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Protecting Roofs From Nature’s Fury Using FM Global’s Data Sheets

May 15, 2008

As more and more of the world’s
population moves into windstorm-
prone areas, the frequency
and severity of windrelated
catastrophes are on
the rise. When property owners
put themselves in the path of Mother
Nature, they want to feel confident that
their facilities are built soundly, with robust
materials that meet or exceed codes derived
to minimize loss.
Roof consultants play a key role in
ensuring the integrity of commercial property.
This be gins with understanding how
wind can compromise a roof and result in
exposing a building’s valuable contents.
More than 30 years ago, before
American Society of Civil Engineers (ASCE)
Standard 7, Minimum Design Loads for
Buildings and Other Structures, the wind
standard used in the United
States was American National
Standards Institute (ANSI)
A58.1. Many felt this standard
was difficult to apply to commercial
roofs. In response to this
need to simplify the load calculations,
commercial property
insurer FM Global developed the
first issue of Property Loss
Prevention Data Sheet 1-28,
“Wind Design.” Its sole purpose
was to simplify wind-load calculations
for flat commercial roofs
with the development of a very
simple and easy-to-use table.
By simply knowing the wind
speed, roof height, and exposure, the roof
pressure could be found quickly, without
any calculations. Although there are many
good computer-based programs on the
market to help calculate roof pressure, the
tables developed by FM Global are still
widely used today due to their simplicity
and ease of use.
The key factor in any wind-load calculation
is that the pressure across the entire
roof is not uniform. There are higher pressures
in the perimeter and corners of the
roof. The perimeter can see a 68 percent
increase, while the roof corners can experience
up to a 153 percent increase—not
small increases, to say the least. If the roof
cover or roof deck is not designed for these
higher pressures, the roof can be lost in
moderate winds.
“There are higher pressures
in the perimeter and
corners of the roof.”
When calculating the wind load on a
commercial roof system, there are two items
often discussed but not always understood.
These are the need for a factor of safety and
the role the “Importance Factor” has in
commercial roofing.
Scientific research and product testing
conducted by FM Global have shown that
fastener placement on insulation boards
has a significant effect on the wind-uplift
resistance of the assembly. In one example,
re search conducted with a 4-ft x 4-ft x 1.5-
in polyisocyanurate insulation board covered
with a built-up roof (BUR) showed a
dramatic change in performance simply by
rearrangement of the fasteners
on the board. Fasten ers evenly
placed 12 inches from the
board’s edge failed at 105 psf by
fracture of the insulation board.
When fasteners were evenly
spaced six inches from the edge
of the board, the test failed at
160 psf by the screws pulling
out of the deck (160 psf /105 psf
= 1.52). As actual fastener placement
can vary widely from board
to board, this difference illustrates
the need for a safety factor
to be used in the design.
Using a 2.0 factor of safety
will allow for small variations in
Failure started in the perimeter. © 2008 FM Global. Reprinted
with permission. All rights reserved.
MA R C H 2008 I N T E R FA C E • 7
fastener placement and a small amount of loss (0.48) due to weathering
over the life of the roof system.
“A minimum safety factor of 2.0 should
always be used in roof design.”
Wind-speed maps used in today’s building codes—referenced in
FM Global’s data sheets—all come from the same source, ASCE 7-
05. But the wind speeds shown on these maps are based on a 50-
A safety factor should be used in the design. © 2008 FM Global.
Reprinted with permission. All rights reserved.
Product testing and certification engineers conduct a wind-uplift test at the FM Global Research Campus in West Gloucester, RI. © 2008 FM
Global. Reprinted with permission. All rights reserved.
8 • IN T E R FA C E MA R C H 2008
year mean recurrence interval (MRI). Sometimes confusing, the term
50-year MRI means there is a two percent chance in any given year
that these wind speeds will be exceeded. Because these probabilities
are statistically independent, during the approximately 50-year life of
the building, there is a 64 percent chance that the wind speeds
shown on the map will be exceeded.
Not only does changing the Importance Factor from I = 1.0 to I =
1.15 (recommended by FM Global) increase the pressure by 15 percent,
it also changes the MRI from 50 to 100 years, or a one percent
chance that the wind speeds will be exceeded in any given year. Note
that these probabilities can add up quickly.
During the course of 50 years, there is a 40
percent chance that the wind speeds shown
on the map will be exceeded.
A building with a 50-year life has a 64
percent chance that the wind speeds will be
exceeded. Changing the Importance Factor
to 1.15 reduces the probability to 40 percent.
Once adjustments have been made to
the wind load (Safety Factor and Impor –
tance Factor), a roof system can be selected
that has been tested to meet or exceed the
expected uplift pressures. FM Approvals, a
business unit of FM Global, tests roof systems
and provides a simple rating system
based on fire testing and uplift resistance.
The FM 1-90 rating has become a common
specification for commercial roofing. The
number “1” represents Class 1 interior fire
classification, and the “90” denotes the
wind load in the field of the roof at 90 psf—
not 90 mph.
In the past, building codes have always
provided some method of calculating the
uplift pressure in the field of the roof.
However, they have only made slight reference
to the higher wind pressures in the
perimeter and corners of the roof system,
with few, if any, guidelines on how to
address those higher loads. This lack of
direction by the building codes and loss history
prompted FM Global to develop FM
Global Data Sheet 1-29, Roof Deck
Securement and Above-Deck Roof
Components, and a simple prescriptive fastening
method for the perimeter and corners
of the roof.
Although the types of roofing systems
have grown over the years and have become
more complex in design, FM Global Data
Sheet 1-29 continues to provide a simple,
prescriptive solution for insulation boards,
base sheets, and single-ply covers.
Today, FM Global Data Sheet 1-29 is fo –
cused on existing roof systems and details,
Edge flashing failure. © 2008 FM Global.
Reprinted with permission. All rights reserved.
MA R C H 2008 I N T E R FA C E • 9
how to best increase the uplift resistance,
and to meet increasingly higher wind-load
requirements. All modes of failure and all
components of the roof system must be
reviewed, as the mode of failure can quickly
move from one section of the roof to
another. The entire roof system, in cluding
the deck as well as any adhered sections of
the assembly, must be tested to know the
uplift limitations of the entire system.
At one time, 20-gauge, narrow-rib steel
decks with joist spacing 5 ft on center were
common. Today, typical commercial roof
decks are 22-gauge, wide-rib, with joist
spacing 6 ft apart or greater. This allows
much greater upward deflection into the
roofing components, which affects securement
of the steel deck and all of the abovedeck
roofing components.
The roof deck, which provides the structural
support for the roof assembly, should
have adequate strength and rigidity to prevent
the above-deck layers from flexing significantly
and pulling apart due to wind
uplift. Also, as the width of single-ply membranes
continues to increase, the roof cover
places a greater concentrated load (line
load) back on selected sections of the steel
deck. FM Global Data Sheet 1-29 provides
simple prescriptive methods to ensure adequate
steel-deck attachment.
Understand wind dynamics.
Remember that there are substantially
higher uplift pressures in the perimeter and
corners of a roof. If these higher pressures
are not addressed, the roof could be severely
damaged in high winds. FM Global Data
Sheet 1-28 helps the professional to calculate
the uplift load, and FM Global Data
Sheet 1-29 provides simple prescriptive
solutions to best resist these higher pressures
for both the roof deck and the abovedeck
Additional information on all of the items
discussed in this article can be found inside
FM Global Data Sheets 1-28 and 1-29. Free
copies of many of the wind- and roof-related
data sheets can be obtained by visiting
David B. Cox is the corporate windstorm specialist for FM
Global, responsible for coordination of windstorm activities
for over 1,500 engineers worldwide. Dave is involved in consulting
with policyholders and account teams on windstorm
exposures, impact analysis, and mitigation techniques. He
began his career with FM Global in 1981 as a field engineer.
In the 27 years since, he has been a staff engineer, a senior
engineer, and a loss investigator. Dave is involved in both
engineering and underwriting. His responsibilities include research and preparation of
FM Global Property Loss Prevention Data Sheets dealing with windstorm, commercial
roofing systems, construction, and the building envelope. He has a B.S. degree in engineering
from the University of South Florida, and is a member of NRCA, RCI, and ASCE.
David B. Cox
10 • I N T E R FA C E MA R C H 2008
The Homeowner Protection Office
(HPO) of Vancouver, British Columbia,
has urged municipalities not to man –
date green roofs on residential pro –
jects. The announcement grew out of
a task force organized in 2007 on the
subject of green roofs. The group
determined that while it is “feasible to
design, install, and maintain a green
roof that performs as well as a
conventional roof system,” it has not
been demonstrated that a green roof
is as “cost-beneficial” from a “triple
bottom line” perspective. The HPO
cited limited skilled workers, few
accepted standards, minimum levels
of quality control, and concern that
maintenance would be improperly