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.
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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