There is still a common misconception in the design community that Factory Mutual (FM) I-90 represents winds of 90 mph. In fact, the standard requires the roof system to withstand 90 pounds per square foot (psf) of wind uplift tested pressure.
There are actually many calculations that go into the wind design process-all based on the original ASCE-7 standard for building envelope pressures. Building location, height, ground roughness and other factors all enter into the wind uplift equation.
In general, designing for 90 psf is overkill except in high wind areas. However, I-90 is frequently rubber-stamped in up to 80 percent of roofing specifications where 75 or 60 psf would be adequate and more cost effective. Currently, for building design pressures up to and including 45 psf, using an FM safety factor of 2, Factory Mutual now requires the roof assembly to withstand the following tested differential ultimate pressures:
- Field of Roof: 90 psf
- Perimeter of Roof: 150 psf
- Corner areas of Roof: 225 psf
- Field of Roof: 120 psf
- Perimeter of Roof: 195 psf
- Corner areas of Roof: 300 psf
As one can see from the charts above, the required uplift resistance at the corners of the roof is much higher than the field of the roof. This is because the roof edges were the determining factor in failures, according to FM’s and RICOWI’s research. However, there are other methods of strengthening the edges and corners of roof systems, such as using a “peel stop” on roofing membranes. (For more information on peel stops, visitwww.spri.org.)
When these changes took place in 2006, only a handful of the multitude of fully adhered specifications were able to meet the new FM standards, and many roof system suppliers had to re-test.
One way to meet the new FM requirements and increase the “strength” of the roofing assembly is to specify a 1/2-inch pre-primed gypsum cover board directly fastened to the roof deck. In recent testing, a three-ply built-up roof (BUR) with 1.5 inches of loose laid polyiso insulation achieved an impressive FM rating of 315 psf - 15 psf higher than the elevated corner pressures required for an FM I-120 listing.
Similarly, a 45-mil, fleece-backed EPDM single-ply membrane fully adhered with hot asphalt tested at 285 psf. A number of membrane manufacturers have tested various types of gypsum cover boards with their adhered membranes and have achieved a variety of high uplift results.