MIL-HDBK-1001-5 Roofing And Waterproofing S/S By Mil-Std-3007This handbook sets forth guidelines for design of roofing, LAYOUT 1 and waterproofing systems for building construction. It covers design of new low-slope and steep roof systems, reroofing of existing buildings, and waterproofing, and dampproofing systems.

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MIL-HDBK-1001/SA

Section 12: WIND-UPLIFT RESISTANCE

12.1      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 pressures.

12.1.2    Resistance.  Wind-uplift resistance on low slope roofs is based on two radically different principles:

a)   Resistance from fasteners and/or adhesives anchoring the various roof system components to the supporting structural deck;

b)   Ballast, in the form of loose gravel or concrete pavers, designed to overcome wind-uplift pressure through gravitational counter pressures.

12.2      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 deck.

Although the 10 to 2S 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.

12.3      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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