March 2005

Disoriented about orientation

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Consider wind-uplift issues when installing mechanically attached single-ply membrane roof systems

by Joan P. Crowe, AIA
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Many roof assemblies are constructed with mechanically attached single-ply membrane roof systems over rigid board roof insulation over steel roof decks supported by open web steel joists. Although this particular roof assembly is prevalent, there continues to be some confusion with regard to proper membrane orientation relative to the direction of steel deck flutes. This confusion with orientation sometimes can lead to underestimating wind-uplift design loads.

Wind resistance

For a mechanically attached single-ply membrane roof system over rigid board roof insulation over a steel deck, the steel deck typically is welded or mechanically fastened to the steel joists.

However, wind-uplift resistance with regard to the steel deck and joists must be considered when designing a single-ply roof membrane system. To understand wind uplift, wind-load fundamentals must be explained. When there is no wind, air pressure on the topside surface of a roof is the same as that on the underside of a roof. In this case, a roof assembly is in a state of equilibrium when the wind speed is zero. However, as soon as wind moves across the roof, it decreases the air pressure on the topside surface. At the same time, the air pressure on the underside of the roof remains the same. Because the air pressure is greater on the inside of the building, it exerts a force on the roof assembly that causes it to lift up, which is why this phenomenon is called "wind uplift." The load imposed on the roof system from this difference in air pressure is...

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