Wind Speed Versus Wind Load Design
August 4, 2015
Wind Speed Versus Wind Load Design

So much confusion surrounds wind speed when designing a roof system. Designers ask for and specify a particular wind “speed” which will ultimately be the wind speed at which a roofing material manufacturer will warrant the roof assembly. The problem arises because designing for “wind speed” rather than a wind load design means the requirements for roof assembly attachment are very different. The result is a disparity in fastening, since some manufacturers will assume the designer really means wind load when he says wind speed. Other manufacturers will provide a system that will meet the warranty requirements of “wind speed.”

First, let’s clarify the difference in semantics. As mentioned, “wind speed” when used for roofing is the wind speed at which a roofing material manufacturer will warrant the assembly. In reality “wind speed” is only one of the factors used in designing the fastening pattern for a roof assembly so that it accommodates “wind loads.” The “wind load” design is a result of calculating several factors in the building’s design.

Calculating for Wind Load Design

When designing a roof assembly, there are several steps that are required. Key pieces of information are needed as follows:

●The terrain of the area where the building is located, designated B, C, or D (see FM Global Loss Prevention sheet 1-28).

●The historical wind speed (based on a 3 second gust) of the geographical area as noted on the wind speed maps (see FM Global Loss Prevention sheet 1-28), including additional factors for exposure to flying debris.

●The measurements of the building, including length, width, and height, with additional factors that may affect wind uplift such as the size and number of openings in the building and the presence of a parapet. The slope of the roof is also included in the calculation.

From these numbers, the “wind uplift” is calculated. Once the wind uplift is calculated, the actual fastening pattern of the roof assembly may be determined. Uplift numbers are provided in pounds per square foot (psf) and coordinate to fastening patterns for the building’s field, perimeter, and corners. This is the calculation the roof designer needs to know. Don’t worry about where each of the factors is input into the equation because there are calculators available online via NRCA, RoofNav, or a variety of other locations. Once the wind uplift numbers are known, the designer can research approved appropriate roof assemblies.

Wind Load Design

In summary, wind uplift (psf) is the design number that will actually go into the roofing specification. “Wind speed” is just a factor in the calculation to get you there.

About the author

Lynn Jardinico is an talented writer whose unique life perspective gives her fuel to write with hard biting realism and just the right blend of tongue in cheek humor. She covers subjects as diverse as technical roofing industry articles, and open and honest articles on Faith, Family & Americanism that are sure to delight your senses.

One comment
Doug Kirk
August 5, 2015

Thank you

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