Wind resistance
of asphalt shingles
by Mark S. Graham
Asphalt shingle roof system performance during high-wind
conditions has come under increased scrutiny during recent
years. Following the relatively poor performance of
asphalt shingles during a number of high-wind events,
including Hurricane Andrew in Florida in 1992 and
Hurricane Bertha in South Carolina in 1996, building
insurers, code officials and roofing professionals have
taken a renewed interest in asphalt shingle roof system
wind resistance.
ASTM D 3161/UL 997
Currently, asphalt shingle roof system wind resistance is tested
in accordance with two similar, recognized standards:
the American Society for Testing and Materials (ASTM) Standard
D 3161, "Standard Test Method for Wind-Resistance of Asphalt
Shingles (Fan-Induced Method)" or Underwriters Laboratories Inc.
(UL) 997, "Standard for Wind Resistance of Prepared Roof
Covering Materials."
For both these test method standards, asphalt shingle test panels
that measure 50 inches (1.3 m) wide by 66 inches (1.7 m) long
are subjected to sustained fan-induced wind speeds of 60 mph
(27 m/s) for two hours. A test panel assembly is considered to
have passed the test when the full shingle tabs are restrained from lifting or the locking ears
(e.g., interlocking shingles) are not torn loose or disengaged
during the test.
Building codes
The model building codes (i.e., Uniform Building Code, Standard
Building Code [SBC] and The BOCA [Building Officials and Code
Administrators
International Inc.] National Building Code [BNBC]) historically
have addressed wind resistance of asphalt shingle roof systems
with prescriptive, rather than performance-based, requirements.
This language dictates, for example, that asphalt strip shingles be secured to a roof deck with a minimum of four fasteners per strip shingle. For certain high-wind situations, several model building codes require the use of six fasteners for
asphalt strip shingle installation.
Although the model building codes typically have allowed asphalt
shingle roof systems to be installed according to these
prescriptive fastening requirements, most other roof systems
are required to be performance-tested to withstand specific
wind
uplift pressures. These uplift pressures are based, in part, on
code-prescribed design wind speeds of
80 to 110 mph (36 to 49 m/s) for
the continental United States.
BNBC's 1999 edition includes significant revisions
impacting the wind resistance requirements for asphalt shingle
roof systems. Specifically, the provision allowing the
prescriptive fastening requirements (e.g., four fasteners per
strip shingle) to satisfy the wind uplift requirements has
been removed.
Asphalt shingle roof systems now must be designed to resist basic
wind speeds (i.e., 80 to 110 mph [36 to 49 m/s]) adjusted for
building height above grade, building exposure, wind-gust effect
and wind-importance factor (i.e., a classification of a
building's relative hazard risk) as prescribed by the code.
Similarly, the Southern Building Code Congress International
Inc. included a change in SBC's 1999 edition.
As a result of these code changes, existing wind test results
for asphalt shingles based on ASTM D 3161 or UL 997 are deficient
in satisfying the intent of the building codes' wind uplift
resistance provisions.
Roofing professionals should realize there currently is little or
no scientific correlation between using four or six fasteners to
attach asphalt shingles and satisfying the codes' prescribed wind
speeds or wind
uplift pressure requirements.
Further research
Several asphalt shingle manufacturers, along with the Asphalt
Roofing Manufacturers Association, are conducting research to
determine and test the wind resistances of asphalt shingle roof
systems for the purpose of compliance with building code
requirements.
The initial phase of this research work is complete, and a
second phase, intended to address specific shingle products,
currently is under way. (For more information, readers should
refer to "Symposium offers technical information," March issue,
page 60.) The researchers indicate they intend to complete the
research and have the necessary documentation available to submit
to building code officials by early 2000.
Cause for concern
Until further research is completed and accepted by building code
officials in jurisdictions that adopt BNBC 1999 or
SBC 1999, roofing professionals should realize they
probably will not be able to install asphalt shingles and comply with building codes' intent. Because noncompliance with a building code has specific legal implications, including
potential liability for related failure, this issue is of
particular concern
to the roofing industry.
Roofing professionals are urged
to encourage asphalt shingle manufacturers and suppliers to
supply asphalt shingle products and installation instructions
to comply with the building codes' intent. Also, roofing
professionals should attempt to have asphalt shingle
manufacturers and suppliers share their potential code-related
liability. One way to accomplish this is to include
hold-harmless language in purchase orders or contracts in
which roofing professionals specify or purchase asphalt
shingle products.
Mark S. Graham is NRCA's associate
executive director of technical services.
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