(by Harry Poirier)
Due to the inherent similarity with wind speed design and uplift pressures, there is often confusion regarding the impact that these factors have on the wind speed warranty for the roofing system. Chapter 16 of the International Building Code, Section 1609 states that building structures (i.e. Roofs) shall be designed to withstand minimum wind uplift pressures. As laid out in the IBC, these pressures are determined in accordance with Chapters 26 to 30 of ASCE 7 or provisions of the alternate all-heights method in Section 1609.6. The wind speed charts and tables used in building codes are but a single factor, albeit a large contributing one, in calculating wind uplift pressures on a roof system. The code further states that the registered design professional shall show the values on the drawing and a structural engineer can provide these uplift pressures on the building.
Low-slope roof systems uplift pressure resistance classifications generally are based on testing performed according to Factory Mutual or Underwriters Laboratories (UL) Inc. guidelines. Using FM Approvals’ classification, the numeric values represent the tested design uplift resistances for field roof areas while taking into account a safety factor of 2.0 or more. For example, an FM I-90 classification designates a tested design uplift resistance of 45 psf of the roofing system. Therefore, the values in a roof systems’ uplift resistance is based on pounds per square foot (psf), not design wind speeds. The design wind speed or basic wind speed is used in the calculation of the design uplift pressure itself. It is important to remember that meeting a building code requirement is not a warranty requirement and should not be part of the warranty language.
Sika Sarnafil has many systems to meet today’s stringent building codes. We offer design assistance to help determine the proper application based on your building’s attributes.”