Getting the Edge on Roof Wind Design

May 15, 2003

April 2003 Interface • 23
While there are
many items to
include in roof
system selection and
design, one of the most
important is allowing for
adequate wind resistance.
Proper design takes into
account the building location
and specific building
attributes such as height,
deck construction, parapet
or roof edge configuration,
and even building use.
With this information, a
roof system designer can
design the roof to meet
anticipated roof uplift
forces. However, unless
considerable attention is
paid to roof design details
(more specifically, the edge
of the roof), the roof system
will still be vulnerable
to blow-off. This paper will
focus on edge details that
can help complete the
wind design.
United States model codes (and therefore state and municipal
codes) provide directives to allow for anticipated wind loads. For
example, the International Building Code (IBC) states that,
“Buildings, structures, and parts thereof shall be designed to
withstand the minimum loads described herein. Wind loads on
every building shall be determined in accordance with Section 6 of
This is not dissimilar to other requirements for which the
building designer anticipates. Licensed professional designers rely
heavily on one document for guidelines – ASCE 7-02. This document
provides guidance for calculating loads on structures,
including roofs.
At the heart of the ASCE 7-02 methodology for calculating
wind forces on buildings is a simple formula that relates velocity
pressure to air mass and velocity:
Q = 0.00256 V2
Untested, insecure edge detail.
24 • Interface April 2003
Since the constant 0.00256 contains the density of air, the
only value needed to satisfy this equation is the velocity of the
wind. For most locations, ASCE 7-98 provides basic design
wind velocity. Some locations are omitted from the ASCE map
because of local anomalies such as mountains. In these areas,
the designer must obtain the basic wind speed from a local
weather or code authority. The wind speed shown on the
ASCE map is the wind speed recorded at 10 meters above
ground and in an open area such as an airport. The basic
wind is then adjusted for other types of terrain and for elevations
other than 10 meters above ground level. ASCE 7-02
provides formulas and tables to make further modifications
based on the importance of the building and on unusual topographical
features such as a building on the edge of a cliff.
With all of this information in place, the velocity pressure
(i.e., the pressure equivalent of air at the design velocity) can
be identified. However, the effect of this velocity pressure on
buildings is not easily calculated. Instead, wind tunnel tests
have been performed on model buildings to evaluate the actual
effect on the buildings. These wind tunnel tests have provided
the basis to estimate forces on buildings resulting from
various velocity pressures. From these studies, it is shown
that wind can exert a force down on a roof (useful to the
structural engineer) and an uplift force, useful to the roof
designer as well as the structural engineer. Also, the uplift
pressure can be increased by internal pressures. These internal
pressures are a function of openings in the building – how
many and facing in which direction. This relationship is
shown by the equation:
p = qh [(GCp) – (GCpi)] where
p = wind uplift pressure
qh = velocity pressure
GCp = external pressure coefficient
GCpi = internal pressure coefficient
It should be noted that external pressure coefficients are
not uniform across the roof. At a given velocity pressure, uplift
is greatest in the corner of the roof, second greatest along the
perimeters, and least in the field of the roof. This should be
the first hint as to the reason why the edge of the roof and its
design are so important. The internal pressure coefficient
(GCpi) is dependent on whether the building is a closed building,
an open building, or a partially enclosed building. This
internal pressure coefficient is assumed to act uniformly
throughout the roof, perimeter, and field of roof alike.
From these considerations, the wind uplift pressure on a
roof system is determined by the roof system designer. If the
building is insured by an insurance company affiliated with
Factory Mutual (FM) or if the roof designer wants to meet FM
requirements in addition to local code, FM also provides wind
calculation techniques. As of January 2002, this process has
become easier for the designer because the FM calculation
methods are now almost identical to ASCE 7-98. The major
difference between FM and ASCE 7-98 is that FM treats all
buildings as having an importance factor of 1.15. For this rea-
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April 2003 Interface • 25
son, FM wind design will
usually result in higher
wind uplift design forces
than ASCE 7-98.
With the uplift pressure
known, where is the
roof designer to obtain a
specific roof system meeting
his or her needs?
Unlike the structural
engineer who can choose
a steel, wood, or aluminum
beam of varying
dimensions to bear a load,
the roof designer can only
choose a complete roof
assembly that has been
tested at various uplift
pressures. Most frequently,
designers select systems
that have been
recommended by roofing
materials manufacturers
and tested by Factory
Mutual. Some roof systems
have been tested by
Underwriters Laboratories
(UL). Uplift resistances for
individual roof system components have not been tabulated and
are not available for the designer to engineer a unique system.
Fortunately, the designer has available many roof systems that
have been tested by Factory Mutual and by testing organizations
in high wind regions such as South Florida.
Even with all this work and the selection of an appropriate
system, the roof may fail during a high wind event due to poor
edge design or other factors in the field of the roof. The roof may
fail due to a metal edge that became dislodged, allowing the wind
to peel the roof back. It is possible that the wood blocking at a
perimeter will let go, allowing wind under the membrane to initiate
a blow off. Galvanic corrosion can cause sheet metal or fastener
failure, leading to a loose edge detail. Perhaps the perimeter is
not sealed properly and the high positive pressure on the outside
of the wall is allowed to pressurize the roof system, leading to failure.
Perhaps rooftop equipment is not properly anchored, allowing
dislodgement during a high wind event. This could allow air under
the membrane at that point or could tear holes in the roof membrane
as sheet metal tumbles across the roof. Blow off may also
be initiated by holes in the deck, allowing localized pressure
under the membrane. All of these aforementioned defects could
cause a roof to fail in high winds, even though the system was
tested for the uplift pressures experienced and easily passed the
Where can a designer turn to obtain support with these edge
and field of roof details? There are several sources for information
and assistance. Following is a list of some of these sources and a
summary of the assistance they can provide:
1. Sheet Metal & Air Conditioning Contractors National
Assoc. (SMACNA)
PO Box 221230
Chantilly, VA 22022-1230
Architectural Sheet Metal Manual
SMACNA has provided detail assistance to architects and
specifiers for many years. The association provides standard
sheet metal roof edge, coping, counterflashing, gutter,
and other miscellaneous details. Its details are time
proven. They are designed to be secure although there are
no performance numbers to match job site requirements.
SMACNA provides guidance on sheet metal gauges, cleat
gauge and engagement, fastener placement, and more. The
gutter design section provides information on resisting gutter
wind uplift that can initiate air entry under the roof
membrane. SMACNA also provides information on galvanic
corrosion. Additional information on this topic is provided
by the National Roofing Contractors Association.
2. National Roofing Contractors Association (NRCA)
O’Hare International Center
10255 West Higgins Road, Suite 600
Rosemont, IL 60018-5607
Roofing and Waterproofing Manual
The NRCA Roofing and Waterproofing Manual provides a
wealth of information on many topics, including edge
details. Details are available for most perimeter configurations
with information on suitable metal, fastener types,
and frequency. Like SMACNA, these details are tried and
Delamination in the field of the roof may be the result of defects other than the edge.
26 • Interface April 2003
proven with considerable roofing contractor input. NRCA
recognizes that the information and details provided are
good but general in nature. The uplift resistance in psf or
lbs/lin. ft. for each of the guide details is not known. For
the special situation where high wind forces are expected,
NRCA provides the following caution:
“Consideration must be given to
wind zone and local conditions
for the selection of metal gauge,
profile, and fastening schedule.
Severe conditions or code and
regulatory bodies may require
more conservative designs. When
using the above (NRCA) table,
additional items should be considered,
such as fastening pattern.”
Excellent information is provided to the
designer on the potential for galvanic corrosion.
Especially useful are the tables providing
information on the appropriate
fasteners to use with various metals to
avoid incompatibility.
3. Factory Mutual Global
1151 Boston-Providence Turnpike
Norwood, MA 02063
Loss Prevention Data Sheet 1-49: Perimeter Flashing
This document recognizes the potential loss to the insurer
for wind blow off of roof systems caused by inadequate
perimeter flashings. Factory Mutual states in this Data Sheet,
“The majority of roof failures resulting from windstorms
involve improperly designed or constructed perimeter flash-
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Proper edge design is critical.
April 2003 Interface • 27
ings.” To reduce liability
in this area, FM
provides specific
guidelines for perimeter
flashing details.
The document is
unique in at least two
ways. First, up until
the issuance of
ANSI/SPRI ES-1, this
had been the only
guidance to flashing
design to meet specific
wind uplift forces
for a particular building.
The guidance
provided starts with
wind uplift forces
determined using FM
Data Sheet 1-28. For
roof exposures
requiring 1-60 or
1-90 wind uplift tested
assemblies, FM 1-
49 provides guidance
for maximum dimensions
of flashing elements
for various
types and gauges of
sheet metal. For
exposures having
velocity pressures
greater than 45 psf
(requiring roof systems
rated higher
than 1-90), FM provides
uplift and outward
forces on
flashings in psf but
does not provide specific
design guidance
or testing methods.
The second area of
uniqueness associated
with the 1-49 Data
Sheet is the guidance
it provides for attachment
of perimeter
wood nailers. This
document provides
specific bolt diameters,
frequency, and
fastening patterns for
wood nailers and
wood cant strips. The SMACNA and NRCA designs assume
secure attachment of the nailers. The ANSI/SPRI ES-1 document,
described next, also assumes the nailers are sufficiently
attached. This is not always the case, and roof systems do
blow off due to poor design or construction of perimeter
structural members. Therefore, FM 1-49 is a necessary design
and specifying tool for the perimeter flashing system designer.
Wind damage can be very expensive.
Workers prepare for reroofing after extensive wind damage.
28 • Interface April 2003
4. American National Standards Institute (ANSI)
1430 Broadway
New York, NY 10018
5. Sheet Membrane and Component Suppliers to the
Commercial Roofing Industry (SPRI)
200 Reservoir Street, Suite 309A
Needham, MA 02194
ANSI/SPRI ES-1-98: Wind Design Standard for Edge
Systems Used with Low Slope Roofing Systems
This is a relatively new document that provides a
methodology for calculating upward and outward forces
on metal edge and coping details. It also provides specific
testing procedures to evaluate the performance of
nearly all envisioned edge details – testing procedures
that had not existed in the past.
The calculation methodology used by the ANSI/SPRI
ES-1-98 standard is closely tied to those detailed in
ASCE-7-98: Minimum Design Loads for Buildings and
Other Structures. In fact, the ANSI/SPRI documents follow
the same wind map, exposure categories, velocity
pressures, and velocity pressure coefficients as the
ASCE-7 document. The only simplification is that the
ANSI/SPRI perimeter flashing calculations assume no
unusual topographical features and that all buildings
are classified with an Importance Factor = 1.0.
For buildings on an isolated hill or cliff, or buildings
deemed more or less important than “normal” buildings,
the roof edge designer can use the same multipliers as
provided in the ASCE-7 document. However, this should
rarely be needed.
The testing methods outlined in ANSI/SPRI ES-1-98
are test methods RE-1, RE-2, and RE-3. Respectively,
they provide the test methods to evaluate the resistance
of sheet membrane pull out at metal edges, the outward
force resistance of fascia edge details, and the upward,
inward, and outward force resistance of wall coping systems.
The test methods detail attachment to the flashing
element, sample size, rate of load application, and identification
of end of test. These tests are being conducted
by independent testing labs and by individual flashing
manufacturers (overseen by an independent professional
Initially, the ES-1 tests were promoted only by manufacturers
of roofing edge systems. However, specifying
edge systems using the ES-1 criteria is truly a nonrestrictive
specification, as any flashing design can be
evaluated using this standard. Even roofing contractorfabricated
edge systems that have been evaluated by
NRCA and contractors authorized by NRCA to produce
the tested designs may qualify under an ES-1 specification.
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So where is the system designer to turn to “get the edge” on
roof design for wind resistance? First of all, to meet the building
code, the designer should use ASCE-7-98 or the code stipulated
alternative to evaluate wind uplift forces on the roof membrane. If
the building is Factory Mutual insured, the FM Loss Prevention
Data Sheet 1-28 should also be utilized to evaluate uplift forces.
With uplift forces determined, the designer should look for a tested
roof assembly appropriate for the calculated forces. Normally,
this means choosing a system tested by Factory Mutual Research
Corporation or by Underwriters Laboratory.
For the edge system design, it will take a combination of
SMACNA, NRCA, FM 1-49, and ANSI/SPRI ES-1-98 to properly
detail and specify the roof edge. Increasingly, specifiers will use
the ANSI/SPRI ES-1-98 criteria to obtain performance numbers to
match calculated wind pressures on metal edge details. This will
become more and more common as the new International
Building Code (IBC) provisions become local code. These new provisions
require edge designs according to the ANSI/SPRI ES-1
American Society of Civil Engineers, Minimum Design Loads for
Buildings and Other Structures, ASCE 7-95, New York, NY:
ASCE, 1998.
Factory Mutual Research Corporation, Factory Mutual Property
Loss Prevention Data Sheet 1-49 – Perimeter Flashing,
Norwood, MA: 1998.
Sheet Metal, Air Conditioning Contractors National
Association, Architectural Sheet Metal Manual, Fifth
Single-Ply Roofing Institute, ANSI/SPRI ES-1-98 – American
National Standard for Edge Systems Used with Low Slope
Roofing Systems, Needham, MA: SPRI, 1998.
Underwriters Laboratories Roofing Materials and Systems
Directory, updated annually.
Gerry Teitsma, RRC, RRO, CDT, is
Director of Educational Services for
the Roof Consultants Institute.
Teitsma spent 17 years with Dow
Chemical, where he was responsible
for the development of the IRMA Roof
System. He later owned his own roof
consulting firm and was employed
with the the Roofing Industry
Educational Institute (RIEI). He is
also a registered Consultant
Document Technologist and a member
of ASHRAE and CSI.
April 2003 Interface • 29
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