Understanding wind-resistant design

Proper wind design of low-slope roof systems can be easier than you think

FM Global's 2006 revisions to its wind-related guidelines have renewed interest and caused considerable discussion within the U.S. roofing industry regarding proper wind-resistant design for low-slope roof systems. For more than 20 years, many in the industry have referred to FM Global's product testing and certifications subsidiary, FM Approvals, for guidance when designing, specifying and installing low-slope roof systems. However, because of FM Global's revisions to its guidelines, some in the industry are rethinking their reliance on FM Global and FM Approvals and taking a different, more fundamental approach to designing roof systems' wind resistances.

The fundamentals

The fundamental concept of wind design as it applies to roof systems is that the design wind-resistance (uplift-resistance) capacity of a building's roof system should be greater than or equal to the design wind loads that will act upon the roof system. This relationship is expressed mathematically as: wr = wl where wr = design wind resistance and wl = design wind load.

In the event actual wind loads exceed a roof system's wind-resistance capacity, the roof system may not be considered wind-resistant and blow-off is possible.