- Q. We build along the coast in North Carolina, which is considered a high-wind zone with gusts up to 120 mph. We’re interested in using standing-seam metal roofing panels, but we are concerned about blow-offs. Can you tell us what details and panel specifications we should require from our roofing subs before accepting a bid?
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.