Tech Today

Comparing wind speeds and uplift pressures


A frequent question posed to NRCA's Technical Services Section is whether the numeric values in low-slope roof systems' uplift-resistance classifications (such as I-90, Class 60) represent buildings' design wind speeds. If you believe they do or are unclear about how the values relate to one another, continue reading.

Uplift pressures

Design wind-uplift pressures on roof systems are determined based on a number of considerations, including a specific building's mean roof height and basic wind speed. The fundamental equation for determining the design wind pressure in the field of a low-slope roof area is qh = 0.00256(Kh)(Kzt)(Kd)(V2)(I).

The variable qh represents the calculated design velocity (uplift) pressure at a specific height above grade, which is designated in pounds per square foot (psf). V is the basic wind speed designated in miles per hour. Kh is a velocity pressure coefficient; Kzt is a topographic factor; Kd is a wind directionality factor; and I is an importance factor.