically addressed in ASCE 7. For example, various 
buildings on a hospital campus, such as medical office buildings 
that are integrally connected to the hospital and various types of 
non-emergency treatment facilities (such as storage, cancer treatment, 
physical therapy, and dialysis) are not specifically required 
by ASCE 7 to be designed with a 1.15 factor. This manual recommends 
a value of 1.15 for all critical facilities.

In the past, design professionals seldom 
performed load calculations on the 
building envelope (i.e., roof and wall 
coverings, doors, windows, and skylights) 
and rooftop equipment. These building 
components are the ones that have failed 
the most during past wind events. In 
large part they failed because of the 
lack of proper load determination and 
inappropriate design of these elements. 
It is imperative that design professionals 
determine the loads for the building 
envelope and rooftop equipment, and 
design them to accommodate such loads.

Uplift loads on roof assemblies can also 
be determined from FM Global (FMG) 
Data Sheets. If the critical facility is FMG 
insured, and the FMG-derived loads are 
higher than those derived from ASCE 7 or 
the building code, the FMG loads should 
govern. However, if the ASCE 7 or code-
derived loads are higher than those from 
FMG, the ASCE 7 or code-derived loads 
should govern (whichever procedure 
results in the highest loads). 

Wind directionality factor: The ASCE 7 wind load calculation procedure 
incorporates a wind directionality factor (kd). The 
directionality factor accounts for the reduced probability of maximum 
winds coming from any given direction. By applying the 
prescribed value of 0.85, the loads are reduced by 15 percent. Because 
hurricane winds can come from any direction, and because 
of the historically poor performance of building envelopes and 
rooftop equipment, this manual recommends a more conservative 
approach for critical facilities in hurricane-prone regions. A directionality 
factor of 1.0 is recommended for the building envelope 
and rooftop equipment (a load increase over what is required by 
ASCE 7). For the MWFRS, a directionality factor of 0.85 is recommended 
(hence, no change for MWFRS).

Step 2: Determine Load Resistance

When using allowable stress design, after loads have been determined, 
it is necessary to select a reasonable safety factor in 
order to determine the minimum required load resistance. 
For building envelope systems, a minimum safety factor of 2 is 
recommended. For anchoring exterior-mounted mechanical, 
electrical, and communications equipment (such as satellite 
dishes), a minimum safety factor of 3 is recommended. When 
using strength design, load combinations and load factors specified 
in ASCE 7 are used.

ASCE 7 provides criteria for combining wind loads with other types 
of loads (such as dead and flood loads) using allowable stress design.

For structural members and cladding elements 
where strength design can be used, 
load resistance can be determined by calculations. 
For other elements where allowable 
stress design is used (such as most types of 
roof coverings), load resistance is primarily 
obtained from system testing.

The load resistance criteria need to be provided 
in contract documents. For structural 
elements, the designer of record typically accounts 
for load resistance by indicating the 
material, size, spacing, and connection of the 
elements. For nonstructural elements, such 
as roof coverings or windows, the load and 
safety factor can be specified. In this case, the 
specifications should require the contractor’s 
submittals to demonstrate that the system will 
meet the load resistance criteria. This performance 
specification approach is necessary if, 
at the time of the design, it is unknown who 
will manufacture the system.

Regardless of which approach is used, it is important that the 
designer of record ensure that it can be demonstrated, via calculations 
or tests, that the structure, building envelope, and 
nonstructural systems (exterior-mounted mechanical, electrical, 
and communications equipment) have sufficient 
strength to resist design wind loads.

Step 3: Detailed Design

It is vital to design, detail, and specify the 
structural system, building envelope, and 
exterior-mounted mechanical, electrical, 
and communications equipment to meet 
the factored design loads (based on appropriate 
analytical or test methods). It 
is also vital to respond to the risk assessment 
criteria discussed in Section 3.2.2, as 
appropriate.

When using allowable stress design, a 
safety factor is applied to account for 
reasonable variations in material strengths, 
construction workmanship, and conditions 
when the actual wind speed somewhat 
exceeds the design wind speed. For 
design purposes, the ultimate resistance 
an assembly achieves in testing is reduced 
by the safety factor. For example, if a 
roof assembly resisted an uplift pressure 
of 100 pounds per square foot (psf), after 
applying a safety factor of 2, the assembly 
would be suitable where the design load 
was 50 psf or less. Conversely, if the 
design load is known, multiplying it by the 
safety factor equals the minimum required 
test pressure (e.g., 50 psf design load 
multiplied by a safety factor of 2 equals a 
minimum required test pressure of 100 psf). 

Connections are a key aspect of load 
path continuity between various structural 
and nonstructural building elements. In a 
window, for example, the glass must be 
strong enough to resist the wind pressure 
and must be adequately anchored to the 
window frame, the frame adequately 
anchored to the wall, the wall to the 
foundation, and the foundation to the 
ground. As loads increase, greater 
load capacity must be developed in the 
connections.

As part of the detailed design effort, load path continuity 
should be clearly indicated in the contract documents via illustration 
of connection details. Load paths need to accommodate 
design uplift, racking, and overturning loads. Load path continuity 
obviously applies to MWFRS elements, but it also applies 
to building envelope elements. Figure 3-20 shows a load path 
discontinuity between a piece of HVAC equipment and its 
equipment stand. The equipment on this new Federal courthouse 
blew away because it was resting on vibration isolators 
that provided lateral resistance, but no uplift resistance (also see 
Figure 3-75).

Figure 3-21 illustrates the load path concept. Members are sized 
to accommodate the design loads. Connections are designed to 
transfer uplift loads applied to the roof, and the positive and negative 
loads applied to the exterior bearing walls, down to the 
foundation and into the ground. The roof covering (and wall 
covering, if there is one) is also part of the load path. To avoid 
blow-off, the nonstructural elements must also be adequately attached 
to the structure.

Figure 3-20: Temporary coverings placed over two large 
openings in the roof that were left after the ductwork 
blew away. Hurricane Katrina (Mississippi, 2005)

As part of the detailed design process, special consideration 
should be given to the durability of materials and water 
infiltration.

Figure 3-21: Illustration of load path continuity 

Durability: Because some locales have very aggressive atmospheric 
corrosion (such as areas near oceans), special attention needs 
to be given to the specification of adequate protection for ferrous 
metals, or to specify alternative metals such as stainless 
steel. FEMA Technical Bulletin, Corrosion 
Protection for Metal Connectors in Coastal Areas 
(FIA-TB-8, 1996) contains information on 
corrosion protection. Attention also needs 
to be given to dry rot avoidance, for example, 
by specifying preservative-treated 
wood or developing details that avoid excessive 
moisture accumulation. Appendix J 
of the Coastal Construction Manual, (FEMA 
55, 2000) presents information on wood 
durability.

Durable materials are particularly important 
for components that are inaccessible 
and cannot be inspected regularly (such as 
fasteners used to attach roof insulation). 
Special attention also needs to be given to 
details. For example, details that do not 
allow water to stand at connections or sills 
are preferred. Without special attention to 
material selection and details, the demands 
on maintenance and repair will be increased, 
along with the likelihood of failure 
of components during high winds.

Water infiltration (rain): Although prevention 
of building collapse and major building 
damage is the primary goal of wind-resistant 
design, consideration should also be 
given to minimizing water damage and subsequent 
development of mold from the 
penetration of wind-driven rain. To the extent 
possible, non-load-bearing walls and 
door and window frames should be designed 
in accordance with rain-screen 
principles. With this approach, it is assumed 
that some water will penetrate past the face 
of the building envelope. The water is intercepted 
in an air-pressure equalized cavity 
that provides drainage from the cavity to the outer surface of 
the building. See Sections 3.3.3.2 and 3.3.3.3, and Figure 3-40 
for further discussion and an example. 

Coastal environments are conducive to 
metal corrosion, especially in buildings 
within 3,000 feet of the ocean. Most 
jurisdictions require metal building 
hardware to be hot-dipped galvanized 
or stainless steel. Some local codes 
require protective coatings that are thicker 
than typical “off-the-shelf” products. For 
example, a G90 zinc coating (0.75 mil 
on each face) may be required. Other 
recommendations include the following:

 Use hot-dipped galvanized or stainless 
steel hardware. Reinforcing steel should 
be fully protected from corrosion by the 
surrounding material (masonry, mortar, 
grout, or concrete). Use galvanized 
or epoxy-coated reinforcing steel 
in situations where the potential for 
corrosion is high.

 Avoid joining dissimilar metals, 
especially those with high galvanic 
potential.

 Avoid using certain wood preservatives 
in direct contact with galvanized metal. 
Verify that wood treatment is suitable 
for use with galvanized metal, or use 
stainless steel.

 Metal-plate-connected trusses should 
not be exposed to the elements. Truss 
joints near vent openings are more 
susceptible to corrosion and may 
require increased corrosion protection.

Note: Although more resistant than other 
metals, stainless steel is still subject to 
corrosion. 

In conjunction with the rain-screen principle, 
it is desirable to avoid using sealant 
as the first or only line of defense against 
water infiltration. When sealant joints are 
exposed, obtaining long-lasting watertight 
performance is difficult because of the 
complexities of sealant joint design and installation 
(see Figure 3-40, which shows the sealant protected by a 
removable stop).

Step 4: Peer Review

If the design team’s wind expertise and experience is limited, 
wind design input and/or peer review should be sought from a 
qualified individual. The design input or peer review could be arranged 
for the entire building, or for specific components such as 
the roof or glazing systems, that are critical and beyond the design 
team’s expertise. 

Regardless of the design team’s expertise and experience, peer review 
should be considered when a critical facility:

m Is located in an area where the basic wind speed is greater 
than 90 mph (peak gust)—particularly if the facility is a 
hospital, or will be used as an EOC or hurricane shelter.
m Will incorporate a tornado shelter.
3.3.1.3 Construction Contract Administration

After a suitable design is complete, the design team should endeavor 
to ensure that the design intent is achieved during 
construction. The key elements of construction contract administration 
are submittal reviews and field observations, as 
discussed below.

Further information on the rain-screen 
principle can be found in the National 
Institute of Building Sciences’ Building 
Envelope Design Guide (www.wbdg.org/
design/envelope.php).

Submittal reviews: The specifications need to stipulate the submittal 
requirements. This includes specifying what systems require 
submittals (e.g., windows) and test data (where appropriate). Each 
submittal should demonstrate the development of a load path 
through the system and into its supporting element. For example, 
a window submittal should show that the glazing has sufficient 
strength, its attachment to the frame is adequate, and the attachment 
of the frame to the wall is adequate.

During submittal review, it is important for the designer of record 
to be diligent in ensuring that all required documents are 
submitted and that they include the necessary information. The 
submittal information needs to be thoroughly checked to ensure 
its validity. For example, if an approved method used to 
demonstrate compliance with the design load has been altered 
or incorrectly applied, the test data should be rejected, unless 
the contractor can demonstrate the test method was suitable. 
Similarly, if a new test method has been developed by a manufacturer 
or the contractor, the contractor should demonstrate 
its suitability.

Field observations: It is recommended that the design team analyze 
the design to determine which elements are critical to ensuring 
high-wind performance. The analysis should include the structural 
system and exterior-mounted electrical equipment, but it 
should focus on the building envelope and exterior-mounted 
mechanical and communications equipment. After determining 
the list of critical elements to be observed, observation frequency 
and the need for special inspections by an inspection firm 
should be determined. Observation frequency and the need for 
special inspections will depend on the results of the risk assessment 
described in Section 3.2.2, complexity of the facility, and 
the competency of the general contractor, subcontractors, and 
suppliers.

See Section 3.4 for additional information pertaining to critical facilities 
located in hurricane-prone regions.

3.3.1.4 Post-Occupancy Inspections, Periodic Maintenance, Repair, and 
Replacement

The design team should advise the building owner of the importance 
of periodic inspections, maintenance, and timely repair. It 
is important for the building owner to understand that a facility’s 
wind resistance will degrade over time due to exposure to weather 
unless it is regularly maintained and repaired. The goal should 
be to repair or replace items before they fail in a storm. This 
approach is less expensive than waiting for failure and then repairing 
the failed components and consequential damage. 

The building envelope and exterior-mounted equipment should 
be inspected once a year by persons knowledgeable of the 
systems/materials they are inspecting. Items that require maintenance, 
repair, or replacement should be documented and 
scheduled for work. For example, the deterioration of glazing is 
often overlooked. After several years of exposure, scratches and 
chips can become extensive enough to weaken the glazing. Also, if 
an engineered film was surface-applied to glazing for wind-borne 
debris protection, the film should be periodically inspected and 
replaced before it is no longer effective.

A special inspection is recommended following unusually high 
winds. The purpose of the inspection is to assess whether the 
strong storm caused damage that needs to be repaired to maintain 
building strength and integrity. In addition to inspecting 
for obvious signs of damage, the inspector should determine if 
cracks or other openings have developed that may allow water infiltration, 
which could lead to corrosion or dry rot of concealed 
components.

See Section 3.4 for additional information pertaining to buildings 
located in hurricane-prone regions.

3.3.2 Structural Systems

Based on post-storm damage evaluations, with the exception of 
strong and violent tornado events, the structural systems (i.e., 
MWFRS and structural components such as roof decking) of critical 
facilities have typically performed quite well during design 
wind events. There have, however, been notable exceptions; in 
these cases, the most common problem has been blow-off of the 
roof deck, but instances of collapse have also been documented 
(see Figure 3-22). The structural problems have primarily been 
caused by lack of an adequate load path, with connection failure 
being a common occurrence. Problems have also been caused by 
reduced structural capacity due to termites, workmanship errors 
(commonly associated with steel decks attached by puddle welds), 
and limited uplift resistance of deck connections in roof perimeters 
and corners (due to lack of code-required enhancement in 
older editions of the model codes).

With the exception of strong and violent tornado events, structural 
systems designed and constructed in accordance with the 
IBC should typically offer adequate wind resistance, provided attention 
was given to load path continuity and to the durability 
of building materials (with respect to corrosion and termites). 
However, the greatest reliability is offered by cast-in-place concrete. 
There are no known reports of any cast-in-place concrete 
buildings experiencing a significant structural problem during 
wind events, including the strongest hurricanes (Category 5) 
and tornadoes (F5). 

Figure 3-22: Collapse of a large portion of a new pre-engineered 
metal building used as a shelter for approximately 
1,400 people. Hurricane Charley (Florida, 2004)

The following design parameters are recommended for structural 
systems (see Section 3.4.2 for critical facilities located in hurricane-
prone regions):

 If a pre-engineered metal building is being contemplated, 
special steps should be taken to ensure the structure has more 
redundancy than is typically the case with pre-engineered buildings.
7 Steps should be taken to ensure the structure is not vulnerable 
to progressive collapse in the event a primary bent (steel 
moment frame) is compromised or bracing components fail.
 Exterior load-bearing walls of masonry or precast concrete 
should be designed to have sufficient strength to resist external 
and internal loading when analyzed as C&C. CMU walls should 
have vertical and horizontal reinforcing and grout to resist wind 
loads. The connections of precast concrete wall panels should 
be designed to have sufficient strength to resist wind loads.
 For roof decks, concrete, steel, plywood, or oriented strand 
board (OSB) is recommended. 
m For steel roof decks, it is recommended that a screw 
attachment be specified, rather than puddle welds or 
powder-driven pins. Screws are more reliable and much less 
susceptible to workmanship problems. Figure 3-23 shows 
decking that was attached with puddle welds. At most of the 
welds, there was only superficial bonding of the metal deck to 
the joist, as illustrated by this example. Only a small portion of 
the deck near the center of the weld area (as delineated by the 
circle) was well fused to the joist. Figures 3-24 and 3-25 show 
problems with acoustical decking attached with powder-driven 
pins. The pin shown on the left of Figure 3-25 is properly 
seated. However, the pin at the right did not penetrate far 
enough into the steel joist below. 
 For attaching wood sheathed roof decks, screws, ring-shank, 
or screw-shank nails are recommended in the corner regions 
of the roof. Where the basic wind speed is greater than 90 
mph, these types of fasteners are also recommended for the 
perimeter regions of the roof.

7. The structural system of pre-engineered metal buildings is composed of rigid 
steel frames, secondary members (including roof purlins and wall 
girts made of Z- or C-shaped members) and bracing.

 For precast concrete decks it is recommended that the deck 
connections be designed to resist the design uplift loads 
because the deck dead load itself is often insufficient to resist 
the uplift. The deck in Figure 3-26 had bolts to provide uplift 
resistance; however, anchor plates and nuts had not been 
installed. Without the anchor plates, the dead load of the deck 
was insufficient to resist the wind uplift load.

Figure 3-23: View looking down at the top of a steel joist after the metal 
decking blew away. Only a small portion of 
the deck was well fused to the joist (circled area). 
Tornado (Oklahoma, 1999)

Figure 3-25: View looking along a sidelap of a deck attached 
with powder-driven pins. The right pin does not provide adequate uplift and 
shear resistance.

Figure 3-24: Looking down at a sidelap of a deck attached 
with powder-driven pins. The washer at the top pin 
blew through the deck.

 For precast Tee decks, it is recommended that the reinforcing 
be designed to accommodate the uplift loads in addition 
to the gravity loads. Otherwise, large uplift forces can cause 
member failure due to the Tee’s own pre-stress forces after 
the uplift load exceeds the dead load of the Tee. This type 
of failure occurred at one of the roof panels shown in Figure 
3-27, where a panel lifted because of the combined effects of 
wind uplift and pre-tension. Also, because the connections 
between the roof and wall panels provided very little uplift 
load resistance, several other roof and wall panels collapsed.

Figure 3-26: Portions of this waffled 
precast concrete roof deck were blown off. Typhoon 
Paka (Guam, 1997)

Figure 3-27: Twin-Tee roof panel lifted as a result of the combined 
effects of wind uplift and pre-tension. Tornado 
(Missouri, May 2003)

 For buildings that have mechanically attached single-ply or 
modified bitumen membranes, designers should refer to 
the decking recommendations presented in the Wind Design 
Guide for Mechanically Attached Flexible Membrane Roofs, B1049 
(National Research Council of Canada, 2005).
 If an FMG-rated roof assembly is specified, the roof deck also 
needs to comply with the FMG criteria.
 Walkway and entrance canopies are often damaged during 
high winds (see Figure 3-28). Wind-borne debris from 
damaged canopies can damage nearby buildings and injure 
people, hence these elements should also receive design and 
construction attention. 

Figure 3-28: The destroyed walkway canopy in front of a school 
became wind-borne debris. Hurricane Ivan (Florida, 2004)

ASCE 7-05 provides pressure coefficients for open canopies of various slopes 
(referred to as “free roofs” in ASCE 7). The free roof figures for MWFRS in ASCE 
7-05 (Figures 6-18A to 6-18D) include two load cases, Case A and Case B. While 
there is no discussion describing the two load cases, they pertain to 
fluctuating loads and are intended to represent upper and lower limits of 
instantaneous wind pressures. Loads for both cases must be calculated to 
determine the critical loads. Figures 6-18A to 6-18C are for a wind direction 
normal to the ridge. For wind direction parallel to the ridge, use Figure 6-18D 
in ASCE 7-05.

3.3.3 Building Envelope 

The following section highlights the design considerations for 
building envelope components that have historically sustained the 
greatest and most frequent damage in high winds.

3.3.3.1 Exterior Doors

This section addresses primary and secondary 
egress doors, sectional (garage) doors, and 
rolling doors. Although blow-off of personnel 
doors is uncommon, it can cause serious 
problems (see Figure 3-29). Blown-off doors 
allow entrance of rain and tumbling doors 
can damage buildings and cause injuries. 

Blown off sectional and rolling doors are quite common. These 
failures are typically caused by the use of door and track assemblies 
that have insufficient wind resistance, or by inadequate 
attachment of the tracks or nailers to the wall. At the relatively 
new fire station shown in Figure 3-30, two 
of the windward doors were pushed out of 
their tracks. At the third door, the track was 
pushed out from the nailer. With the collapse 
of these doors, the apparatus bay was 
fully pressurized. Because the connections 
between the trusses and the beam were too 
weak to accommodate the uplift load, the 
entire roof structure over the apparatus bay 
blew off. 

See Section 3.4.3.1 for critical facilities located in hurricane-
prone regions.

Loads and Resistance

The IBC requires that the door assembly (i.e., door, hardware, 
frame, and frame attachment to the wall) be of sufficient strength 
to resist the positive and negative design wind pressure. Design 
professionals should require that doors comply with wind 
load testing in accordance with ASTM E 1233. Design professionals should also 
specify the attachment 
of the door frame to the wall (e.g., type, 
size, spacing, and edge distance of frame 
fasteners). For sectional and rolling doors attached 
to wood nailers, design professionals 
should also specify the attachment of the 
nailer to the wall.

For further general information on doors, 
see “Fenestration Systems” in the National 
Institute of Building Sciences’ Building 
Envelope Design Guide (www.wbdg.org/
design/envelope.php)

Particular attention should be given to 
the design and installation of fire station 
apparatus bay doors which have been 
blown-in or blown-out frequently (see 
Figure 3-30). If doors blow inward, they 
can damage fire engines and ambulances 
and impair emergency response. 

For design guidance on attachment of door 
frames, see Technical Data Sheet #161, 
Connecting Garage Door Jambs to Building 
Framing, published by the Door & Access 
Systems Manufacturers Association, 2003 
(available at www.dasma.com).

Figure 3-29: Door on a hospital penthouse blown off its 
hinges during Hurricane Katrina (Mississippi, 2005)


Figure 3-30: The roof structure over the apparatus bay blew 
off following the failure of sectional doors. Hurricane 
Charley (Florida, 2004)

Water Infiltration

Heavy rain that accompanies high winds (e.g., thunderstorms, 
tropical storms, and hurricanes) can cause significant wind-
driven water infiltration problems. The magnitude of the 
problem increases with the wind speed. Leakage can occur between 
the door and its frame, the frame 
and the wall, and between the threshold 
and the door. When wind speeds approach 
120 mph, some leakage should be anticipated 
because of the very high wind 
pressures and numerous opportunities for 
leakage path development. 

The following recommendations should be considered to minimize 
infiltration around exterior doors. 

Vestibule: Adding a vestibule allows both the inner and outer doors 
to be equipped with weatherstripping. The vestibule can be designed 
with water-resistant finishes (e.g., concrete or tile) and the 
floor can be equipped with a drain. In addition, installing exterior 
threshold trench drains can be helpful (openings must be small 
enough to avoid trapping high-heeled shoes). Note that trench 
drains do not eliminate the problem, since water can still penetrate 
at door edges.

Door swing: Out-swinging doors have weatherstripping on the interior 
side of the door, where it is less susceptible to degradation, 
which is an advantage when compared to 
in-swinging doors. Some interlocking weatherstripping 
assemblies are available for 
out-swinging doors.

The successful integration of the door frame 
and the wall is a special challenge when 
designing doors. See Section 3.3.3.2 for discussion 
of this juncture. 

ASTM E 2112 provides information pertaining to the installation 
of doors, including the use of sill pan flashings with end dams and 
rear legs (see Figure 3-31). It is recommended that designers use 
ASTM E 2112 as a design resource.

Where corrosion is problematic, anodized 
aluminum or galvanized doors and frames, 
and stainless steel frame anchors and 
hardware are recommended. 

For primary swinging entry/exit doors, 
exit door hardware is recommended to 
minimize the possibility of the doors being 
pulled open by wind suction. Exit hardware 
with top and bottom rods is more secure 
than exit hardware that latches at the jamb.

Weatherstripping

A variety of pre-manufactured weatherstripping components is 
available, including drips, door shoes and bottoms, thresholds, 
and jamb/head weatherstripping. 

Drips: These are intended to shed water away from the opening between 
the frame and the door head, and the opening between the 
door bottom and the threshold (see Figures 3-32 and 3-33). Alternatively, 
a door sweep can be specified (see Figure 3-33). For 
high-traffic doors, periodic replacement of the neoprene components 
will be necessary.

Figure 3-31: Door sill pan flashing with end dams, rear leg, 
and turned-down front leg 

Figure 3-33: Door shoe with drip and vinyl seal (left). 
Neoprene door bottom sweep (right)

Figure 3-32: Drip at door head and drip with hook at head

Door shoes and bottoms: These are intended to minimize the gap 
between the door and the threshold. Figure 3-33 illustrates a door 
shoe that incorporates a drip. Figure 3-34 illustrates an automatic 
door bottom. Door bottoms can be surface-mounted or mortised. 
For high-traffic doors, periodic replacement of the neoprene components 
will be necessary.

Thresholds: These are available to suit a variety of conditions. 
Thresholds with high (e.g., 1-inch) vertical offsets offer enhanced 
resistance to wind-driven water infiltration. However, 
the offset is limited where the thresholds are required to comply 
with the Americans with Disabilities Act (ADA), or at high-traffic 
doors. At other doors, high offsets are preferred. 

Thresholds can be interlocked with the door (see Figure 3-
35), or thresholds can have a stop and seal (see Figure 3-36). In 
some instances, the threshold is set directly on the floor. Where 
this is appropriate, setting the threshold in butyl sealant is recommended 
to avoid water infiltration between the threshold 
and the floor. In other instances, the threshold is set on a pan 
flashing, as previously discussed in this section. If the threshold 
has weep holes, specify that the weep holes not be obstructed 
(see Figure 3-35). 

Figure 3-34: Automatic door bottom

Figure 3-36: Threshold with stop and seal

Figure 3-35: Interlocking threshold with drain pan

Adjustable jamb/head weatherstripping: This type of weatherstripping 
is recommended because the wide sponge neoprene offers 
good contact with the door (see Figure 3-37). The adjustment 
feature also helps to ensure good contact, provided the proper adjustment 
is maintained.

Meeting stile: At the meeting stile of pairs of doors, an overlapping 
astragal weatherstripping offers greater protection than weatherstripping 
that does not overlap. 

3.3.3.2 Windows and Skylights

This section addresses general design considerations for exterior 
windows and skylights in critical facilities. For additional information 
on windows and skylights in critical facilities located in 
hurricane-prone regions, see Section 3.4.3.2, and for those in tornado-
prone regions, see Section 3.5.

Loads and Resistance

The IBC requires that windows, curtain walls, 
and skylight assemblies (i.e., the glazing, 
frame, and frame attachment to the wall 
or roof) have sufficient strength to resist 
the positive and negative design wind pressure 
(see Figure 3-38). Design professionals 
should specify that these assemblies comply 
with wind load testing in accordance with ASTM E 1233. It is 
important to specify an adequate load path and to check its continuity 
during submittal review.

Water Infiltration 

Heavy rain accompanied by high winds can cause wind-driven 
water infiltration problems. The magnitude of the problem 
increases with the wind speed. Leakage can occur at the glazing/
frame interface, the frame itself, or between the frame and wall. 
When the basic wind speed is greater than 120 mph, because of 
the very high design wind pressures and numerous opportunities 
for leakage path development, some leakage should be anticipated 
when the design wind speed conditions are approached.

Figure 3-37: Adjustable jamb/head weatherstripping

For further general information on windows, 
see the National Institute of Building 
Sciences’ Building Envelope Design Guide 
(www.wbdg.org/design/envelope.php).

The successful integration of windows and curtain walls into exterior 
walls is a challenge in protecting against water infiltration. 
To the extent possible when detailing the interface between 
the wall and the window or curtain wall units, designers should 
rely on sealants as the secondary line of defense 
against water infiltration, rather than 
making the sealant the primary protection. 
If a sealant joint is the first line of defense, a 
second line of defense should be designed to 
intercept and drain water that drives past the 
sealant joint.

When designing joints between walls and windows and curtain wall 
units, consider the shape of the sealant joint (i.e., a square joint 
is typically preferred) and the type of sealant 
to be specified. The sealant joint should be 
designed to enable the sealant to bond on 
only two opposing surfaces (i.e., a backer rod 
or bond-breaker tape should be specified). 
Butyl is recommended as a sealant for concealed 
joints, and polyurethane for exposed 
joints. During installation, cleanliness of the 
sealant substrate is important (particularly if 
polyurethane or silicone sealants are specified), 
as is the tooling of the sealant. ASTM 

Figure 3-38: Two complete windows, including frames, blew out as a result of an 
inadequate number of fasteners. Typhoon Paka (Guam, 1997)

Where corrosion is a problem, use of 
anodized aluminum or stainless steel 
frames and stainless steel frame anchors 
and screws are recommended.

The maximum test pressure used in the 
current ASTM test standard for evaluating 
resistance of window units to wind-driven 
rain is well below design wind pressures. 
Therefore, units that demonstrate adequate 
wind-driven rain resistance during testing 
may experience leakage during actual 
wind events.

E 2112 provides guidance on the design of 
sealant joints, as well as other information 
pertaining to the installation of windows, including 
the use of sill pan flashings with end 
dams and rear legs (see Figure 3-39). Windows 
that do not have nailing flanges should 
typically be installed over a pan flashing. It 
is recommended that designers use ASTM E 
2112 as a design resource. 

Sealant joints can be protected with a removable 
stop, as illustrated in Figure 3-40. 
The stop protects the sealant from direct 
exposure to the weather and reduces the 
possibility of wind-driven rain penetration. 

Where water infiltration protection is particularly 
demanding and important, it is recommended that onsite 
water infiltration testing in accordance with ASTM E 1105 be 
specified.


Figure 3-39: View of a typical window sill pan flashing with 
end dams and rear legs 
Source: ASTM E 2112

Figure 3-40: Protecting sealant with a stop retards weathering 
and reduces the exposure to wind-driven rain.

3.3.3.3 Non-Load-Bearing Walls, Wall Coverings, and Soffits

This section addresses exterior non-load-bearing walls, exterior 
wall coverings, and soffits, as well as the underside of elevated 
floors, and provides guidance for interior 
non-load-bearing masonry walls. See Section 
3.4.3.3 for additional information pertaining 
to critical facilities located in hurricane-
prone regions, and Section 3.5 for additional 
information pertaining to critical facilities located 
in tornado-prone regions. 

Loads and Resistance

The IBC requires that soffits, exterior non-load-bearing walls, and 
wall coverings have sufficient strength to resist the positive and 
negative design wind pressures.

For further general information on non-
load-bearing walls and wall coverings, see 
the National Institute of Building Sciences’ 
Building Envelope Design Guide 
(www.wbdg.org/design/envelope.php).

To ensure the continuity of elevator service, elevator penthouse walls must 
possess adequate 
wind and water resistance. If the walls blow away or water leaks through the 
wall system, the 
elevator controls and/or motors can be destroyed. Loss of elevators may 
critically affect facility 
operations because the restoration can take weeks even with expedited work (see 
Figure 3-41).

Figure 3-41: The wall covering blew off the penthouse at this 
hospital complex allowing rainwater to destroy 
the elevator controls. Hurricane Ivan (Florida, 2004)

Soffits: Depending on the wind direction, 
soffits can experience either positive or negative 
pressure. Besides the cost of repairing 
the damaged soffits, wind-borne soffit debris 
can cause property damage and injuries 
(see Figure 3-42). Failed soffits may also provide 
a convenient path for wind-driven rain 
to enter the building. Storm-damage research 
has shown that water blown into attic 
spaces after the loss of soffits caused significant 
damage and the collapse of ceilings. 
At the relatively new fire station shown in 
Figure 3-43, essentially all of the perforated 
aluminum soffit was blown away. Wind-
driven water entered the attic and saturated the batt insulation, 
which caused the ceiling boards to collapse. After the storm, the 
fire station was evacuated solely because of this damage. Even 
in instances where soffits remain in place, water can penetrate 
through soffit vents and cause damage. At this time, there are no 
known specific test standards or design guidelines to help design 
wind- and water-resistant soffits and soffit vents.

Where corrosion is a problem, stainless 
steel fasteners are recommended for wall 
and soffit systems. For other components 
(e.g., furring, blocking, struts, and hangers), 
nonferrous components (such as wood), 
stainless steel, or steel with a minimum of 
G-90 hot-dipped galvanized coating are 
recommended. Additionally, access panels 
are recommended so components within 
soffit cavities can be periodically inspected 
for corrosion or dry rot.

Figure 3-42: This suspended metal 
soffit was not designed for upward-acting wind 
pressure. Typhoon Paka (Guam, 1997)

Exterior non-load-bearing masonry walls: Particular care 
should be given to the design and construction of 
exterior non-load-bearing masonry walls. Although 
these walls are not intended to carry gravity loads, 
they should be designed to resist the external and 
internal loading for components and cladding in order to avoid 
collapse. When these types of walls collapse, they represent a severe 
risk to life because of their great weight (see Figure 3-15). 

Interior non-load-bearing masonry walls: Special consideration 
should also be given to interior non-load-bearing masonry walls. 
Although these walls are not required by building codes to be designed 
to resist wind loads, if the exterior glazing is broken, or 
the exterior doors are blown away, the interior walls could be subjected 
to significant load as the building rapidly becomes fully 
pressurized. To avoid casualties, it is recommended that interior 
non-load-bearing masonry walls adjacent to occupied areas be designed 
to accommodate loads exerted by a design wind event, 
using the partially enclosed pressure coefficient (see Figure 3-44). 
By doing so, wall collapse may be prevented if the building envelope 
is breached. This recommendation is applicable to critical 
facilities located in areas with a basic wind speed greater than 120 
mph, those used for hurricane shelters, and to critical facilities in 
tornado-prone regions that do not have shelter space designed in 
accordance with FEMA 361.

Figure 3-43: This fire station was 
abandoned after Hurricane Charley 
because of soffit failure. (Florida, 2004)
Wall Coverings

There are a variety of exterior wall coverings. Brick veneer, exterior 
insulation finish systems (EIFS), stucco, metal wall panels, and 
aluminum and vinyl siding have often exhibited poor wind performance. 
Veneers (such as ceramic tile and stucco) over concrete, 
stone veneer, and cement-fiber panels and siding have also blown 
off. Wood siding and panels rarely blow off. Although tilt-up precast 
walls have failed during wind storms, precast wall panels 
attached to steel or concrete framed buildings typically offer excellent 
wind performance.

Brick veneer:8 Brick veneer is frequently blown off walls during 
high winds. When brick veneer fails, wind-driven water can enter 
and damage buildings, and building occupants can be vulnerable 
to injury from wind-borne debris (particularly if the walls are 
sheathed with plastic foam insulation or wood fiberboard in lieu 
of wood panels). Pedestrians in the vicinity of damaged walls can 
also be vulnerable to injury from falling bricks (see Figure 3-45). 
Common failure modes include tie (anchor) fastener pull-out 
(see Figure 3-46), failure of masons to embed ties into the mortar 
(Figure 3-47), poor bonding between ties and mortar, a mortar of 
poor quality, and tie corrosion.

Figure 3-44: The red arrows show the original location of 
a CMU wall that nearly collapsed following a 
rolling door failure. Hurricane Charley (Florida, 
2004)

8. The brick veneer discussion is from Attachment of Brick Veneer in High-Wind 
Regions - Hurricane Katrina Recovery Advisory (FEMA, 
December 2005). 

Figure 3-45: The brick veneer failure 
on this building was attributed to tie corrosion. 
Hurricane Ivan (Florida, 2004)

Figure 3-46: This tie remained embedded in the mortar 
joint while the smooth-shank nail pulled from the stud.

Ties are often installed before brick laying begins. When this is 
done, ties are often improperly placed above or below the mortar 
joints. When misaligned, the ties must be angled up or down to 
be embedded into the mortar joints (Figure 3-48). Misalignment 
not only reduces the embedment depth, but also reduces the 
effectiveness of the ties, because wind forces do not act in parallel 
direction to the ties.

Corrugated ties typically used in residential and nursing home veneer 
construction provide little resistance to compressive loads. 
The use of compression struts would likely be beneficial, but off-
the-shelf devices do not currently exist. Two-piece adjustable ties 
(Figure 3-49) provide significantly greater compressive strength 
than corrugated ties.

Figure 3-47: These four ties were never embedded into the 
mortar joint. 

Figure 3-48: Misalignment of the tie reduces the embedment 
and promotes veneer failure.

The following Brick Industry Association (BIA) technical notes 
provide guidance on brick veneer: Technical Notes 28: Anchored 
Brick Veneer, Wood Frame Construction (2002); Technical Notes 
28B: Brick Veneer/Steel Stud Walls (2005); and Technical Notes 44B: 
Wall Ties (2003) (available online at www.bia.org). These technical 
notes provide attachment recommendations; however, they are 
not specific for high-wind regions. To enhance wind performance 
of brick veneer, the following are recommended: 

m Calculate wind loads and determine tie spacing in accordance 
with the latest edition of the Building Code Requirements for 
Masonry Structures, ACI 530/ASCE 5/TMS 402 (ACI 530, 
1996)). A stud spacing of 16 inches on center is recommended 
so that ties can be anchored at this spacing.
m Ring-shank nails are recommended in lieu of smooth-shank nails 
for wood studs. A minimum embedment of 2 inches is suggested.
m For use with wood studs, two-piece adjustable ties are 
recommended. However, where corrugated steel ties are 
used, they should be 22-gauge minimum, 7/8-inch wide by 6-
inch long, and comply with ASTM A 1008, with a zinc coating 
complying with ASTM A 153 Class B2. For ties used with steel 
studs, see BIA Technical Notes 28B, Brick Veneer/Steel Stud Walls. 
Stainless steel ties should be used for both wood and steel 
studs in areas within 3,000 feet of the coast.

Figure 3-49: Examples of two-piece adjustable ties

 Install ties as the brick is laid so that the ties are properly 
aligned with the mortar joints.
 Locate ties within 8 inches of door and window openings, and 
within 12 inches of the top of veneer sections.
 Although corrugated ties are not recommended, if used, bend 
the ties at a 90-degree angle at the nail head to minimize tie 
flexing when the ties are loaded in tension or compression 
(Figure 3-50).
 Embed ties in joints so that the mortar completely 
encapsulates the ties. Embed a minimum of 1½ inches into 
the bed joint, with a minimum mortar cover of 5/8-inch to the 
outside face of the wall (see Figure 3-51). 

Figure 3-50: Bend ties at nail heads

Figure 3-51: Tie embedment 

To avoid water leaking into the building, it is important that weep 
holes be adequately spaced and not be blocked during brick installation, 
and that through-wall flashings be properly designed 
and installed. At the hospital shown in Figure 3-52, water leaked 
into the building along the base of many of the brick veneer walls. 
When high winds accompany heavy rain, a substantial amount of 
water can be blown into the wall cavity. 

EIFS: Figure 3-53 shows typical EIFS assemblies. Figures 3-41, 3-54, 
and 4-6 show EIFS blow-off. In these cases, the molded expanded 
polystyrene (MEPS) was attached to gypsum board, which in turn 
was attached to metal studs. The gypsum board detached from 
the studs, which is a common EIFS failure. When the gypsum 
board on the exterior side of the studs is blown away, it is common 
for gypsum board on the interior side to also be blown off. The 
opening allows the building to become fully pressurized and allows 
the entrance of wind-driven rain. Other common types of failure 
include wall framing failure, separation of the MEPS from its substrate, 
and separation of the synthetic stucco from the MEPS. 

Figure 3-52: Water leaked inside along 
the base of the brick veneer walls. Hurricane 
Katrina (Louisiana, 2005) 

At the hospital shown in Figure 3-55, the EIFS was applied over a 
concrete wall. The MEPS debonded from the concrete. In general, 
a concrete substrate prevents wind and water from entering a 
building, but if the EIFS debonds from the concrete, EIFS debris 
can break unprotected glazing. 

Figure 3-53: Typical EIFS assemblies

Wind-borne EIFS debris can be devastating to unprotected 
glazing. At the hospital shown in Figure 3-56, the hospital’s original 
concrete wall panels had been furred with metal hat channels 
and covered with EIFS. In a large corner area, the EIFS and 
gypsum board substrate blew off the hat channels and broke a 
large number of windows in the multi-story connecting walkway 
between the hospital and the medical office building (MOB). The 
EIFS debris also broke a large number of windows in the MOB 
(see Figure 3-56). Glass shards from the MOB punctured the roof 
membrane over the dialysis unit. The costly damage resulted in 
loss of several rooms in the MOB and hampered functioning of 
the hospital complex.

Figure 3-54: EIFS blow-off at building 
corner. In places, metal fascia was also blown in. 
Tornado (Oklahoma, 1999)

Figure 3-55: EIFS blown off a cast-in-
place concrete wall. Note the damaged rooftop 
ductwork. Hurricane Ivan (Florida, 2004)

Reliable wind performance of EIFS is very demanding 
on the designer and installer, as well 
as the maintenance of EIFS and associated 
sealant joints in order to minimize the reduction 
of EIFS’ wind resistance due to water 
infiltration. It is strongly recommended that 
EIFS be designed with a drainage system that 
allows the dissipation of water leaks. For further 
information on EIFS performance during 
high winds and design guidance see FEMA 489 
and 549.

Another issue associated with EIFS is the potential for judgment errors. 
EIFS applied over studs is sometimes mistaken for a concrete 
wall, which may lead people to seek shelter behind it. However, instead 
of being protected by several inches of concrete, only two 
layers of gypsum board (i.e., one layer on each side of the studs) 
and a layer of MEPS separate the occupants from the impact of 
wind-borne debris that can easily penetrate such a wall and cause 
injury.

Figure 3-56:EIFS debris blown off 
the hospital building in the background (red 
square) broke numerous windows in the MOB in 
the foreground. Hurricane Ivan (Florida, 2004) 

Stucco: Wind performance of traditional stucco walls is similar to 
the performance of EIFS, as shown in Figure 3-57. In several areas 
the metal stud system failed; in other areas the gypsum sheathing 
blew off the studs; and in other areas, the metal lath blew off the 
gypsum sheathing. The failure shown in Figure 3-57 illustrates the 
importance of designing and constructing wall framing (including 
attachment of stud tracks to the building and attachment of the 
studs to the tracks) to resist the design wind loads.

Metal wall panels: Wind performance of metal wall panels is highly 
variable. Performance depends on the strength of the specified 
panel (which is a function of material and thickness, panel profile, 
panel width, and whether the panel is a composite) and the adequacy 
of the attachment (which can either be by concealed clips 
or exposed fasteners). Excessive spacing between clips/fasteners 
is the most common problem. Clip/fastener spacing should be 
specified, along with the specific type and size of fastener. Figures 
3-14 and 3-58 illustrate metal wall panel problems. At the school 
shown in Figure 3-58 (which was being used as a shelter), the 
metal panels were attached with concealed fasteners. The panels 
unlatched at the standing seams. In addition to generating wind-
borne debris, loss of panels allowed wind-driven rain to enter the 
building.

Figure 3-57: The stucco wall failure was 
caused by inadequate attachment between 
the stud tracks and the building’s structure. 
Hurricane Ivan (Florida, 2004)

To minimize water infiltration at metal wall panel joints, it is recommended 
that sealant tape be specified at sidelaps when the 
basic wind speed is in excess of 90 mph. However, endlaps should 
be left unsealed so that moisture behind the panels can be wicked 
away. Endlaps should be a minimum of 3 inches (4 inches where 
the basic wind speed is greater than 120 mph) to avoid wind-
driven rain infiltration. At the base of the 
wall, a 3-inch (4-inch) flashing should also be 
detailed, or the panels should be detailed to 
overlap with the slab or other components by 
a minimum of 3 inches (4 inches).

Vinyl siding: Vinyl siding blow-off is typically 
caused by nails spaced too far apart and/or 
the use of vinyl siding that has inadequate 
wind resistance. Vinyl siding is available 
with enhanced wind resistance features, 
such as an enhanced nailing hem, greater 
interlocking area, and greater thickness. 

Figure 3-58: The loss of metal wall 
panels allowed a substantial amount of wind-
driven rain to penetrate this building. Hurricane 
Ivan (Florida, 2004) 

The Vinyl Siding Institute (VSI) sponsors 
a Certified Installer Program that 
recognizes individuals with at least 1 
year of experience who can demonstrate 
proper vinyl siding application. If vinyl 
siding is specified, design professionals 
should consider specifying that the siding 
contractor be a VSI-certified installer. For 
further information on this program, see 
www.vinylsiding.org.

Secondary line of protection: Almost all wall coverings permit the 
passage of some water past the exterior surface of the covering, 
particularly when the rain is wind-driven. For this reason, most 
wall coverings should be considered water-shedding, rather than 
waterproofing coverings. To avoid moisture-related problems, 
it is recommended that a secondary line of protection with a 
moisture barrier (such as housewrap or asphalt-saturated felt) 
and flashings around door and window openings be provided. 
Designers should specify that horizontal laps of the moisture 
barrier be installed so that water is allowed to drain from the 
wall (i.e., the top sheet should lap over the bottom sheet so that 
water running down the sheets remains on their outer surface). 
The bottom of the moisture barrier needs to be designed to 
allow drainage. Had the metal wall panels shown in Figure 3-58 
been applied over a moisture barrier and sheathing, the amount 
of water entering the building would have likely been eliminated 
or greatly reduced. 

In areas that experience frequent wind-driven rain, incorporating 
a rain screen design, by installing vertical furring strips 
between the moisture barrier and siding materials, will facilitate 
drainage of water from the space between the moisture barrier 
and backside of the siding. In areas that frequently experience 
strong winds, enhanced flashing is recommended. Enhancements 
include use of flashings that have extra-long flanges, 
and the use of sealant and tapes. Flashing design should recognize 
that wind-driven water could be pushed up vertically. 
The height to which water can be pushed increases with wind 
speed. Water can also migrate vertically and horizontally by capillary 
action between layers of materials (e.g., between a flashing 
flange and housewrap). Use of a rain screen design, in conjunction 
with enhanced flashing design, is recommended in areas 
that frequently experience wind-driven rain or strong winds. It 
is recommended that designers attempt to determine what type 
of flashing details have successfully been used in the area where 
the facility will be constructed.

Underside of Elevated Floors

If sheathing is applied to the underside of joists or trusses elevated 
on piles (e.g., to protect insulation installed between the 
joists/trusses), its attachment should be specified in order to 
avoid blow-off. Stainless steel or hot-dip galvanized nails or screws 
are recommended. Since ASCE 7 does not provide guidance for 
load determination, professional judgment in specifying attachment 
is needed.

3.3.3.4 Roof Systems

Because roof covering damage has historically 
been the most frequent and the costliest 
type of wind damage, special attention needs 
to be given to roof system design. See Section 
3.4.3.4 for additional information pertaining 
to critical facilities located in hurricane-
prone regions, and Section 3.5 for critical 
facilities located in tornado-prone regions.

Code Requirements 

The IBC requires the load resistance of the roof assembly to 
be evaluated by one of the test methods listed in IBC’s Chapter 
15. Design professionals are cautioned that designs that deviate 
from the tested assembly (either with material substitutions or 
change in thickness or arrangement) may adversely affect the 
wind performance of the assembly. The IBC does not specify a 
minimum safety factor. However, for the roof system, a safety 
factor of 2 is recommended. To apply the safety factor, divide 
the test load by 2 to determine the allowable design load. 
Conversely, multiply the design load by 2 to determine the minimum 
required test resistance.

For structural metal panel systems, the IBC requires test methods 
UL 580 or ASTM E 1592. It is recommended 
that design professionals specify use of E 
1592, because it gives a better representation 
of the system’s uplift performance capability. 

For further general information on roof 
systems, see the National Institute of 
Building Sciences’ Building Envelope 
Design Guide (www.wbdg.org/design/
envelope.php).

The roof of the elevator penthouse must 
possess adequate wind and water 
resistance to ensure continuity of elevator 
service. It is recommended that a 
secondary roof membrane, as discussed 
in Section 3.3.3.3, be specified over the 
elevator penthouse roof deck. 

Load Resistance 

Specifying the load resistance is commonly done by specifying a 
Factory Mutual Research (FMR) rating, such as FM 1-75. The first 
number (1) indicates that the roof assembly passed the FMR tests 
for a Class 1 fire rating. The second number (75) indicates the uplift 
resistance in pounds per square foot (psf) that the assembly 
achieved during testing. With a safety factor of two this assembly 
would be suitable for a maximum design uplift load of 37.5 psf.

The highest uplift load occurs at the roof corners because of 
building aerodynamics as discussed in Section 3.1.3. The perimeter 
has a somewhat lower load, while the field of the roof has the 
lowest load. FMG Property Loss Prevention Data Sheets are formatted 
so that a roof assembly can be selected for the field of the 
roof. For the perimeter and corner areas, FMG 
Data Sheet 1-29 provides three options: 1) use 
the FMG Approval Guide listing if it includes 
a perimeter and corner fastening method; 2) 
use a roof system with the appropriate FMG Approval 
rating in the field, perimeter, and corner, 
in accordance with Table 1 in FMG Data Sheet 
1-29; or 3) use prescriptive recommendations 
given in FMG Data Sheet 1-29. 

When perimeter and corner uplift resistance 
values are based on a prescriptive method rather 
than testing, the field assembly is adjusted to 
meet the higher loads in the perimeter and corners 
by increasing the number of fasteners or 
decreasing the spacing of adhesive ribbons by 
a required amount. However, this assumes that 
the failure is the result of the fastener pulling 
out from the deck, or that the failure is in the vicinity 
of the fastener plate, which may not be the case. Also, the 
increased number of fasteners required by FMG may not be sufficient 
to comply with the perimeter and corner loads derived from 
the building code. Therefore, if FMG resistance data are specified, 
it is prudent for the design professional to specify the resistance for 
each zone of the roof separately. Using the example cited above, 
if the field of the roof is specified as 1-75, the perimeter would be 
specified as 1-130 and the corner would be specified as 1-190. 

FM Global (FMG) is the name of the 
Factory Mutual Insurance Company and its 
affiliates. One of FMG’s affiliates, Factory 
Mutual Research (FMR) provides testing 
services, produces documents that can be 
used by designers and contractors, and 
develops test standards for construction 
products and systems. FMR evaluates 
roofing materials and systems for resistance 
to fire, wind, hail, water, foot traffic and 
corrosion. Roof assemblies and components 
are evaluated to establish acceptable levels 
of performance. Some documents and 
activities are under the auspices of FMG 
and others are under FMR. 

If the roof system is fully adhered, it is not possible to increase 
the uplift resistance in the perimeter and corners. Therefore, for 
fully adhered systems, the uplift resistance requirement should be 
based on the corner load rather than the field load.

Roof System Performance 

Storm-damage research has shown that sprayed polyurethane 
foam (SPF) and liquid-applied roof systems are very reliable 
high-wind performers. If the substrate to which the SPF or liquid-
applied membrane is applied does not lift, it is highly unlikely 
that these systems will blow off. Both systems are also more resistant 
to leakage after missile impact damage than most other 
systems. Built-up roofs (BURs) and modified bitumen systems 
have also demonstrated good wind performance provided the 
edge flashing/coping does not fail (which happens frequently). 
The exception is aggregate surfacing, which is prone to blow-off 
(see Figures 3-12 and 3-13). Modified bitumen applied to a concrete 
deck has demonstrated excellent resistance to progressive 
peeling after blow-off of the metal edge flashing. Metal panel performance 
is highly variable. Some systems are very wind-resistant, 
while others are quite vulnerable. 

Of the single-ply attachment methods, the paver-ballasted and 
fully adhered methods are the least problematic. Systems with 
aggregate ballast are prone to blow-off, unless care is taken in 
specifying the size of aggregate and the parapet height (see Figure 
3-9). The performance of protected membrane roofs (PMRs) with 
a factory-applied cementitious coating over insulation boards is 
highly variable. When these boards are installed over a loose-laid 
membrane, it is critical that an air retarder be incorporated to prevent 
the membrane from ballooning and disengaging the boards. 
ANSI/SPRI RP-4 (which is referenced in the IBC) provides wind 
guidance for ballasted systems using aggregate, pavers, and cementitious-
coated boards. 

The National Research Council of Canada, Institute for Research 
in Construction’s Wind Design Guide for Mechanically Attached 
Flexible Membrane Roofs (B1049, 2005) provides recommendations 
related to mechanically attached single-ply and modified 
bituminous systems. B1049 is a comprehensive wind design guide 
that includes discussion on air retarders. Air retarders can be 
effective in reducing membrane flutter, in addition to being beneficial 
for use in ballasted single-ply systems. When a mechanically 
attached system is specified, careful coordination with the structural 
engineer in selecting deck type and thickness is important. 

If a steel deck is selected, it is critical to specify that the membrane 
fasteners be attached in rows perpendicular to the steel flanges 
to avoid overstressing the attachment of the deck to the deck support 
structure. At the school shown in Figure 3-59, the fastener 
rows of the mechanically attached single-ply membrane ran parallel 
to the top flange of the steel deck. The deck fasteners were 
overstressed and a portion of the deck blew off and the membrane 
progressively tore. At another building, shown in Figure 3-60, the 
membrane fastener rows also ran parallel to the top flange of the 
steel deck. When membrane fasteners run parallel to the flange, 
the flange with membrane fasteners essentially carries all the uplift 
load because of the deck’s inability to transfer any significant 
load to adjacent flanges. Hence, at the joists shown in Figure 3-60, 
the deck fasteners on either side of the flange with the membrane 
fasteners are the only connections to the joist that are carrying 
substantial uplift load.

Figure 3-59: The orientation of the 
membrane fastener rows led to blow-off of the steel 
deck. Hurricane Marilyn (U.S. Virgin Islands, 1995)

For metal panel roof systems, the following are recommended:

 When clip or panel fasteners are attached to nailers, detail the 
connection of the nailer to the nailer support (including the 
detail of where nailers are spliced over a support). 
 When clip or panel fasteners are loaded in withdrawal 
(tension), screws are recommended in lieu of nails. 
 For concealed clips over a solid substrate, it is recommended 
that chalk lines be specified so that the clips are correctly 
spaced.
 When the basic wind speed is 110 mph or greater, it is 
recommended that two clips be used along the eaves, ridges, 
and hips.
 For copper panel roofs in areas with a basic wind speed greater 
than 90 mph, it is recommended that Type 316 stainless steel 
clips and stainless steel screws be used in lieu of copper clips.
 Close spacing of fasteners is recommended at hip and ridge 
flashings (e.g., spacing in the range of 3 to 6 inches on center, 
commensurate with the design wind loads.)

Figure 3-60: View of the underside of 
a steel deck showing the mechanically attached 
single-ply membrane fastener rows running 
parallel to, instead of across, the top flange of 
the deck. 

Edge Flashings and Copings

Roof membrane blow-off is almost always a result of lifting and 
peeling of the metal edge flashing or coping, which serves to 
clamp down the membrane at the roof edge. Therefore, it is important 
for the design professional to carefully consider the 
design of metal edge flashings, copings, and the nailers to which 
they are attached. The metal edge flashing on the modified 
bitumen membrane roof shown in Figure 3-61 was installed underneath 
the membrane, rather than on top of it, and then stripped 
in. In this location, the edge flashing was unable to clamp the 
membrane down. At one area, the membrane was not sealed to 
the flashing. An ink pen was inserted into the opening prior to 
photographing to demonstrate how wind could catch the opening 
and lift and peel the membrane. 

ANSI/SPRI ES-1, Wind Design Standard for Edge Systems Used in Low 
Slope Roofing Systems (2003) provides general design guidance including 
a methodology for determining the outward-acting load 
on the vertical flange of the flashing/coping (ASCE 7 does not 
provide this guidance). ANSI/SPRI ES-1 is referenced in the IBC. 
ANSI/SPRI ES-1 also includes test methods for assessing flashing/
coping resistance. This manual recommends a minimum safety 
factor of 3 for edge flashings, copings, and nailers for critical facil
ities. For FMG-insured facilities, FMR-approved flashing should be 
used and FM Data Sheet 1-49 should also be consulted. 

Figure 3-61: The ink pen shows an 
opening that the wind can catch, and cause 
lifting and peeling of the membrane. 

The traditional edge flashing/coping attachment method relies 
on concealed cleats that can deform under wind load and lead to 
disengagement of the flashing/coping (see Figure 3-62) and subsequent 
lifting and peeling of the roof membrane (as shown in 
Figure 3-12). When a vertical flange disengages and lifts up, the 
edge flashing and membrane are very susceptible to failure. Normally, 
when a flange lifts such as shown in Figure 3-62, the failure 
continues to propagate and the metal edge flashing and roof 
membrane blows off.

Storm-damage research has revealed that, in lieu of cleat attachment, 
the use of exposed fasteners to attach the vertical flanges 
of copings and edge flashings has been found to be a very effective 
and reliable attachment method. The coping shown in 
Figure 3-63 was attached with ¼-inch diameter stainless steel concrete 
spikes at 12 inches on center. When the fastener is placed 
in wood, #12 stainless steel screws with stainless steel washers are 
recommended. The fasteners should be more closely spaced in 
the corner areas (the spacing will depend upon the design wind 
loads). ANSI/SPRI ES-1 provides guidance on fastener spacing 
and thickness of the coping and edge flashing.

Figure 3-62: The metal edge flashing 
disengaged from the continuous cleat and 
the vertical flange lifted. Hurricane Hugo (South 
Carolina, 1989)

Gutters 

Storm-damage research has shown that gutters are seldom constructed 
to resist wind loads (see Figure 3-64). When a gutter lifts, 
it typically causes the edge flashing that laps into the gutter to lift 
as well. Frequently, this results in a progressive lifting and peeling 
of the roof membrane. The membrane blow-off shown in Figure 
3-65 was initiated by gutter uplift. The gutter was similar to that 
shown in Figure 3-64. The building, housing the county Sheriff’s 
office, suffered water leakage that shut down the county 911 call 
center, destroyed the crime lab equipment, and caused significant 
interior water damage.

Figure 3-63: Both vertical faces of the 
coping were attached with exposed fasteners instead 
of concealed cleats. Typhoon Paka (Guam, 1997)

Figure 3-64: This gutter, supported by a type of bracket that 
provides no significant uplift resistance, failed 
when wind lifted it together with the metal 
edge flashing that lapped into the gutter. Hurricane 
Francis (Florida, 2004)

Special design attention needs to be given to attaching gutters 
to prevent uplift, particularly for those in excess of 6 inches in 
width. Currently, there are no standards pertaining to gutter 
wind resistance. It is recommended that the designer calculate 
the uplift load on gutters using the overhang coefficient from 
ASCE 7. There are two approaches to resist gutter uplift.

 Gravity-support brackets can be designed to resist uplift 
loads. In these cases, in addition to being attached at its 
top, the bracket should also be attached at its low end to the 
wall. The gutter also needs to be designed so it is attached 
securely to the bracket in a way that will effectively transfer 
the gutter uplift load to the bracket. Bracket spacing will 
depend on the gravity and uplift load, the bracket’s strength, 
and the strength of connections between the gutter/bracket 
and the bracket/wall. With this option, the bracket’s top will 
typically be attached to a wood nailer, and that fastener will 
be designed to carry the gravity load. The bracket’s lower 
connection will resist the rotational force induced by gutter 
uplift. Because brackets are usually spaced close together 
to carry the gravity load, developing adequate connection 
strength at the lower fastener is generally not difficult. 
 The other option is to use gravity-support brackets only to 
resist gravity loads, and use separate sheet-metal straps at 45-
degree angles to the wall to resist uplift loads. Strap spacing 
will depend on the gutter uplift load and strength of the 
connections between the gutter/strap and the strap/wall. 
Note that FMG Data Sheet 1-49 recommends placing straps 
10 feet apart. However, at that spacing with wide gutters, 
fastener loads induced by uplift are quite high. When straps 
are spaced at 10 feet, it can be difficult to achieve sufficiently 
strong uplift connections. 

Figure 3-65: The original modified 
bitumen membrane was blown away after the 
gutter lifted in the area shown by the red arrow 
(the black membrane is a temporary roof). Hurricane 
Francis (Florida, 2004)

When designing a bracket’s lower connection to a 
wall or a strap’s connection to a wall, designers should 
determine appropriate screw pull-out values. With this 
option, a minimum of two screws at each end of a strap is 
recommended. At a wall, screws should be placed side by 
side, rather than vertically aligned, so the strap load is carried 
equally by the two fasteners. When fasteners are vertically 
aligned, most of the load is carried by the top fastener.
Since the uplift load in the corners is much higher than the load 
between the corners, enhanced attachment is needed in corner 
areas regardless of the option chosen. ASCE 7 provides guidance 
about determining a corner area’s length.

Parapet Base Flashings 

Information on loads for parapet base flashings was first introduced 
in the 2002 edition of ASCE 7. The loads on base flashings 
are greater than the loads on the roof covering if the parapet’s exterior 
side is air-permeable. When base flashing is fully adhered, 
it has sufficient wind resistance in most cases. However, when base 
flashing is mechanically fastened, typical fastening patterns may 
be inadequate, depending on design wind conditions (see Figure 
3-66). Therefore, it is imperative that the base flashing loads be 
calculated, and attachments be designed to accommodate these 
loads. It is also important for designers to specify the attachment 
spacing in parapet corner regions to differentiate them from the 
regions between corners.

Steep-Slope Roof Coverings 

For a discussion of wind performance of asphalt shingle and tile 
roof coverings, see FEMA 488 (2005), 489 (2005), and 549 (2006). 
For recommendations pertaining to asphalt shingles and tiles, see 
Fact Sheets 19, 20, and 21 in FEMA 499 (2005).

3.3.4 Nonstructural Systems and Equipment

Nonstructural systems and equipment include all components 
that are not part of the structural system or building envelope. Exterior-
mounted mechanical equipment (e.g., exhaust fans, HVAC 
units, relief air hoods, rooftop ductwork, and boiler stacks), electrical 
equipment (e.g., light fixtures and lightning protection 
systems), and communications equipment (e.g., antennae and 
satellite dishes) are often damaged during high winds. Damaged 
equipment can impair the operation of the facility, the equipment 
can detach and become wind-borne missiles, and water can enter 
the facility where equipment was displaced (see Figures 3-20, 3-67, 
3-68, 3-72, 3-76, and 3-78). The most common problems typically 
relate to inadequate equipment anchorage, inadequate strength 
of the equipment itself, and corrosion.

Figure 3-66: If mechanically attached 
base flashings have an insufficient number 
of fasteners, the base flashing can be blown 
away. Hurricane Andrew (Florida, 1992) 

See Section 3.4.4 for additional information pertaining to critical 
facilities located in hurricane-prone regions.

3.3.4.1 Exterior-Mounted Mechanical Equipment

This section discusses loads and attachment methods, as well as 
the problems of corrosion and water infiltration.

Figure 3-67: Toppled rooftop mechanical equipment. 
Hurricane Andrew (Florida, 1992)

Figure 3-68: This gooseneck was attached with only two 
small screws. A substantial amount of water was 
able to enter the building during Hurricane Francis. 
(Florida, 2004)

Loads and Attachment Methods 

Information on loads on rooftop equipment was first introduced 
in the 2002 edition of ASCE 7. For guidance on load calculations, 
see “Calculating Wind Loads and Anchorage Requirements for 
Rooftop Equipment” (ASHRAE, 2006). A minimum safety factor 
of 3 is recommended for critical facilities. Loads and resistance 
should also be calculated for heavy pieces of equipment since the 
dead load of the equipment is often inadequate to resist the design 
wind load. The 30’ x 10’ x 8’ 18,000-pound HVAC unit shown 
in Figure 3-69 was attached to its curb with 16 straps (one screw 
per strap). Although the wind speeds were estimated 
to be only 85 to 95 miles per hour 
(peak gust), the HVAC unit blew off the medical 
office building. 

To anchor fans, small HVAC units, and relief 
air hoods, the minimum attachment 
schedule provided in Table 3-2 is recommended. 
The attachment of the curb to the 
roof deck also needs to be designed and constructed 
to resist the design loads. 

Mechanical penetrations through the 
elevator penthouse roof and walls must 
possess adequate wind and water 
resistance to ensure continuity of elevator 
service (see Section 3.3.3.3). In addition 
to paying special attention to equipment 
attachment, air intakes and exhausts 
should be designed and constructed to 
prevent wind-driven water from entering 
the penthouse. 

Figure 3-69: Although this 18,000-
pound HVAC unit was attached to its curb with 
16 straps, it blew off during Hurricane Ivan. 
(Florida, 2004)

9. Discussion is based on: Attachment of Rooftop Equipment in High-wind Regions—
Hurricane Katrina Recovery Advisory (May 2006, 
revised July 2006)
Table 3-2: Number of #12 Screws for Base Case Attachment of Rooftop Equipment

Fan cowling attachment: Fans are frequently 
blown off their curbs because they are poorly 
attached. When fans are well attached, the 
cowlings frequently blow off (see Figure 
3-70). Blown off cowlings can tear roof membranes 
and break glazing. Unless the fan 
manufacturer specifically engineered the 
cowling attachment to resist the design wind 
load, cable tie-downs (see Figure 3-71) are 
recommended to avoid cowling blow-off. 
For fan cowlings less than 4 feet in diameter, 
1/8-inch diameter stainless steel cables 
are recommended. For larger cowlings, use 
3/16-inch diameter cables. When the basic 
wind speed is 120 mph or less, specify two cables. 
Where the basic wind speed is greater 
than 120 mph, specify four cables. To minimize 
leakage potential at the anchor point, 
it is recommended that the cables be adequately anchored to 
the equipment curb (rather than anchored to the roof deck). 
The attachment of the curb itself also needs to be designed and 
specified. 

To avoid corrosion-induced failure (see 
Figure 3-78), it is recommended that 
exterior-mounted mechanical, electrical, 
and communications equipment be 
made of nonferrous metals, stainless 
steel, or steel with minimum G-90 hot-dip 
galvanized coating for the equipment 
body, stands, anchors, and fasteners. 
When equipment with enhanced corrosion 
protection is not available, the designer 
should advise the building owner that 
periodic equipment maintenance and 
inspection is particularly important to 
avoid advanced corrosion and subsequent 
equipment damage during a windstorm.

Figure 3-70: Cowlings blew off two of the fans on a police 
building that housed the county’s EOC. Hurricane 
Ivan (Florida, 2004) 

Ductwork: To avoid wind and wind-borne debris damage to rooftop 
ductwork, it is recommended that ductwork not be installed on 
the roof (see Figure 3-72). If ductwork is installed on the roof, it 
is recommended that the ducts’ gauge and the method of attachment 
be able to resist the design wind loads.

Figure 3-71: Cables were attached to prevent the cowling from blowing off. 
Typhoon Paka (Guam, 1997)

Figure 3-72: Two large openings remained (circled area 
and inset to the left) after the ductwork on this roof 
blew away. Hurricane Katrina (Mississippi, 2005)

Condenser attachment: In lieu of placing 
rooftop-mounted condensers on wood 
sleepers resting on the roof (see Figure 3-
73), it is recommended that condensers 
be anchored to equipment stands. The attachment 
of the stand to the roof deck also 
needs to be designed to resist the design 
loads. In addition to anchoring the base of 
the condenser to the stand, two metal straps 
with two side-by-side #12 screws or bolts 
with proper end and edge distances at each 
strap end are recommended when the basic 
wind speed is greater than 90 mph (see 
Figure 3-74).

Vibration isolators: If vibration isolators are used to mount equipment, 
only those able to resist design uplift loads should be 
specified and installed, or an alternative means to accommodate 
uplift resistance should be provided (see Figure 3-75). 

Three publications pertaining to seismic 
restraint of equipment provide general information 
on fasteners and edge distances: 

 Installing Seismic Restraints for 
Mechanical Equipment (FEMA 412, 
2002) 

 Installing Seismic Restraints for 
Electrical Equipment (FEMA 413, 
2004)

 Installing Seismic Restraints for Duct 
and Pipe (FEMA 414, 2004) 

Figure 3-73: Sleeper-mounted condensers displaced by 
high winds. Hurricane Katrina (Mississippi, 2005) 

Figure 3-74: This condenser had supplemental attachment 
straps (see red arrows). Typhoon Paka (Guam, 1997)

Figure 3-75: Failure of vibration isolators that provided lateral resistance but 
no uplift resistance caused equipment damage. 
A damaged vibration isolator is shown in the 
inset. Hurricane Katrina (Mississippi, 2005)

Boiler and exhaust stack attachment: To avoid wind damage to boiler 
and exhaust stacks, wind loads on stacks should be calculated and 
guy-wires should be designed and constructed to resist the loads. 
Toppled stacks, as shown at the hospital in Figure 3-76, can allow 
water to enter the building at the stack penetration, damage the 
roof membrane, and become wind-borne debris. The designer 
should advise the building owner that guy-wires should be inspected 
annually to ensure they are taut.

Access panel attachment: Equipment access panels frequently blow 
off. To minimize this, job-site modifications, such as attaching 
hasps and locking devices like carabiners, are recommended. 
The modification details need to be customized. Detailed design 
may be needed after the equipment has been delivered to the job 
site. Modification details should be approved by the equipment 
manufacturer.

Equipment screens: Screens around rooftop equipment are frequently 
blown away (see Figure 3-77). Screens should be designed 
to resist the wind load derived from ASCE 7. Since the effect of 
screens on equipment wind loads is unknown, the equipment attachment 
behind the screens should be designed to resist the 
design load. 

Figure 3-76: Three of the five stacks that did not have guy-
wires were blown down. Hurricane Marilyn (U.S. 
Virgin Islands, 1995) 

Water Infiltration

During high winds, wind-driven rain can be driven through air 
intakes and exhausts unless special measures are taken. Louvers 
should be designed and constructed to prevent leakage between 
the louver and wall. The louver itself should be designed to avoid 
water being driven past the louver. However, it is difficult to prevent 
infiltration during very high winds. Designing sumps with 
drains that will intercept water driving past louvers or air intakes 
should be considered. ASHRAE 62.1 (2004) provides some information 
on rain and snow intrusion. The Standard 62.1 User’s 
Manual provides additional information, including examples and 
illustrations of various designs.

3.3.4.2 Exterior-Mounted Electrical and Communications Equipment

Damage to exterior-mounted electrical equipment is infrequent, 
mostly because of its small size (e.g., disconnect switches). Exceptions 
include communication towers, surveillance cameras, 
electrical service masts, satellite dishes, and lightning protection 
systems. The damage is typically caused by inadequate mounting 
as a result of failure to perform wind load calculations and anchorage 
design. Damage is also sometimes caused by corrosion 
(see Figure 3-78 and the text box on corrosion in Section 3.3.4.1).

Figure 3-77: Equipment screen panels, such as these blown away 
at a hospital, can break glazing, puncture roof 
membranes, and cause injury. Hurricane Ivan 
(Florida, 2004)

Communication towers and poles: ANSI/C2 provides guidance for 
determining wind loads on power distribution and transmission 
poles and towers. AASHTO LTS-4-M (amended by LTS-4-12, 
2001 and 2003, respectively) provides guidance for determining 
wind loads on light fixture poles (standards).

Both ASCE 7 and ANSI/TIA-222-G contain wind load provisions 
for communication towers (structures). The IBC allows the use 
of either approach. The ASCE wind load provisions are generally 
consistent with those contained in ANSI/TIA-222-G. ASCE 7, 
however, contains provisions for dynamically sensitive towers that 
are not present in the ANSI/TIA standard. ANSI/TIA classifies 
towers according to their use (Class I, Class II, and Class III). This 
manual recommends that towers (including antennae) that are 
mounted on, located near, or serve critical facilities be designed as 
Class III structures.

Figure 3-78: Collapsed hospital light fixtures caused by severe 
corrosion (see inset). Hurricane Ivan (Florida, 2004)

Collapse of both large and small communication towers at emergency 
operation centers, fire and police stations, and hospitals 
is quite common during high-wind events (see Figures 3-79 and 
3-80). These failures often result in complete loss of communication 
capabilities. In addition to the disruption of communications, 
collapsed towers can puncture roof membranes and allow water 
leakage into the facilities, unless the roof system incorporated 
a secondary membrane (as discussed in Section 3.4.3.4). At the 
tower shown in Figure 3-79 the anchor bolts were pulled out of 
the deck, which resulted in a progressive peeling of the fully adhered 
single-ply roof membrane. Tower collapse can also injure or 
kill people. 

See Section 3.3.1.1 regarding site considerations for light fixture 
poles, power poles, and electrical and communications towers.

Figure 3-79: The collapse of the antenna 
tower caused progressive peeling of the roof 
membrane. Also note that the exhaust fan blew off the 
curb, but the high parapet kept it from blowing off the 
roof. Hurricane Andrew (Florida, 1992) 

Electrical service masts: Service mast failure is typically caused by 
collapse of overhead power lines, which can be avoided by using 
underground service. Where overhead service is provided, it is 
recommended that the service mast not penetrate the roof. Otherwise, 
a downed service line could pull on the mast and rupture the 
roof membrane. 

Satellite dishes: For the satellite dish shown in Figure 3-81, the 
dish mast was anchored to a large metal pan that rested on the 
roof membrane. CMU was placed on the pan to provide overturning 
resistance. This anchorage method should only be used 
where calculations demonstrate that it provides sufficient resistance. 
In this case the wind approached the satellite dish in 
such a way that it experienced very little wind pressure. In hurricane-
prone regions, use of this anchorage method is not 
recommended (see Figure 3-82). 

Lightning protection systems: For attachment of lightning protection 
systems on buildings higher than 100 feet above grade, and 
for buildings located where the basic wind speed is in excess of 90 
mph, see Section 3.4.4.3.

Figure 3-80: The antenna tower at this fire station buckled. 
Hurricane Katrina (Mississippi, 2005) 

The recommendations given in Section 3.3 are summarized in 
Table 3-3.

Figure 3-81: Common anchoring method for satellite dish. 
Hurricane Ivan (Florida, 2004)

Figure 3-82: A satellite dish anchored 
similarly to that shown in Figure 3-81 was 
blown off of this five-story building. Hurricane 
Charley (Florida, 2004) 

Table 3-3: Risk Reduction Design Methods

3.4 BEST PRACTICES IN HURRICANE-
PRONE REGIONS

This section presents the general design and construction 
practice recommendations for critical facilities located in 
hurricane-prone regions. These recommendations are additional 
to the ones presented in Section 3.3 and in many cases 
supersede those recommendations. Critical facilities located in 
hurricane-prone regions require special design and construction 
attention because of the unique characteristics of this type 
of windstorm. Hurricanes can bring very high winds that last for 
many hours, which can lead to material fatigue failures. The variability 
of wind direction increases the probability that the wind will 
approach the building at the most critical angle. Hurricanes also 
generate a large amount of wind-borne debris, which can damage 
various building components and cause injury and death. 
Although all critical facilities in hurricane-prone regions require 
special attention, three types of facilities are particularly important 
because of their function or occupancy: EOCs, healthcare 
facilities, and shelters. EOCs serve as centralized management 
hubs for emergency operations. The loss of an EOC can severely 
affect the overall response and recovery in the area. Healthcare 
facilities normally have vulnerable occupants (patients) at the 
time of a hurricane, and afterwards, many injured people seek 
medical care. Significant damage to a facility can put patients at 
risk and jeopardize delivery of care to those seeking treatment. 
Shelters often have a large number of occupants. The collapse 
of a shelter building or entrance of wind-borne debris into 
a shelter has the potential to injure or kill many people. See 
Chapter 4 for information on the performance of some EOCs, 
healthcare facilities, shelters, and other types of critical facilities 
that were affected by Hurricane Katrina.

In order to ensure continuity of service during and after hurricanes, 
the design, construction, and maintenance of the following 
critical facilities should be very robust to provide sufficient resiliency 
to withstand the effects of hurricanes.

EOCs: Communications are important for most types of critical 
facilities, but for EOCs they are vital. To inhibit disruption of operations, 
water infiltration that could damage electrical equipment 
must be prevented, antenna towers need to be strong enough 
to resist the wind, and the emergency and standby power system 
needs to remain operational.

Healthcare facilities: Full or partial evacuation of a hospital prior 
to, during, or after a hurricane, is time consuming, expensive, 
and for some patients, potentially life-threatening. Water infiltration 
that could damage electrical equipment 
or medical supplies, or inhibit the use of 
critical areas (such as operating rooms and 
nursing floors) needs to be prevented. The 
emergency and standby power systems need 
to remain operational and be adequately 
sized to power all needed circuits, including 
the HVAC system. Provisions are needed for 
water and sewer service in the event of loss of 
municipal services, and antenna towers need 
to be strong enough to resist the wind.

Shelters: During and after hurricanes, these facilities are often 
occupied by more than 1,000 people. The primary purpose of 
shelters is to protect occupants from injury or death as a result 
of building collapse or entrance of wind-borne debris. However, 
beyond meeting this basic requirement, providing a degree of 
occupant comfort during a stressful time is important. The building’s 
design and construction should avoid significant water 
infiltration and provide at least a minimum level of lighting and 
mechanical ventilation using emergency generators. Shelters 
should also have provisions for sewage service (such as portable 
toilets) in the event of loss of municipal water or sewer service. 

Nursing homes are often no more 
hurricane-resistant than residential buildings. 
Evacuating these facilities (particularly 
skilled nursing homes and facilities caring 
for patients with Alzheimer’s disease) can 
be difficult. Except for antenna towers, the 
issues identified for hospitals are applicable 
to nursing homes.

FEMA recommends that shelters be designed in accordance with 
FEMA 361, Design and Construction Guidance for Community Shelters 
(2000).

3.4.1 Site and General Design Considerations

Via ASCE 7, the 2006 edition of the IBC has only one special wind-
related provision pertaining to Category III and IV buildings in 
hurricane-prone regions. It pertains to glazing protection within 
wind-borne debris regions (as defined in ASCE 7). This single additional 
requirement does not provide adequate protection for 
occupants of a facility during a hurricane, nor does it ensure a 
critical facility will remain functional during and after a hurricane. 
A critical facility may comply with IBC but still remain vulnerable 
to water and missile penetration through the roof or walls. To provide 
occupant protection, the exterior walls and the roof must be 
designed and constructed to resist wind-borne debris as discussed 
in Sections 3.4.2 and 3.4.3. 

The following recommendations are made regarding siting:

 Locate poles, towers, and trees with trunks larger than 6 inches 
in diameter away from primary site access roads so that they do 
not block access to, or hit, the facility if toppled.
 Determine if existing buildings within 1,500 feet of the new 
facility have aggregate surfaced roofs. If roofs with aggregate 
surfacing are present, it is recommended that the aggregate be 
removed to prevent it from impacting the new facility. Aggregate 
removal may necessitate reroofing or other remedial work 
in order to maintain the roof’s fire or wind resistance.
 In cases where multiple buildings, such as hospitals or school 
campuses, are occupied during a storm, it is recommended 
that enclosed walkways be designed to connect the buildings. 
The enclosed walkways (above- or below-grade) are particularly 
important for protecting people moving between buildings 
during a hurricane (e.g., to retrieve equipment or supplies) or 
for situations when it is necessary to evacuate occupants from 
one building to another during a hurricane.

3.4.2 Structural Systems 

Because of the exceptionally good wind performance and wind-
borne debris resistance that reinforced cast-in-place concrete 
structures offer, a reinforced concrete roof deck and reinforced 
concrete or reinforced and fully grouted CMU exterior walls are 
recommended as follows: 

Roof deck: A minimum 4-inch thick cast-in-place reinforced concrete 
deck is the preferred deck. Other recommended decks 
are minimum 4-inch thick structural concrete topping over steel 
decking, and precast concrete with an additional minimum 4-inch 
structural concrete topping.

If these recommendations are not followed 
for critical facilities located in areas where 
the basic wind speed is 100 mph or greater, 
it is recommended that the roof assembly 
be able to resist complete penetration of the 
deck by the “D” missile specified in ASTM E 
1996 (2005, see text box in Section 3.4.3.1).

Figure 3-83: Open walkways do not provide protection 
from wind-borne debris. Hurricane Katrina 
(Mississippi, 2005)

If precast concrete is used for the roof or 
wall structure, the connections should be 
carefully designed, detailed, and 
constructed. 

Exterior load-bearing walls: A minimum 6-inch thick cast-in-place 
concrete wall reinforced with #4 rebars at 12 inches on center 
each way is the preferred wall. Other recommended walls are a 
minimum 8-inch thick fully grouted CMU reinforced with #4 rebars 
in each cell, and precast concrete that is a minimum 6 inches 
thick and reinforced equivalent to the recommendations for cast-
in-place walls.

3.4.3 Building Envelope 

The design considerations for building envelope components of 
critical facilities in hurricane-prone regions include a number of 
additional recommendations. The principal concern that must be 
addressed is the additional risk from wind-borne debris and water 
leakage, as discussed below.

3.4.3.1 Exterior Doors 

Although the ASCE-7 wind-borne debris provisions only apply 
to glazing within a portion of hurricane-prone regions, it is recommended 
that all critical facilities located where the basic 
wind speed is 100 mph or greater comply with the following 
recommendations: 

 To minimize the potential for missiles penetrating exterior 
doors and striking people inside the facility, it is recommended 
that doors (with and without glazing) be designed to resist the 
“E” missile load specified in ASTM E 1996. The doors should 
be tested in accordance with ASTM E 1886 (2005). The test 
assembly should include the door, door frame, and hardware. 
 It is recommended that the doors on shelters meet the wind 
pressure and missile resistance criteria found in FEMA 361. 
Information on door assemblies that meet these criteria is 
included in FEMA 361. 

3.4.3.2 Windows and Skylights 

Exterior glazing that is not impact-resistant (such as laminated 
glass or polycarbonate) or protected by shutters is extremely susceptible 
to breaking if struck by wind-borne debris. Even small, 
low-momentum missiles can easily break glazing that is not protected 
(see Figures 3-84 and 3-85). At the hospital shown in 
Figure 3-84, approximately 400 windows were broken. Most of 
the breakage was caused by wind-blown aggregate from the hospital’s 
aggregate ballasted single-ply membrane roofs, and aggregate 
from built-up roofs. With broken windows, a substantial amount of 
water can be blown into a building, and the internal air pressure 
can be greatly increased (as discussed in Section 3.1.3) which may 
damage the interior partitions and ceilings. 

ASTM E 1996 specifies five missile categories, A through E. The missiles are of 
various weights and fired at various velocities during testing. Building type 
(critical or non-critical) and basic wind speed determine the missiles required 
for testing. Of the five missiles, the E missile has the greatest momentum. 
Missile E is required for critical facilities located where the basic wind 
speed is greater than or equal to 130 mph. Missile D is permitted where the 
basic wind speed is less than130 mph. FEMA 361 also specifies a missile for 
shelters. The shelter missile has much greater momentum than the D and E 
missiles, as shown below:

Missile
Missile Weight
Impact Speed
Momentum
ASTM E 1996—D
9 pound 2x4 lumber
50 feet per second 
(34 mph)
14 lbf -s*
ASTM E 1996—E
9 pound 2x4 lumber
80 feet per second
(55 mph)
22 lbf -s*
FEMA 361 (Shelter Missile)
15 pound 2x4 lumber
147 feet per second 
(100 mph)
68 lbf -s*
*lbf -s = pounds force per second

In order to minimize interior damage, the IBC, through ASCE 7, 
prescribes that exterior glazing in wind-borne debris regions be 
impact-resistant, or be protected with an impact-resistant covering 
(shutters). For Category III and IV buildings in areas with a basic 
wind speed of 130 mph or greater, the glazing is required to resist 
a larger momentum test missile than would Category II buildings 
and Category III and IV buildings in areas with wind speeds of less 
than 130 mph.

Figure 3-84: Plywood panels (black continuous bands) installed 
after the glass spandrel panels were broken by 
roof aggregate. Hurricane Katrina (Mississippi, 2005) 

Figure 3-85: A small piece of asphalt 
shingle (red arrow) broke the window at this nursing 
home. Hurricane Katrina (Mississippi, 2005)

ASCE 7 refers to ASTM E 1996 for missile loads and to ASTM E 
1886 for the test method to be used to demonstrate compliance 
with the E 1996 load criteria. In addition to testing impact resistance, 
the window unit is subjected to pressure cycling after test 
missile impact to evaluate whether the window can still resist 
wind loads. If wind-borne debris glazing protection is provided 
by shutters, the glazing is still required by ASCE 7 to meet the 
positive and negative design air pressures.

Although the ASCE 7 wind-borne debris provisions only apply 
to glazing within a portion of hurricane-prone regions, it is recommended 
that all critical facilities located where the basic 
wind speed is 100 mph or greater comply with the following 
recommendations: 

 To minimize the potential for missiles penetrating exterior 
glazing and injuring people, it is recommended that exterior 
glazing up to 60 feet above grade be designed to resist the 
test Missile E load specified in ASTM E 1996 (see text box 
in Section 3.4.3.1). In addition, if roofs with aggregate 
surfacing are present within 1,500 feet of the facility, glazing 
above 60 feet should be designed to resist the test Missile A 
load specified in ASTM E 1996. The height of the protected 
glazing should extend a minimum of 30 feet above the 
aggregate surfaced roof per ASCE 7. 
Because large missiles are generally flying at lower 
elevations, glazing that is more than 60 feet above grade and 
meets the test Missile A load should be sufficient. However, 
if the facility is within a few hundred feet of another 
building that may create debris such as EIFS, tiles, or rooftop 
equipment, it is recommended that the test Missile E load be 
specified instead of the Missile A for the upper-level glazing. 
 For those facilities where glazing resistant to bomb blasts 
is desired, the windows and glazed doors can be designed 
to accommodate wind pressure, missile loads, and blast 
pressure. However, the window and door units need to be 
tested for missile loads and cyclic air pressure, as well as for 
blast. A unit that meets blast criteria will not necessarily meet 
the E 1996 and E 1886 criteria, and vice versa. 

With the advent of building codes requiring 
glazing protection in wind-borne debris 
regions, a variety of shutter designs have entered 
the market. Shutters typically have a 
lower initial cost than laminated glass. However, 
unless the shutter is permanently 
anchored to the building (e.g., an accordion 
shutter), storage space will be needed. Also, 
when a hurricane is forecast, costs will be incurred each time shutters 
are installed and removed. The cost and difficulty of shutter 
deployment and demobilization on upper-level glazing may be 
avoided by using motorized shutters, although laminated glass 
may be a more economical solution. For further information on 
shutters, see Section 3.6.2.2.

3.4.3.3 Non-Load-Bearing Walls, Wall Coverings, and Soffits 

In order to achieve enhanced missile resistance 
of non-load-bearing exterior walls, the 
wall types discussed in Section 3.4.2 (i.e., reinforced 
concrete, or reinforced and fully 
grouted CMU) are recommended. 

To minimize long-term problems with exterior 
wall coverings and soffits, it is recommended 
that they be avoided to the maximum extent 
possible. Exposed or painted reinforced concrete 
or CMU offers greater reliability (i.e., 
they have no coverings that can blow off and 
become wind-borne debris). 

For all critical facilities located where the 
basic wind speed is 100 mph or greater that 
are not constructed using reinforced concrete 
or reinforced and fully grouted CMU 
(as is recommended in this manual), it is recommended 
that the wall system selected be 
sufficient to resist complete penetration of 
the wall by the “E” missile specified in ASTM 
E 1996. 

For further information on designing 
glazing to resist blast, see the “Blast 
Safety” resource pages of the National 
Institute of Building Sciences' Building 
Envelope Design Guide (www.wbdg.org/
design/enve-lope.php).

For buildings not constructed using concrete 
roof decks and concrete or CMU walls (as 
recommended), shelters can be constructed 
within buildings for occupant protection. 
FEMA 320—Taking Shelter From the 
Storm: Building a Safe Room Inside Your 
Home (2004) describes how restrooms 
and storage rooms can be designed for 
sheltering inside new and existing buildings.

It is recommended that wood-framed and 
pre-engineered metal buildings in areas 
with a basic wind speed of 100 mph or 
greater, that will be occupied during a 
hurricane, have a designated storage 
room(s), office(s), or small conference 
room(s) designed in accordance with FEMA 
320 to protect the occupants. Although 
FEMA 320 is intended for residential 
construction, the guidance is suitable for 
small shelters inside critical facilities such as 
fire and police stations. For large shelters, 
FEMA 361 criteria are recommended.

For interior non-load-bearing masonry walls in critical facilities located 
where the basic wind speed is greater than 120 mph, see the 
recommendations given in Section 3.3.3.3. 

3.4.3.4 Roof Systems 

The following types of roof systems are recommended for critical 
facilities in hurricane-prone regions, because they are more likely 
to avoid water infiltration if the roof is hit by wind-borne debris, 
and also because these systems are less likely to become sources of 
wind-borne debris:

 In tropical climates where insulation is not needed above the 
roof deck, specify either liquid-applied membrane over cast-in-
place concrete deck, or modified bitumen membrane torched 
directly to primed cast-in-place concrete deck.
 Install a secondary membrane over a concrete deck (if 
another type of deck is specified, a cover board may be 
needed over the deck). Seal the secondary membrane at 
perimeters and penetrations. Specify rigid insulation over 
the secondary membrane. Where the basic wind speed is 
up to 110 mph, a minimum 2-inch thick layer of insulation 
is recommended. Where the speed is between 110 and 130 
mph, a total minimum thickness of 3 inches is recommended 
(installed in two layers). Where the speed is greater than 130 
mph, a total minimum thickness of 4 inches is recommended 
(installed in two layers). A layer of 5/8-inch thick glass mat 
gypsum roof board is recommended over the insulation, 
followed by a modified bitumen membrane. A modified 
bitumen membrane is recommended for the primary 
membrane because of its somewhat enhanced resistance to 
puncture by small missiles compared with other types of roof 
membranes. 

The purpose of the insulation and gypsum roof board is to 
absorb missile energy. If the primary membrane is punctured 
or blown off during a storm, the secondary membrane should 
provide watertight protection unless the roof is hit with 
missiles of very high momentum that penetrate the insulation 
and secondary membrane. Figure 3-86 illustrates the merit of 
specifying a secondary membrane. The copper roof blew off 
the hospital’s intensive care unit (ICU). Patients and staff were 
frightened by the loud noise generated by the metal panels as 
they banged around during the hurricane. Fortunately there 
was a very robust underlayment (a built-up membrane) that 
remained in place. Since only minor leakage occurred, the 
ICU continued to function. 
 For an SPF roof system over a concrete deck, where the basic 
wind speed is less than 130 mph, it is recommended that 
the foam be a minimum of 3 inches thick to avoid missile 
penetration through the entire layer of foam. Where the 
speed is greater than 130 mph, a 4-inch minimum thickness is 
recommended. It is also recommended that the SPF be coated, 
rather than protected with an aggregate surfacing.
 For a PMR, it is recommended that pavers weighing a minimum 
of 22 psf be specified. In addition, base flashings should 
be protected with metal (such as shown in Figure 3-93) to 
provide debris protection. Parapets with a 3 foot minimum 
height (or higher if so indicated by ANSI/SPRI RP-4, 2002) 
are recommended at roof edges. This manual recommends 
that PMRs not be used for critical facilities in hurricane-prone 
regions where the basic wind speed exceeds 130 mph. 

Figure 3-86: Because this roof system 
incorporated a secondary membrane, the ICU 
was not evacuated after the copper roof blew 
off. Hurricane Andrew (Florida, 1992) 

 For structural metal roofs, it is recommended 
that a roof deck be specified, 
rather than attaching the panels directly to 
purlins as is commonly done with pre-engineered 
metal buildings. If panels blow off 
buildings without roof decking, as shown 
in Figure 3-17, wind-borne debris and rain 
are free to enter the building. 

Structural standing seam metal roof panels with concealed 
clips and mechanically seamed ribs spaced at 12 inches on 
center are recommended. If the panels are installed over a 
concrete deck, a modified bitumen secondary membrane is 
recommended if the deck has a slope less than ½:12. If the 
panels are installed over a steel deck or wood sheathing, a 
modified bitumen secondary membrane (over a suitable cover 
board when over steel decking) is recommend, followed by 
rigid insulation and metal panels. Where the basic wind speed 
is up to 110 mph, a minimum 2-inch thick layer of insulation 
is recommended. Where the speed is between 110 and 130 
mph, a total minimum thickness of 3 inches is recommended. 
Where the speed is greater than 130 mph, a total minimum 
thickness of 4 inches is recommended. Although some clips 
are designed to bear on insulation, it is recommended that 
the panels be attached to wood nailers attached to the deck, 
because nailers provide a more stable foundation for the clips. 
If the metal panels are blown off or punctured during a 
hurricane, the secondary membrane should provide watertight 
protection unless the roof is hit with missiles of very high 
momentum. At the roof shown in Figure 3-87, the structural 
standing seam panel clips bore on rigid insulation over a steel 
deck. Had a secondary membrane been installed over the steel 
deck, the membrane would have likely prevented significant 
interior water damage and facility disruption.
 Based on field performance of architectural metal panels in 
hurricane-prone regions, exposed fastener panels are recommended 
in lieu of architectural panels with concealed clips. 
For panel fasteners, stainless steel screws are recommended. A 
secondary membrane protected with insulation is recommended, 
as discussed above for structural standing seam systems. 
Roofs over rooms used to store important 
records (such as police station evidence 
rooms) should incorporate secondary 
membranes to avoid water leakage damage. 
To preclude water infiltration damage from 
exterior walls, avoid locating important 
storage rooms adjacent to exterior walls. 

In order to avoid the possibility of roofing components blowing 
off and striking people arriving at a critical facility during a storm, 
the following roof systems are not recommended: aggregate surfacings 
either on BUR (shown in Figure 3-12), single-plies (shown 
in Figure 3-9), or SPF; lightweight concrete pavers; cementitious-
coated insulation boards; slate; and tile (see Figure 3-88). Even 
when slates and tiles are properly attached to resist wind loads, 
their brittleness makes them vulnerable to breakage as a result 
of wind-borne debris impact. The tile and slate fragments can be 
blown off the roof, and fragments can damage other parts of the 
roof causing a cascading failure. 

Figure 3-87: Significant interior water 
damage and facility interruption occurred 
after the standing seam roof blew off. Hurricane 
Marilyn (U.S. Virgin Islands, 1995)

Figure 3-88: Brittle roof coverings, 
like slate and tile, can be broken by missiles, and 
tile debris can break other tiles. Hurricane Charley 
(Florida, 2004)

Mechanically attached and air-pressure equalized single-ply membrane 
systems are susceptible to massive progressive failure after 
missile impact, and are therefore not recommended for critical 
facilities in hurricane-prone regions. At the building shown in 
Figure 3-89, a missile struck the fully adhered low-sloped roof and 
slid into the steep-sloped reinforced mechanically attached single-
ply membrane in the vicinity of the red arrow. A large area of the 
mechanically attached membrane was blown away as a result of 
progressive membrane tearing. Fully adhered single-ply membranes 
are very vulnerable to missiles (see Figure 3-90) and are 
not recommended unless they are ballasted with pavers. 

Figure 3-89: Mechanically attached 
single-ply membrane progressively tore after 
being cut by wind-borne debris. Hurricane 
Andrew (Florida, 1992)

Figure 3-90: Fully adhered single-ply 
roof membrane struck by a large number of missiles. 
Hurricane Marilyn (U.S. Virgin Islands, 1995)

Edge flashings and copings: If cleats are used for attachment, it is 
recommended that a “peel-stop” bar be placed over the roof membrane 
near the edge flashing/coping, as illustrated in Figure 3-91. 
The purpose of the bar is to provide secondary protection against 
membrane lifting and peeling in the event that edge flashing/
coping fails. A robust bar specifically made for bar-over mechanically 
attached single-ply systems is recommended. The bar needs 
to be very well anchored to the parapet or the deck. Depending 
on design wind loads, spacing between 4 and 12 inches on center 
is recommended. A gap of a few inches should be left between 
each bar to allow for water flow across the membrane. After the 
bar is attached, it is stripped over with a stripping ply.

Walkway pads: Roof walkway pads are frequently blown off during 
hurricanes (Figure 3-92). Pad blow-off does not usually damage 
the roof membrane. However, wind-borne pad debris can damage 
other building components and injure people. Walkway pads are 
therefore not recommended in hurricane-prone regions.

Figure 3-91: A continuous peel-stop bar 
over the membrane may prevent a catastrophic 
progressive failure if the edge flashing or coping is 
blown off. (Modified from FEMA 55, 2000)

Parapets: For low-sloped roofs, minimum 3-foot high parapets are 
recommended. With parapets of this height or greater, the uplift 
load in the corner region is substantially reduced (ASCE 7 permits 
treating the corner zone as a perimeter zone). Also, a high 
parapet (as shown in Figures 3-78 and 3-118) may intercept wind-
borne debris and keep it from blowing off the roof and damaging 
other building components or injuring people. To protect base 
flashings from wind-borne debris damage and subsequent water 
leakage, it is recommended that metal panels on furring strips be 
installed over the base flashing (Figure 3-93). Exposed stainless 
steel screws are recommended for attaching the panels to the furring 
strips because using exposed fasteners is more reliable than 
using concealed fasteners or clips (as were used for the failed 
panels shown in Figure 3-58). 

Figure 3-92: To avoid walkway pad 
blow off, as occurred on this hospital roof, 
walkway pads are not recommended. Hurricane 
Charley (Florida, 2004)

3.4.4 Nonstructural Systems and Equipment

Nonstructural systems and equipment include all components 
that are not part of the structural system or building envelope. 
Exterior-mounted equipment is especially vulnerable to hurricane-
induced damage, and special attention should be paid to 
positioning and mounting of these components in hurricane-
prone regions. 

3.4.4.1 Elevators 

Where interruption of elevator service would significantly disrupt 
facility operations, it is recommended that elevators be placed in 
separate locations within the building and be served by separate 
elevator penthouses. This is recommended, irrespective of the 
elevator penthouse enhancements recommended in Sections 3.3.3 
and 3.3.4, because of the greater likelihood that at least one of the 
elevators will remain operational and therefore allow the facility to 
function as intended. 

Figure 3-93: Base flashing protected by 
metal panels attached with exposed screws. Hurricane 
Katrina (Mississippi, 2005)

3.4.4.2 Mechanical Penthouses

By placing equipment in mechanical penthouses rather than 
leaving them exposed on the roof, equipment can be shielded 
from high-wind loads and wind-borne debris. Although screens 
(such as shown in Figure 3-77) could be designed and constructed 
to protect equipment from horizontally-flying debris, they are not 
effective in protecting equipment from missiles that have an angular 
trajectory. It is therefore recommended that mechanical 
equipment be placed inside mechanical penthouses. The penthouse 
itself should be designed and constructed in accordance 
with the recommendations given in Sections 3.4.2 and 3.4.3.

3.4.4.3 Lightning Protection Systems (LPS)

Lightning protection systems frequently become disconnected 
from rooftops during hurricanes. Displaced LPS components can 
puncture and tear roof coverings, thus allowing water to leak into 
buildings (see Figures 3-94 and 3-95). Prolonged and repeated 
slashing of the roof membrane by loose conductors (“cables”) and 
puncturing by air terminals (“lightning rods”) can result in lifting 
and peeling of the membrane. Also, when displaced, the LPS is no 
longer capable of providing lightning protection in the vicinity of 
the displaced conductors and air terminals. 

Figure 3-94: A displaced air terminal 
that punctured the membrane in several 
locations. Hurricane Marilyn (U.S. Virgin 
Islands, 1995) 

Lightning protection standards such as NFPA 780 and UL 96A 
provide inadequate guidance for attaching LPS to rooftops in 
hurricane-prone regions, as are those recommendations typically 
provided by LPS and roofing material manufacturers. LPS conductors 
are typically attached to the roof at 3-foot intervals. The 
conductors are flexible, and when they are exposed to high winds, 
the conductors exert dynamic loads on the conductor connectors 
(“clips”). Guidance for calculating the dynamic loads does not 
exist. LPS conductor connectors typically have prongs to anchor 
the conductor. When the connector is well-attached to the roof 
surface, during high winds the conductor frequently bends back 
the malleable connector prongs (see Figure 3-96). Conductor connectors 
have also debonded from roof surfaces during high winds. 
Based on observations after Hurricane Katrina and other hurricanes, 
it is apparent that pronged conductor connectors typically 
have not provided reliable attachment.

Figure 3-95: View of an end of a conductor that became 
disconnected. Hurricane Katrina (Mississippi, 2005) 

Figure 3-96: The conductor deformed 
the prongs under wind pressure, and pulled 
away from the connector. Hurricane Katrina 
(Mississippi, 2005)

To enhance the wind performance of LPS, the following are 
recommended: 

Parapet attachment: When the parapet is 12 inches high or greater, 
it is recommended that the air terminal base plates and conductor 
connectors be mechanically attached with #12 screws 
that have minimum 1¼-inch embedment into the inside face of 
the parapet nailer and are properly sealed for watertight protection. 
Instead of conductor connectors that have prongs, it is 
recommended that mechanically attached looped connectors be 
installed (see Figure 3-97). 

Attachment to built-up, modified bitumen, and single-ply membranes: 
For built-up and modified bitumen membranes, attach the air 
terminal base plates with asphalt roof cement. For single-ply membranes, 
attach the air terminal base plates with pourable sealer (of 
the type recommended by the membrane manufacturer). 

In lieu of attaching conductors with conductor connectors, it is 
recommended that conductors be attached with strips of membrane 
installed by the roofing contractor. For built-up and 
modified bitumen membranes, use strips of modified bitumen 
cap sheet, approximately 9 inches wide at a minimum. If strips 
are torch-applied, avoid overheating the conductors. For single-
ply membranes, use self-adhering flashing strips, approximately 9 
inches wide at a minimum. Start the strips approximately 3 inches 
from either side of the air terminal base plates. Use strips that are 
approximately 3 feet long, separated by a gap of approximately 3 
inches (see Figure 3-98).

Figure 3-97: This conductor was attached to 
the coping with a looped connector. Hurricane 
Katrina (Mississippi, 2005)

10. Discussion is based on Rooftop Attachment of Lightning Protection Systems in 
High-Wind Regions—Hurricane Katrina Recover Advisory 
(May 2006, Revised July 2006).

As an option to securing the conductors with stripping plies, 
conductor connectors that do not rely on prongs could be used 
(such as the one shown in Figure 3-99). However, the magnitude 
of the dynamic loads induced by the conductor is unknown, 
and there is a lack of data on the resistance 
provided by adhesively-attached connectors. For this 
reason, attachment with stripping plies is the preferred 
option, because the plies shield the conductor from 
the wind. If adhesive-applied conductor connectors 
are used, it is recommended that they be spaced more 
closely than the 3-foot spacing required by NFPA 780 
and UL 96A. Depending on wind loads, a spacing of 6 
to 12 inches on center may be needed in the corner regions 
of the roof, with a spacing of 12 to 18 inches on 
center at roof perimeters (see ASCE 7 for the size of 
corner regions).

Figure 3-98: Plan showing conductor attachment 

Figure 3-99: Adhesively-attached conductor connector that does not use prongs 

Mechanically attached single-ply membranes: It is recommended that 
conductors be placed parallel to, and within 8 inches of, membrane 
fastener rows. Where the conductor falls between or is 
perpendicular to membrane fastener rows, install an additional 
row of membrane fasteners where the conductor will be located, 
and install a membrane cover-strip over the membrane fasteners. 
Place the conductor over the cover-strip and secure the conductor 
as recommended above.

By following the above recommendations, additional rows of 
membrane fasteners (beyond those needed to attach the membrane) 
may be needed to accommodate the layout of the 
conductors. The additional membrane fasteners and cover-
strip should be coordinated with, and installed by, the roofing 
contractor.

Standing seam metal roofs: It is recommended that pre-manufactured, 
mechanically attached clips that are commonly used to 
attach various items to roof panels be used. After anchoring 
the clips to the panel ribs, the air terminal base plates and conductor 
connectors are anchored to the panel clips. In lieu of 
conductor connectors that have prongs, it is recommended 
that mechanically attached looped connectors be installed (see 
Figure 3-97). 

Conductor splice connectors: In lieu of pronged splice connectors 
(see Figure 3-100), bolted splice connectors are recommended 
because they provide a more reliable connection (see Figure 3-
101). It is recommended that strips of flashing membrane (as 
recommended above) be placed approximately 3 inches from 
either side of the splice connector to minimize conductor movement 
and to avoid the possibility of the 
conductors becoming disconnected. To 
allow for observation during maintenance inspections, 
do not cover the connectors.

It is recommended that the building 
designer advise the building owner to 
have the LPS inspected each spring, to 
verify that connectors are still attached to 
the roof surface, that they still engage the 
conductors, and that the splice connectors 
are still secure. Inspections are also 
recommended after high-wind events.

3.4.5 Municipal Utilities 

Hurricanes typically disrupt municipal electrical service, and often 
they disrupt telephone (both cellular and land-line), water, and 
sewer services. These disruptions may last from several days to several 
weeks. Electrical power disruptions can be caused by damage 
to power generation stations and by damaged lines, such as major 
transmission lines and secondary feeders. Water disruptions can 
be caused by damage to water treatment or well facilities, lack of 
power for pumps or treatment facilities, or by broken water lines 
caused by uprooted trees. Sewer disruptions can be caused by 
damage to treatment facilities, lack of power for treatment facilities 
or lift stations, or broken sewer lines. Phone disruptions can 
be caused by damage at switching facilities and collapse of towers. 
Critical facilities should be designed to prevent the disruption of 
operations arising from prolonged loss of municipal services. 

Figure 3-100: If conductors detach from 
the roof, they are likely to pull out from pronged 
splice connectors. Hurricane Charley (Florida, 2004) 

Figure 3-101: Bolted splice connectors 
are recommended to prevent free ends of 
connectors from being whipped around by 
wind. Hurricane Katrina (Mississippi, 2005)

3.4.5.1 Electrical Power 

It is recommended that critical facilities that will be occupied 
during a hurricane, or will be needed within the first few weeks afterwards, 
be equipped with one or more emergency generators. In 
addition to providing emergency generators, it is recommended 
that one or more additional standby generators be considered, 
particularly for facilities such as EOCs, hospitals and nursing 
homes, shelters, and fire and police stations, where continued 
availability of electrical power is vital. The purpose of providing 
the standby generators is to power those circuits that are not powered 
by the emergency generators. With both emergency and 
standby generators, the entire facility will be completely backed 
up. It is recommended that the emergency generator and standby 
generator systems be electrically connected via manual transfer 
switches to allow for interconnectivity in the event of emergency 
generator failure. The standby circuits can be disconnected from 
the standby generators, and the emergency circuits can be manually 
added. The emergency generators should be rated for prime 
power (continuous operation). 

Running generators for extended time periods frequently results 
in equipment failure. Thus, provisions for back-up generation capacity 
are important, because the municipal power system may 
be out of service for many days or even weeks. Therefore, it is recommended 
that an exterior box for single pole cable cam locking 
connectors be provided so that a portable generator can be connected 
to the facility. With a cam locking box, if one or more of 
the emergency or standby generators malfunction, a portable 
generator can be brought to the facility and quickly connected. 

Back-up portable generators should be 
viewed as a third source of power (i.e., they 
should not replace standby generators), because 
it may take several days to get a back-up 
portable generator to the site. 

Generators should be placed inside wind-
borne debris resistant buildings (see 
recommendations in Sections 3.4.2 and 
3.4.3) so that they are not susceptible to 
damage from debris or tree fall. Locating 
generators outdoors (see Figure 4-44) or inside 
weak enclosures is not recommended. 
It is recommended that wall louvers for generators be capable 
of resisting the test Missile E load specified in ASTM E 1996. Alternatively, 
wall louvers can be protected with a debris-resistant 
screen wall so that wind-borne debris is unable to penetrate the 
louvers and damage the generators.

It is recommended that sufficient onsite fuel storage be provided 
to allow all of the facility’s emergency and standby generators to 
operate at full capacity for a minimum of 96 hours (4 days).11 If 
at any time it appears that refueling won’t occur within 96 hours, 
provision should be made to shut off part or all the standby circuits 
in order to provide longer operation of the emergency 
circuits. For remote facilities or situations where it is believed 
that refueling may not occur within 96 hours, the onsite fuel 
storage capacity should be increased as deemed appropriate. It 
is recommended that fuel storage tanks, piping, and pumps be 
placed inside wind-borne debris resistant buildings, or underground. 
If the site is susceptible to flooding, refer to Chapter 2 
recommendations.

3.4.5.2 Water Service 

It is recommended that critical facilities that rely on water for 
continuity of service (especially hospitals and nursing homes) 
be provided with an independent water supply—a well or onsite 
water storage. Facilities that only need drinking water for 
occupants can have bottled water provided 
instead.

It is recommended that shelters be 
provided with an emergency generator to 
supply power for lighting, exit signs, fire 
alarm system, public address system, and 
for mechanical ventilation. A standby 
generator is also recommended in the 
event that the emergency generator 
malfunctions. A cam locking box is also 
recommended to facilitate connection of a 
back-up portable generator. 

11. The 96-hour fuel supply is based in part on the Department of Veterans 
Affairs criteria.

If water is needed for cooling towers, the independent 
water supply should be sized to 
accommodate the system. It is recommended 
that the well or onsite storage be capable of 
providing an adequate water supply for fire 
sprinklers. Alternatively, it is recommended 
that the building designer should advise the 
building owner to implement a continual fire-watch and provide 
additional fire extinguishers until the municipal water service is 
restored. For hospitals, it is recommended that the well or onsite 
water storage be capable of providing a minimum of 100 gallons 
of potable water per day per patient bed for four days (the 100 gallons 
includes water for cooling towers).12 

It is recommended that pumps for wells or 
onsite storage be connected to an emergency 
power circuit, that a valve be provided 
on the municipal service line, and that onsite 
water treatment capability be provided 
where appropriate.

3.4.5.3 Sewer Service 

It is recommended that critical facilities that rely on sewer service 
for continuity of operations (especially hospitals and 
nursing homes) be provided with an alternative means of waste 
disposal, such as a temporary storage tank which can be pumped 
out by a local contractor. For facilities such as EOCs, fire and police 
stations, and shelters, portable toilets can be placed inside 
the facility before the onset of a hurricane. It is recommended 
that all critical facilities be provided with back-flow preventors.

For critical facilities with boilers, it is 
recommended to store fuel onsite for a 
minimum of 96 hours (4 days). Storage 
tanks, piping, and pumps should be inside 
wind-borne debris resistant buildings or be 
placed underground (if site is susceptible 
to flooding, refer to Chapter 2). 

For hospitals and nursing homes, it is 
recommended that onsite storage of 
medical gases be sized to provide a 
minimum of 96 hours (4 days) of service. 

12. This recommendation is based on the Department of Veterans Affairs criteria.

3.4.6 Post-Design Considerations 

In addition to adequate design, proper attention must be given to 
construction, post-occupancy inspection, and maintenance.

3.4.6.1 Construction Contract Administration 

It is important for owners of critical facilities in hurricane-prone 
regions to obtain the services of a professional contractor who 
will execute the work described in the contract documents in 
a diligent and technically proficient manner. The frequency of 
field observations and extent of special inspections and testing 
should be greater than those employed on critical facilities that 
are not in hurricane-prone regions.

3.4.6.2 Periodic Inspections, Maintenance, and 
Repair 

The recommendations given in Section 3.3.1.4 for post-occupancy 
and post-storm inspections, maintenance, and repair are 
crucial for critical facilities in hurricane-prone regions. Failure 
of a building component that was not maintained properly, repaired 
or replaced, can present a considerable risk of injury or 
death to occupants, and the continued operation of the facility 
can be jeopardized. 

The recommendations given in Section 3.4 are summarized in 
Table 3-4. 

Table 3-4: Recommendations for Design of Critical Facilities 
EOCs, healthcare facilities, and shelters

Design very robustly.

Shelters

Refer to FEMA 361, Design and Construction Guidance for Community 
Shelters.

Walkways between campus buildings

If buildings will be occupied during a hurricane, provide enclosed 
walkways.

Structural systems

Use reinforced cast-in-place concrete. If the roof deck is not cast-in-
place, use precast concrete or concrete topping over steel decking. 

Exterior walls

Use reinforced concrete or fully grouted and reinforced CMU without 
wall coverings, other than paint.

Exterior doors

Use doors designed and tested to resist test missiles.

Exterior windows and skylights

Use laminated glass or shutters designed and tested to resist test 
missiles. If equipped with shutters, glazing is still required to resist wind 
pressure loads.

Roof covering

Design a roof system that can accommodate missiles as recommended 
in Section 3.4.3.4. Avoid aggregate surfacings, lightweight concrete 
pavers, cementitious-coated insulation boards, slate, and tile. Avoid 
single-ply membranes unless ballasted with heavy pavers. 

Parapets 

Use minimum 3-foot high parapets for low-sloped roofs.

Elevators

Place elevators in separate locations served by separate penthouses.

Mechanical penthouses

Place rooftop equipment in penthouses rather than exposed on the 
roof.

Lightning protection systems

Attach LPS to the roof as recommended in Section 3.4.4.3.

Emergency power

Provide emergency power as recommended in Section 3.4.5.1. 

Water service

Provide a water supply independent of municipal supplies. 

Sewer service

Provide a means of waste disposal independent of municipal service.

Construction contract administration

Construction executed by a professional contractor and subcontractors. 
Conduct more frequent field observations, special inspections and 
testing.

Periodic inspections, maintenance, 
and repair

After construction, conduct diligent periodic inspections and special 
inspections after storms. Ensure diligent maintenance and prompt 
repairs. 

3.5 BEST PRACTICES IN TORNADO-PRONE REGIONS

Strong and violent tornadoes may reach wind speeds substantially 
greater than those recorded in the strongest 
hurricanes. The wind pressures that these tornadoes can 
exert on a building are tremendous, and far exceed the minimum 
pressures derived from building codes. Figure 3-102 shows a 
classroom wing in a school in Illinois. All of the exterior windows 
were broken, and virtually all of the cementitious wood-fiber deck 
panels were blown away. Much of the metal roof decking over the 
band and chorus area also blew off. The gymnasium collapsed, as 
did a portion of the multi-purpose room. The school was not in 
session at the time the tornado struck. 

Figure 3-102: A Northern Illinois school 
heavily damaged by a strong tornado in 1990

Strong and violent tornadoes can generate very powerful missiles. 
Experience shows that large and heavy objects, including vehicles, 
can be hurled into buildings at high speeds. The missile sticking 
out of the roof in the foreground of Figure 3-103 is a double 2-
inch by 6-inch wood member. The portion sticking out of the roof 
is 13 feet long. It penetrated a ballasted ethylene propylene diene 
monomer (EPDM) membrane, approximately 3 inches of polyisocyanurate 
roof insulation, and the steel roof deck. The missile 
lying on the roof just beyond is a 2-inch by 10-inch by 16-foot long 
wood member. 

There is little documentation regarding tornado-induced damage 
to critical facilities. Most of the damage reports available pertain 
to schools because schools are the most prevalent type of critical 
facilities and, therefore, are more likely to be struck. A 1978 report 
prepared for the Veterans Administration13 identified four 
hospitals that were struck by tornadoes between 1973 and 1976. 
Table 3-5 (taken from that report) further illustrates the effects 
tornados can have on critical facilities.

Figure 3-103: A violent tornado 
showered the roof of this school with missiles. 
(Oklahoma, 1999)

13. A Study of Building Damage Caused by Wind Forces, McDonald, J.R. and Lea, 
P.A, Institute for Disaster Research, Texas Tech University, 1978. 

Table 3-5: Examples of Ramifications of Tornado Damage at Four Hospitals

For critical facilities located in tornado-prone regions (as defined 
in Section 3.2.2), the following are recommended:

 Incorporate a shelter within the facility to provide occupant 
protection. For shelter design, FEMA 361 criteria are 
recommended. 
 For interior non-load-bearing masonry walls, see the 
recommendations given in Section 3.3.3.3.
 Brick veneer, aggregate roof surfacing, roof pavers, slate, and 
tile cannot be effectively anchored to prevent them from 
becoming missiles if a strong or violent tornado passes near 
a building with these components. To reduce the potential 
number of missiles, and hence reduce the potential for 
building damage and injury to people, it is recommended that 
these materials not be specified for critical facilities in tornado 
prone regions.
 For hospitals, nursing homes, and other critical facilities 
where it is desired to minimize disruption of operations from 
nearby weak tornadoes and from strong and violent tornados 
that are on the periphery of the facility, the following are 
recommended: 
1) For the roof deck, exterior walls, and doors, follow the 
recommendations given in Sections 3.4.2 and 3.4.3. 
2) For exterior glazing, specify laminated glass window 
assemblies that are designed to resist the test Missile E load 
specified in ASTM E 1996, and are tested in accordance with 
ASTM E 1886. Note that missile loads used for designing 
tornado shelters significantly exceed the missile loads used 
for designing glazing protection in hurricane-prone regions. 
Missiles from a strong or violent tornado passing near the 
facility could penetrate the laminated glazing and result in 
injury or interior damage. Therefore, to increase occupant 
safety, even when laminated glass is specified, the facility 
should also incorporate a shelter as recommended above.



Existing Facilities without Tornado Shelters 

Where the number of recorded F3, F4, and F5 tornadoes per 
3,700 square miles is one or greater (see Figure 3-2 and discussion 
of Fujita Scale in Section 3.1.1), the best available refuge areas 
should be identified if the facility does not have a tornado shelter. 
FEMA 431, Tornado Protection, Selecting Refuge Areas in Buildings 
provides useful information for building owners, architects, and 
engineers who perform evaluations of existing facilities.

To minimize casualties in critical facilities, it is very important that 
the best available refuge areas be identified by a qualified architect 
or engineer.14 Once identified, those areas need to be clearly 
marked so that occupants can reach the refuge areas without 
delay. Building occupants should not wait for the arrival of a tornado 
to try to find the best available refuge area in a particular 
facility; by that time, it will be too late. If refuge areas have not 
been identified beforehand, occupants will take cover wherever 
they can, frequently in very dangerous places. Corridors, as shown 
in Figure 3-104, sometimes provide protection, but they can also 
be death traps. 

Figure 3-104: View of school corridor 
after passage of a violent tornado (Oklahoma, 1999) 

14. It should be understood that the occupants of a “best available refuge area” 
are still vulnerable to death and injury if the refuge area was not 
specifically designed as a tornado shelter.

Retrofitting a shelter space inside an existing 
building can be very expensive. An economical 
alternative is an addition that can function 
as a shelter as well as serve another purpose. 
This approach works well for smaller facilities. 
For very large facilities, constructing two or 
more shelter additions should be considered 
in order to reduce the time it takes to reach 
the shelter (often there is ample warning 
time, but sometimes an approaching tornado 
is not noticed until a few minutes before it 
strikes). This is particularly important for hospitals and nursing 
homes because of the difficulty of accommodating patients with 
different medical needs. 

The recommendations given in Section 3.5 are summarized in 
Table 3-6.

Table 3-6: Critical Facilities Located in Tornado-Prone Regions
Proposed FacilityOccupant protection 

Refer to FEMA 361 for design guidance.

Interior non-load-bearing masonry walls

See recommendations in Section 3.3.3.3.

Wind-borne missiles

Avoid use of brick veneer, aggregate surfacing, roof 
pavers, slate, and tile.

Healthcare and other critical facilities where it is desired 
to minimize disruption of operations from nearby weak 
tornadoes

See recommendations in Section 3.5.

Existing facilities without specifically designed tornado shelters
If one or more F3-F5 tornadoes per 3,700 square miles 

Identify best available refuge areas. See Figure 3-2 for 
historical data on frequency, and refer to FEMA 431 
(2003) for identification guidance.

If six or more F3-F5 tornadoes per 3,700 square miles 

Consider incorporating a shelter within the building or 
inside a new building addition. Refer to FEMA 320 and 
FEMA 361 for design guidance.

For small shelters within facilities such 
as fire and police stations, a designated 
storage room(s), office(s), or small 
conference room(s) can be economically 
retrofitted in accordance with FEMA 320 
to protect the occupants. Where it is 
desired to provide a large shelter area, 
FEMA 361 criteria are recommended.

3.6 REMEDIAL WORK ON EXISTING FACILITIES

Many existing critical facilities need to strengthen their 
structural or building envelope components. The reasons 
for this are the deterioration that has occurred 
over time, or inadequate facility strength to resist current design 
level winds. It is recommended that building owners have 
a vulnerability assessment performed by a qualified architectural 
and engineering team. A vulnerability assessment should be 
performed for all facilities older than 5 years. However, as illustrated 
by Figure 3-30 and the case of Garden Park Medical Center 
discussed in Section 4.2, an assessment is recommended for all facilities 
located in areas where the basic wind 
speed is greater than 90 mph (even if the 
facility is younger than five years). It is particularly 
important to perform vulnerability 
assessments on critical facilities located in 
hurricane-prone and tornado-prone regions.
Components that typically make buildings 
constructed before the early 1990s vulnerable 
to high winds are weak non-load-bearing 
masonry walls, poorly connected precast concrete 
panels, long-span roof structures with 
limited uplift resistance (e.g., at gyms), inadequately 
connected roof decks, weak glass 
curtain walls, building envelope, and exterior-mounted equipment. 
Although the technical solutions to these problems are not 
difficult, the cost of the remedial work is typically quite high. If 
funds are not available for strengthening or 
replacement, it is important to minimize the 
risk of injury and death by evacuating areas 
adjacent to weak non-load-bearing walls, 
weak glass curtain walls, and areas below 
long-span roof structures when winds above 
60 mph are forecast. 

American Red Cross (ARC) Publication 
4496, Standards for Hurricane Evacuation 
Shelter Selection (2002) provides 
information regarding assessing existing 
buildings for use as hurricane shelters. 
Unless a facility has been specifically 
designed for use as a shelter, it should 
only be used as a shelter of last resort, and 
even then, only if it meets the criteria given 
in ARC 4496. 

As a result of building code changes and 
heightened awareness, some of the common 
building vulnerabilities have generally been 
eliminated for facilities constructed in the 
mid-1990s or later. Components that typically 
remain vulnerable to high winds are the 
building envelope and exterior-mounted mechanical, 
electrical, and communications 
equipment. Many failures can be averted by 
identifying weaknesses and correcting them. 

By performing a vulnerability assessment, items that need to be 
strengthened or replaced can be identified and prioritized. A proactive 
approach in mitigating weaknesses can save significant sums 
of money and decrease disruption or total breakdown in critical 
facility operations after a storm. For example, 
a vulnerability assessment on a school such as 
that shown in Figure 3-105 can identify weakness 
of exterior classroom walls. Replacing 
walls before a hurricane is much cheaper 
than replacing the walls and repairing consequential 
damages after a storm, and proactive 
work avoids the loss of use while repairs are 
made.

A comprehensive guide for remedial work 
on existing facilities is beyond the scope of 
this manual. However, the following are examples 
of mitigation measures that are often 
applicable.

Critical facilities sometimes occupy 
buildings that have changed their original 
use (see the case of Hancock County EOC, 
discussed in Section 4.4). Buildings that 
were not designed for a critical occupancy 
were likely designed with a 1.0 rather 
than a 1.15 importance factor, and hence 
are not as wind-resistant as needed. It 
is particularly important to perform a 
vulnerability assessment if a facility 
is located in a building not originally 
designed for a critical occupancy, 
especially if the facility is located in a 
hurricane- or tornado-prone region. 

Before beginning remedial work, it is 
necessary to understand all significant 
aspects of the vulnerability of a facility 
with respect to wind and wind-driven rain. 
If funds are not available to correct all 
identified deficiencies, the work should be 
systematically prioritized so that the items 
of greatest need are first corrected. For 
example, at a building such as that shown 
in Figure 3-105, had the windows been 
retrofitted with shutters, that effort would 
have been ineffective, because the walls 
themselves collapsed. Mitigation efforts 
can be very ineffective if they do not 
address all items that are likely to fail.

3.6.1 Structural Systems

As discussed in Section 3.1.4.1, roof decks on many facilities designed 
prior to the 1982 edition of the SBC and UBC and the 
1987 edition of the NBC are very susceptible to failure. Poorly attached 
decks that are not upgraded are susceptible to blow-off, as 
shown in Figure 3-106. Decks constructed of cementitious wood-
fiber, gypsum, and lightweight insulating concrete over form 
boards were commonly used on buildings built in the 1950s and 
1960s. In that era, these types of decks typically had very limited 
uplift resistance due to weak connections to the support structure. 
Steel deck attachment is frequently not adequate because of 
an inadequate number of welds, or welds of poor quality. Older 
buildings with overhangs are particularly susceptible to blow-off, as 
shown in Figure 3-107, because older codes provided inadequate 
uplift criteria.

Figure 3-105:Several walls at this school 
collapsed. Windows were located above a non-
load-bearing masonry wall. Hurricane Katrina 
(Mississippi, 2005)

Figure 3-106: The school’s built-up roof 
blew off after one of the cementitious wood-fiber 
deck panels detached from the joists. Hurricane 
Katrina (Mississippi, 2005) 

Figure 3-107: The cementitious wood-
fiber deck panels detached from the joists along the 
overhangs and caused the school’s built-up 
membranes to lift and peel. Hurricane Katrina 
(Mississippi, 2005)

A vulnerability assessment of the roof deck should include evaluating 
the existing deck attachment, spot checking the structural 
integrity of the deck (including the underside, if possible), and 
evaluating the integrity of the beams/joists. If the deck attachment 
is significantly overstressed under current design wind 
conditions or the deck integrity is compromised, the deck should 
be replaced or strengthened as needed. The evaluation should be 
conducted by an investigator experienced with the type of deck 
used on the building. 

If a low-slope roof is converted to a steep-slope roof, the new 
support structure should be engineered and constructed to resist 
the wind loads and avoid the kind of damage shown in 
Figure 3-108.

Figure 3-108: The school’s wood superstructure installed 
as part of a steep-slope conversion blew away 
because of inadequate attachment. Hurricane 
Katrina (Louisiana, 2005)

3.6.2 Building Envelope 

The following recommendations apply to building envelope components 
of existing critical facilities.

3.6.2.1 Sectional and Rolling Doors

Sectional and rolling doors (e.g., at fire station apparatus bays and 
hospital loading docks), installed in older buildings before attention 
was given to the wind resistance of these elements, are very 
susceptible to being blown away. Although weak doors can be retrofitted, 
it is difficult to ensure that the door, door tracks, and 
connections between the door and tracks are sufficient. It is therefore 
recommended that weak doors and tracks be replaced with 
new assemblies that have been tested to meet the factored design 
wind loads. As part of the replacement work, nailers between the 
tracks and building structure should either be replaced, or their 
attachment should also be strengthened.

If a facility has more than one sectional or rolling door, all doors 
should be replaced, rather than just replacing one of the doors. 
The fire station shown in Figure 3-109 had six sectional doors. 
One door had been replaced before a hurricane. It performed 
very well, but three of the older doors were blown away and two of 
the older doors remained in place but had some wind damage.

Figure 3-109: The new door in the center performed very 
well, but the older doors on either side of it were 
blown away. Hurricane Charley (Florida, 2004)

3.6.2.2 Windows and Skylights

Windows in older facilities may possess inadequate resistance 
to wind pressure. Window failures are typically caused by wind-
borne debris, however, glazing or window frames may fail as a 
result of wind pressure (see Figure 3-110). Failure can be caused 
by inadequate resistance of the glazing, inadequate anchorage 
of the glazing to the frame, failure of the frame itself, or inadequate 
attachment of the frame to the wall. For older windows 
that are too weak to resist the current design pressures, window 
assembly replacement is recommended. Some older window assemblies 
have sufficient strength to resist the design pressure, but 
are inadequate to resist wind-driven rain. If the lack of water resistance 
is due to worn glazing gaskets or sealants, replacing the 
gaskets or sealant may be viable. In other situations, replacing the 
existing assemblies with new, higher-performance assemblies may 
be necessary.

It is recommended that all non-impact-resistant, exterior glazing 
located in hurricane-prone regions (with a basic wind speed of 
100 mph or greater) be replaced with impact-resistant glazing 
or be protected with shutters, as discussed in Section 3.4.3.2. 

Figure 3-110: Wind pressure caused 
the window frames on the upper floor to fail (red 
arrow). Hurricane Katrina (Mississippi, 2005)

Shutters are typically a more economical approach for existing 
facilities. There are a variety of shutter types, all illustrated by 
Figures 3-111 to 3-113. Accordion shutters are permanently 
attached to the wall (Figure 3-111). When a hurricane is forecast, 
the shutters are pulled together and latched into place. Panel 
shutters (Figure 3-112) are made of metal or polycarbonate. When 
a hurricane is forecast, the shutters are taken from storage and 
inserted into metal tracks that are permanently mounted to the 
wall above and below the window frame. The panels are locked 
into the frame with wing nuts or clips. Track designs that have 
permanently mounted studs for the nuts have been shown to be 
more reliable than track designs using studs that slide into the 
track. A disadvantage of panel shutters is the need for storage 
space. Roll-down shutters (Figure 3-113) can be motorized or 
pulled down manually. Figure 3-113 illustrates the benefits of 
shuttering. Two of the unprotected window units experienced 
glass breakage and the third window unit blew in. 

Figure 3-111: This school has accordion 
shutters. Hurricane Ivan (Florida, 2004)

Deploying accordion or panel shutters a few stories above grade is 
expensive. Although motorized shutters have greater initial cost, 
their operational cost should be lower. Other options for providing 
missile protection on upper levels include replacing the existing assemblies 
with laminated glass assemblies, or installing permanent 
impact resistant screens. Engineered films are also available for application 
to the interior of the glass. The film needs to be anchored 
to the frame, and the frame needs to be adequately anchored to 
the wall. The film degrades over time and requires replacement 
(approximately every decade). Use of laminated glass or shutters is 
recommended in lieu of engineered films.

Figure 3-112: Illustrates a metal panel 
shutter. Hurricane Georges (Puerto Rico, 1998) 

Figure 3-113: The lower window 
assembly was protected with a motorized shutter. 
Hurricane Ivan (Florida, 2004)

3.6.2.3 Roof Coverings

For roofs with weak metal edge flashing or coping attachment, 
face-attachment of the edge flashing/coping (as shown in Figure 
3-63) is a cost-effective approach to greatly improve the wind resistance 
of the roof system. 

The vulnerability assessment of roofs ballasted with aggregate, 
pavers, or cementitious-coated insulation boards, should determine 
whether the ballast complies with ASNI/SPRI RP-4. 
Corrective action is recommended for non-compliant, roof coverings. 
It is recommended that roof coverings with aggregate 
surfacing, lightweight pavers, or cementitious-coated insulation 
boards on buildings located in hurricane-prone regions be replaced 
to avoid blow off (see Figure 3-114). 

Figure 3-114: Aggregate from the hospital’s built-up roofs 
broke several windows in the intensive care unit, 
which had to be evacuated during the hurricane. 
Hurricane Charley (Florida, 2004)

When planning the replacement of a roof covering, it is recommended 
that all existing roof covering be removed down to the 
deck rather than simply re-covering the roof. Tearing off the covering 
provides an opportunity to evaluate the structural integrity 
of the deck and correct deck attachment and other problems. For 
example, if a roof deck was deteriorated due to roof leakage (see 
Figure 3-115), the deterioration would likely not be identified if 
the roof was simply re-covered. By tearing off down to the deck, 
deteriorated decking like that shown in Figure 3-115 can be found 
and replaced. In addition, it is recommended that the attachment 
of the wood nailers at the top of parapets and roof edges be 
evaluated and strengthened where needed, to avoid blow-off and 
progressive lifting and peeling of the new roof membrane (see 
Figure 3-116). 

Figure 3-115: The built-up roof on this 
school was blown off after a few of the rotted wood 
planks detached from the joists. Hurricane Katrina 
(Mississippi, 2005)

If the roof has a parapet, it is recommended that the inside of the 
parapet be properly prepared to receive the new base flashing. In 
many instances, it is prudent to re-skin the parapet with sheathing 
to provide a suitable substrate. Base flashing should not be applied 
directly to brick parapets because they have irregular surfaces that 
inhibit good bonding of the base flashing to the brick (see Figure 
3-117). Also, if moisture drives into the wall from the exterior 
side of the parapet with base flashing attached directly to brick, 
the base flashing can inhibit drying of the wall. Therefore, rather 
than totally sealing the parapet with membrane base flashing, the 
upper portion of the brick can be protected by metal panels (as 
shown in Figure 3-93), which permit drying of the brick.

Figure 3-116: The edge nailer on top 
of an old brick wall at a hospital blew off because it 
was inadequately attached. Hurricane Ivan (Florida, 
2004)

3.6.3 Exterior-Mounted Equipment

Exterior-mounted equipment on existing 
critical facilities should be carefully examined 
and evaluated.

3.6.3.1 Antenna (Communications Mast)

Antenna collapse is very common. Besides loss of communications, 
collapsed masts can puncture roof membranes or cause 
other building damage as shown in Figure 3-118. This case also 
demonstrates the benefits of a high parapet. Although the roof 
still experienced high winds that blew off this penthouse door, the 
parapet prevented the door from blowing off the roof.

In hurricane-prone regions, it is recommended that antennae 
strength be evaluated as part of the vulnerability assessment. 
Chapter 15 of ANSI/TIA-222-G provides guidance on the structural 
evaluation of existing towers. Appendix J of that standard 
contains checklists for maintenance and condition assessments. 
Additional bracing, guy-wires, or tower strengthening or replacement 
may be needed.

Figure 3-117: Failed base flashing 
adhered directly to the brick parapet. Hurricane 
Katrina (Louisiana, 2005) 

Fastening rooftop equipment to curbs, 
as discussed in Section 3.3.4.1, is a 
cost-effective approach to minimize wind-
induced problems.

3.6.3.2 Lightning Protection Systems

Adhesively-attached conductor connectors and pronged splice 
connectors typically have not provided reliable attachment during 
hurricanes. To provide more reliable attachment for LPS located 
in hurricane-prone regions where the basic wind speed is 100 mph 
or greater, or on critical facilities in excess of 100 feet above grade, 
it is recommended that attachment modifications based on the 
guidance given in Section 3.3.4.3 be used. 

The recommendations given in Section 3.6 are summarized in 
Table 3-7.

Figure 3-118: The antenna at this hospital collapsed and 
was whipped back and forth across the roof 
membrane. Hurricane Andrew (Florida, 1992) 

Table 3-7: Recommendations for Remedial Work on Existing Critical Facilities
Weakness Recommended remedy
Critical facilities older than 5 years, or any age if 
located in an area with basic wind speed greater than 
90 mph. 

Perform vulnerability assessment with life-safety issues as 
the first priority, and property damage and interruption of 
service as the second priority. 

A building with weak non-load-bearing masonry or 
curtain walls, poorly connected precast concrete panels, 
or weak long-span roof structures.

Implement remedial work on elements with insufficient 
strength to resist wind loads if the facility will be occupied 
during high wind events (e.g., strong thunderstorms). 

Sectional and rolling doors.

Replace weak doors and tracks. 

Worn window gaskets and sealants. 

Replace with new gaskets and sealants, or replace window 
assembly. 

Buildings in a hurricane-prone region where the basic 
wind speed is 100 mph or greater, with non-impact-
resistant exterior glazing. 

Replace with impact-resistant glazing or protect with 
shutters.

Inadequately attached edge flashings or copings.

Face-attach the vertical flanges. See Figure 3-63.

Ballasted single-ply roof membranes.

Take corrective action if non-compliant with ANSI/SPRI RP-4. 

Buildings in a hurricane-prone region with aggregate 
roof surfacing, lightweight pavers, or cementitious-
coated insulation boards. 

Replace roof covering to avoid blow-off.

Rooftop equipment unanchored or poorly anchored.

Add screws or bolts to anchor equipment to curbs. Add 
cables to secure fan cowlings. Add latches to secure 
equipment access panels. See Section 3.3.4.1.

Weak roof deck connections or weak roof structure.

When planning replacement of roof covering, remove roof 
covering and strengthen attachment of deck and/or roof 
structure. See Section 3.6.2.3.

Emergency generators in a hurricane-prone region not 
adequately protected from wind-borne debris. 

Build an enclosure to provide debris protection. See 
Section 3.4.5.1.

Antennae (communication masts) in hurricane-prone 
regions.

Evaluate wind resistance and strengthen as needed. See 
Chapter 15 and Appendix J of ANSI/TIA-222-G.

Lightning protection systems with adhesively-attached 
conductor connectors or pronged splice connectors 
located in hurricane-prone regions where the basic wind 
speed is 100 mph or greater, or on critical facilities in 
excess of 100 feet above grade.

Modify attachment according to recommendations in 
Section 3.4.4.3.

3.7 Checklist for Building Vulnerability of Critical 
Facilities Exposed to High Winds

The Building Vulnerability Assessment Checklist (Table 3-8) 
is a tool that can help in assessing the vulnerability of various 
building components during the preliminary design 
of a new building, or the rehabilitation of an existing building. 
In addition to examining design issues that affect vulnerability to 
high winds, the checklist also examines the potential adverse effects 
on the functionality of the critical and emergency systems 
upon which most critical facilities depend. The checklist is organized 
into separate sections, so that each section can be assigned 
to a subject expert for greater accuracy of the examination. The 
results should be integrated into a master vulnerability assessment 
to guide the design process and the choice of appropriate mitigation 
measures.

Table 3-8: Checklist for Building Vulnerability of Critical Facilities Exposed 
to High Winds
Vulnerability Sections Guidance Observations General

What is the age of the facility, and 
what building code and edition was 
used for the design of the building?

Substantial wind load improvements were made 
to the model building codes in the 1980s. Many 
buildings constructed prior to these improvements 
have structural vulnerabilities. Since the 1990s, 
several additional changes have been made, the 
majority of which pertain to the building envelope. 

Older buildings, not designed and constructed in 
accordance with the practices developed since 
the early 1990s, are generally more susceptible to 
damage than newer buildings.


Is the critical facility older than 5 
years, or is it located in a zone with 
basic wind speed greater than 90 
mph?

In either case, perform a vulnerability 
assessment with life-safety issues as the first 
priority, and property damage and interruption 
of service as the second priority.

Site

What is the design wind speed at the 
site? Are there topographic features 
that will result in wind speed-up?

ASCE 7 and Section 3.1.3.

What is the wind exposure on site?

Avoid selecting sites in Exposure D, and avoid 
escarpments and hills (Section 3.1.3).

Are there trees or towers on site?

Avoid trees and towers near the facility (Section 
3.3.1.1). If the site is in a hurricane-prone 
region, avoid trees and towers near primary 
access roads (Section 3.4.1).

Road access

Provide two separate means of access (Section 
3.3.1.1).

Is the site in a hurricane-prone 
region?

ASCE 7. If yes, follow hurricane-resistant design 
guidance (Section 3.4).

If in a hurricane-prone region, are 
there aggregate surfaced roofs within 
1,500 feet of the facility?

Remove aggregate from existing roofs (Section 
3.6.2.3). If the buildings with aggregate are 
owned by other parties, attempt to negotiate 
the removal of the aggregate (e.g., consider 
offering to pay the reroofing costs).

Architectural 

Will the facility be used as a shelter?

If yes, refer to FEMA 361.

Are there interior non-load-bearing 
walls?

Design for wind load according to Section 
3.3.3.3. 

Are there multiple buildings on site in 
a hurricane-prone region?

Provide enclosed walkways between buildings 
that will be occupied during a hurricane 
(Section 3.4.1).

Are multiple elevators needed for the 
building?

Place elevators in separate locations served by 
separate penthouses (Section 3.4.4.1).

Structural Systems Section 3.3.2

Is a pre-engineered building being 
considered?

If yes, ensure the structure is not vulnerable 
to progressive collapse. If a pre-engineered 
building exists, evaluate to determine if it is 
vulnerable to progressive collapse.

Is precast concrete being considered?

If yes, design the connections to resist wind 
loads. If precast concrete elements exist, verify 
that the connections are adequate to resist the 
wind loads.

Are exterior load-bearing walls being 
considered?

If yes, design as MWFRS and C&C. 

Is an FM Global-rated roof assembly 
specified?

If yes, comply with FM Global deck criteria.

Is there a covered walkway or 
canopy?

If yes, use “free roof” pressure coefficients 
from ASCE 7.Canopy decks and canopy 
framing members on older buildings often have 
inadequate wind resistance. Wind-borne debris 
from canopies can damage adjacent buildings 
and cause injury. 

Is the site in a hurricane-prone 
region?

A reinforced cast-in-place concrete structural 
system, and reinforced concrete or fully grouted 
and reinforced CMU walls, are recommended 
(Section 3.4.2).

Is the site in a tornado-prone region?

If yes, provide occupant protection. See FEMA 
361.

Do portions of the existing facility 
have long-span roof structures (e.g., 
a gymnasium)?

Evaluate structural strength, since older 
long-span structures often have limited uplift 
resistance.

Is there adequate uplift resistance 
of the existing roof deck and deck 
support structure?

The 1979 (and earlier) SBC and UBC, 
and 1984 (and earlier) BOCA/NBC, did 
not prescribe increased wind loads at roof 
perimeters and corners. Decks (except cast-in-
place concrete) and deck support structures 
designed in accordance with these older codes 
are quite vulnerable.The strengthening of the 
deck attachment and deck support structure is 
recommended for older buildings.

Structural Systems Section 3.3.2 (continued)

Are there existing roof overhangs 
that cantilever more than 2 feet?

Overhangs on older buildings often have 
inadequate uplift resistance.

Building Envelope Section 3.3.3

Exterior doors, walls, roof systems, 
windows, and skylights.

Select materials and systems, and detail to resist 
wind and wind-driven rain (Sections 3.3.3.1 to 
3.3.3.4).

Are soffits considered for the 
building?

Design to resist wind and wind-driven water 
infiltration (Section 3.3.3.3). If there are existing 
soffits, evaluate their wind and wind-driven rain 
resistance. If the soffit is the only element preventing 
wind-driven rain from being blown into 
an attic space, consider strengthening the soffit.

Are there elevator penthouses on 
the roof?

Design to prevent water infiltration at walls, 
roof, and mechanical penetrations (Sections 
3.3.3.3, 3.3.3.4, 3.3.4.1, and 3.4.4.1).

Is a low-slope roof considered on a 
site in a hurricane-prone region?

A minimum 3-foot parapet is recommended on 
low-slope roofs (Section 3.4.3.4). 

Is an EOC, healthcare facility, shelter, 
or other particularly important critical 
facility in a hurricane-prone region?

If yes, a very robust building envelope, resistant 
to missile impact, is recommended 
(Section 3.4). 

Is the site in a tornado-prone region?

To minimize generation of wind-borne missiles, 
avoid the use of brick veneer, aggregate roof 
surfacing, roof pavers, slate, and tile 
(Section 3.5). 

Are there existing sectional or rolling 
doors?

Older doors often lack sufficient wind 
resistance. Either strengthen or replace. This is 
particularly important for fire station apparatus 
bay doors.

Does the existing building have large 
windows or curtain walls?

If an older building, evaluate their wind 
resistance.

Does the existing building have 
exterior glazing (windows, glazed 
doors, or skylights)?

If the building is in a hurricane-prone region, 
replace with impact-resistant glazing, or protect 
with shutters.

Building Envelope Section 3.3.3 (continued)

Does the existing building have 
operable windows?

If an older building, evaluate its wind-driven 
rain resistance.

Are there existing exterior non-load-
bearing masonry walls?

If the building is in a hurricane- or tornado-
prone region, strengthen or replace.

Are there existing brick veneer, EIFS, 
or stucco exterior coverings?

If the building is in a hurricane-prone region, 
evaluate attachments. To evaluate wind 
resistance of EIFS, see ASTM E 2359 (2006).

Are existing exterior walls resistant to 
wind-borne debris?

If the building is in a hurricane-prone region, 
consider enhancing debris resistance, particularly 
if dealing with an important critical facility.

Are there existing ballasted single-ply 
roof membranes?

Determine if they are in compliance with ANSI/
SPRI RP-4. If non-compliant, take corrective action.

Does the existing roof have 
aggregate surfacing, lightweight 
pavers, or cementitious-coated 
insulation boards?

If the building is in a hurricane- prone region, 
replace the roof covering to avoid blow-off.

Does the existing roof have edge 
flashing or coping?

Evaluate the adequacy of the attachment. 

Does the existing roof system 
incorporate a secondary membrane?

If not, and if the building is in a hurricane-prone 
region, reroof and incorporate a secondary 
membrane into the new system. 

Does the existing building have a brittle 
roof covering, such as slate or tile?

If the building is in a hurricane-prone region, 
consider replacing with a non-brittle covering, 
particularly if it is an important critical facility.

Exterior-Mounted Mechanical Equipment

Is there mechanical equipment 
mounted outside at grade or the 
roof?

Anchor the equipment to resist wind loads (Section 
3.3.4.1). If there is existing equipment, 
evaluate the adequacy of the attachment, including 
attachment of cowlings and access panels.

Are there penetrations through the 
roof?

Design intakes and exhausts to avoid water 
leakage (Section 3.3.4.1).

Is the site in a hurricane-prone 
region?

If yes, place the equipment in a penthouse, rather 
than exposed on the roof (Section 3.4.4.2).

Exterior-mounted Electrical and Communications Equipment 

Are there antennae (communication 
masts) or satellite dishes?

See Section 3.3.4.2. If there are existing 
antennae or satellite dishes and the building is 
located in a hurricane-prone region, evaluate 
wind resistance. For antennae evaluation, see 
Chapter 15 of ANSI/TIA-222-G-2005.

Does the building have a lightning 
protection system?

See Sections 3.3.4.2 and 3.4.4.3 for lightning 
protection system attachment. For existing 
lightning protection systems, evaluate wind 
resistance (Section 3.6.3.1)

Municipal Utilities

Is the site in a hurricane-prone 
region?

See Section 3.4.5.1 for emergency and standby 
power recommendations.

Is the emergency generator(s) 
housed in a wind- and debris-
resistant enclosure?

If not, build an enclosure to provide debris 
protection in a hurricane-prone region (Section 
3.4.5.1).

Is the emergency generator’s wall 
louver protected from wind-borne 
debris?

If the building is in a hurricane-prone region, 
install louver debris impact protection (Section 
3.4.5.1).

Is the site in a hurricane-prone 
region?

If yes, an independent water supply and 
alternative means of sewer service are 
recommended, independent of municipal 
services (Sections 3.4.5.2 and 3.4.5.3). 

3.8 References and Sources of Additional Information 

NOTE: FEMA publications may be obtained at no cost by 
calling (800) 480-2520, faxing a request to (301) 497-
6378, or downloading from the library/publications 
section online at http://www.fema.gov.

American Association of State Highway and Transportation 
Officials, Standard Specifications for Structural Supports for 
Highway Signs, Luminaires and Traffic Signals, LTS-4-M and LTS-
4-12, January 2001 and October 2003.

American Concrete Institute, Building Code Requirements for Ma-
sonry Structures, ACI 530/ASCE 5/TMS 402, November 1996.

American Institute of Architects, Buildings at Risk: Wind Design 
Basics for Practicing Architects, 1997. 

American National Standards Institute/SPRI RP-4, Wind Design 
Standard for Ballasted Single-Ply Roofing Systems, 2002.

American National Standards Institute/SPRI ES-1, Wind Design 
Standard for Edge Systems Used in Low Slope Roofing Systems, 2003.

American National Standards Institute/Telecommunications 
Industry Association, Structural Standards for Antenna Supporting 
Structures and Antennas, ANSI/TIA-222-G, August 2005.

American Red Cross, Standards for Hurricane Evacuation Shelter 
Selection, Publication 4496, July 1992, rev. January 2002.

American Society of Civil Engineers, Structural Engineering In-
stitute, Minimum Design Loads for Buildings and Other Structures, 
ASCE/SEI 7-05, Reston, VA, 2005.

American Society for Testing and Materials, Standard Test 
Method for Field Determination of Water Penetration of Installed 
Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform or 
Cyclic Static Air Pressure Difference, ASTM E1105, 2000.

American Society for Testing and Materials, Standard Test 
Method for Structural Performance of Sheet Metal Roof and Siding 
Systems by Uniform Static Air Pressure Difference, ASTM E1592, 
2000.

American Society for Testing and Materials, Standard Test 
Method for Structural Performance of Exterior Windows, Curtain 
Walls, and Doors by Cyclic Static Air Pressure Differential, ASTM 
E1233, December 2000.

American Society for Testing and Materials, Standard Practice 
for Installation of Exterior Windows, Doors and Skylights, ASTM 
E2112, 2001.

American Society for Testing and Materials, Standard Test 
Method for Performance of Exterior Windows, Curtain Walls, Doors, 
and Impact Protective Systems Impacted by Missile(s) and Exposed to 
Cyclic Pressure Differentials, ASTM E1886, 2005.

American Society for Testing and Materials, Standard 
Specification for Performance of Exterior Windows, Curtain Walls, 
Doors and Impact Protective Systems Impacted by Windborne Debris in 
Hurricanes, ASTM E1996, 2005.

American Society for Testing and Materials, Standard 
Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, 
ASTM A153, April 2005.

American Society for Testing and Materials, Standard 
Specification for Steel, Sheet, Cold-Rolled, Carbon, Structural, 
High-Strength Low-Alloy, High-Strength Low-Alloy with Improved 
Formability, Solution Hardened, and Bake Hardenable, ASTM 
A1008/A1008M, 2006.

American Society for Testing and Materials, Test Method for Field 
Pull Testing of an In-Place Exterior Insulation and Finish System 
Clad Wall Assembly, ASTM E 2359, 2006.

American Society for Testing and Materials, Standard Test 
Method for Structural Performance of Exterior Windows, Doors, 
Skylights, and Curtain Walls by Cyclic Air Pressure Differential, 
ASTM E1233, 2006.

American Society of Heating, Refrigeration and Air-
Conditioning Engineers, Inc. (ASHRAE), Ventilation for 
Acceptable Indoor Air Quality, Standard 62.1, 2004.

American Society of Heating, Refrigeration and Air-
Conditioning Engineers, Inc. (ASHRAE), Standard 62.1 User’s 
Manual, 2006.

Brick Industry Association, Anchored Brick Veneer-Wood Frame 
Construction, Technical Notes 28, Revised August 2002.

Brick Industry Association, Wall Ties for Brick Masonry, 
Technical Notes 44B, Revised May 2003.

Brick Industry Association, Brick Veneer/Steel Stud Walls, 
Technical Notes 28B, December 2005.

Door & Access Systems Manufacturers Association, Connecting 
Garage Door Jambs to Building Framing, Technical Data Sheet 
#161, December 2003.

FM Global, Loss Prevention Data for Roofing Contractors, 
Norwood, MA, dates vary.

Factory Mutual Research, Approval Guide, Norwood, MA, 
updated quarterly.

FEMA, Building Performance: Hurricane Andrew in Florida, FEMA 
FIA-22, Washington, DC, December 1992.

FEMA, Building Performance: Hurricane Iniki in Hawaii, FEMA 
FIA-23, Washington, DC, January 1993.

FEMA, Corrosion Protection for Metal Connectors in Coastal Areas, 
FEMA Technical Bulletin 8-96, Washington, DC, August 1996. 

FEMA, Typhoon Paka: Observations and Recommendations on 
Building Performance and Electrical Power Distribution System, 
FEMA-1193-DR-GU, Washington, DC, March 1998.

FEMA, Hurricane Georges in Puerto Rico, FEMA 339, Washington, 
DC, March 1999.

FEMA, Oklahoma and Kansas Midwest Tornadoes of May 3, 1999, 
FEMA 342, Washington, DC, October 1999.

FEMA, Coastal Construction Manual, Third Edition, FEMA 55, 
Washington, DC, 2000.

FEMA, Design and Construction Guidance for Community Shelters, 
FEMA 361, Washington, DC, July 2000.

FEMA, Installing Seismic Restraints for Mechanical Equipment, 
FEMA 412, Washington, DC, December 2002.

FEMA, Tornado Protection, Selecting Safe Areas in Buildings, 
FEMA 431, Washington, DC, October 2003.

FEMA, Installing Seismic Restraints for Electrical Equipment, 
FEMA 413, Washington, DC, January 2004. 

FEMA, Installing Seismic Restraints for Duct and Pipe, FEMA 414, 
Washington, DC, January 2004.

FEMA, Taking Shelter From the Storm: Building a Safe Room Inside 
Your House, FEMA 320, Washington, DC, March 2004.

FEMA, MAT Report: Hurricane Charley in Florida, FEMA 488, 
Washington, DC, April 2005.

FEMA, MAT Report: Hurricane Ivan in Alabama and Florida, 
FEMA 489, Washington, DC, August 2005.

FEMA, Home Builder’s Guide to Coastal Construction Technical Fact 
Sheet Series, FEMA 499, August 2005.

FEMA, Attachment of Brick Veneer in High-Wind Regions—
Hurricane Katrina Recovery Advisory, Washington, DC, December 
2005. 

FEMA, MAT Report: Hurricane Katrina in the Gulf Coast, FEMA 
549, Washington, DC, July 2006. 

FEMA, Attachment of Rooftop Equipment in High-Wind Regions—
Hurricane Katrina Recovery Advisory, Washington, DC, May 2006, 
rev. July 2006.

FEMA, Rooftop Attachment of Lighting Protection Systems in High-
Wind Regions—Hurricane Katrina Recovery Advisory, Washington, 
DC, May 2006, rev. July 2006. 

Institute of Electrical and Electronics Engineers, Inc., National 
Electrical Safety Code, ANSI/IEE/C2, 2006.

International Code Council, 2006 International Building Code, 
ICC IBC-2006, March 2006.

McDonald, J.R. and Lea, P.A, A Study of Building Damage Caused 
by Wind Forces, Institute for Disaster Research, Texas Tech 
University, 1978. 

National Institute of Building Sciences, Building Envelope Design 
Guide, Available online at www.wbdg.org/design/envelope.php. 

National Research Council of Canada, Institute for Research 
in Construction, Wind Design Guide for Mechanically Attached 
Flexible Membrane Roofs, B1049, 2005.

Reinhold, Timothy A. “Calculating Wind Loads and Anchorage 
Requirements for Rooftop Equipment,” American Society of 
Heating, Refrigerating and Air-Conditioning Engineers Journal, 
Volume 48, Number 3, March 2006. 

[End of Chapter 3]