Quantcast Section 12: Wind-Uplift Resistance

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Wind-Uplift Hazards.  Wind-uplift hazards to roof
systems depend on the phenomenon known as the Bernoulli
principle: increased air velocity reduces the pressure exerted
perpendicular to the direction of air flow.  As a consequence,
low slope roofs, in particular, experience maximum wind-uplift
Resistance.  Wind-uplift resistance on low slope roofs
is based on two radically different principles:
Resistance from fasteners and/or adhesives
anchoring the various roof system components to the supporting
structural deck;
Ballast, in the form of loose gravel or concrete
pavers, designed to overcome wind-uplift pressure through
gravitational counter pressures.
Failure.  Failure modes for the two different anchorage
techniques are radically different also.  Anchored roof systems
usually fail by blowoffs of membrane and insulation boards, which
are rolled back from roof edges, exposing the deck. Ballasted
systems have a more complex failure mode.  Failure starts with
wind scour, which exposes the loose-laid membrane in areas
subject to highest wind-uplift pressures (generally at building
corners.) Scouring may be followed by membrane ballooning, with
possible membrane tearing, and is sometimes accompanied by
insulation displacement into sub-membrane "hills" on the roof
Although the 10 to 25 psf (49 to 122 kilograms per
square meter) ballast weight may be only a minor fraction of the
wind-uplift forces, ballasted systems seldom allow exposure of
the huge deck areas that are sometimes exposed in failed anchored
systems.  An adhered system usually experiences a blow off from
local failure, like the tensile failure of the weakest link in a
chain.  In contrast, in a ballasted system, any wind-lifted or
ballooned area normally shifts its ballast to an adjacent area,
where the uplift resistance is consequently increased.
Ballasted Systems.  Ballasted systems are limited to
single-ply membranes, usually elastomeric or plastic sheets, some
modified bituminous membranes, and protected membrane roofs.


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