A.Rob Haddock
responds: Metal roofing can hold up
surprisingly well in high wind. I went to South
Dade County two days after Hurricane Andrew to
study the performance of steep-slope steel roofing
in hurricane-force winds. I am happy to report that
I saw less damage to steep-slope, standing-seam
steel roofing than to any other roofing type.
When it comes to wind resistance, metal roofing
has some distinct advantages over conventional
roofing materials. After all, a metal panel is a
structural element; its behavior under load can be
calculated and tested. In addition to being
predictable, metal stays consistent as it ages; its
behavior does not change significantly over time
the way asphaltic, synthetic, and wood roofing
does. In other words, metal roofing panels are
engineered systems that can be designed to
withstand any wind pressure. A panel’s
resistance to blow-off depends on the panel gauge,
sectional geometry, and frequency and method of
attachment.
As wind passes over a house, it typically tries
to suck the roof off. This "uplift" effect
is more pronounced in certain areas of the roof
than in others. The roof’s edges and
particularly corners (for example, the junction
between the eaves and the rake) are especially at
risk. On steeply sloped roofs, the area along the
ridge is also subjected to greater uplift
pressures. In Dade County, the few failures I saw
were at the juncture of the ridge and rake at the
peak.
Conform to test specs.
Metal panel systems are tested or engineered to
meet specific requirements. When they are tested,
the most widely accepted test procedure is
Underwriters Laboratories’ UL-580. For
this test, the panel assembly — including
the metal panels, fasteners, and substrate
— is placed in a 10x10-foot chamber.
Pressure is then applied — cycling between
positive pressures (blowing) and negative pressures
(sucking). Depending on how the assembly responds,
the panel is designated as UL Class 30, UL Class
60, or UL Class 90. (These are comparative levels
of performance; the numbers do not correspond to
any performance criteria.) Class 90 is the highest
rating; these panels survive test cycles that
expose them to as much as 105 pounds per square
foot (psf) of positive and negative pressure.
Since this is the best measure of a
panel’s true resistance to blow-off, it is
important that the assembly is constructed in the
field the way that it was tested. The exact
specifications may be available from the
manufacturer. To be sure, check the UL Building
Materials Directory (available for about $16
from Underwriters Laboratories, Publications Stock,
333 Pfingsten Rd., Northbrook, IL 60062;
708/272-8800). This book comes out periodically,
and shows the details of the roof constructions for
each classification.
Another test procedure has been recently drafted
by the American Society for Testing and Materials
(ASTM). This test, designated ASTM E-1592, uses a
larger test specimen. But it is not yet widely used
or recognized, and there is still some disagreement
within the industry as to how the test results
should be interpreted and used.
Get manufacturer’s
support. The other approach is to
engineer the system. As a rule, wind resistance can
be increased by increasing the panel gauge,
decreasing the panel width, and increasing the
fastening schedule. In any case, the panel
manufacturer should be able to give you some
direction in specifying the gauge, profile, and
fastening. Most panel manufacturers have a
registered structural engineer on staff who can
make recommendations in specific applications.
As a general contractor, you should require your
subs to conform to the test standard. Better yet,
provide them with an engineer-stamped set of
drawings from the panel manufacturer. Also, get a
letter of certification from the panel manufacturer
that verifies that the product and installation is
designed and engineered to meet a specific wind
speed or model code specification.
Rob Haddock is the director of the Metal
Roof Advisory Group in Colorado Springs, Colo., and
a former roofing contractor.