Laserfiche WebLink
ROOF DESIGN CONSIDERATIONS <br /> •The Case Against The Use Of A Vapor Retarder <br /> —The vapor retarder,together with the roofing membrane, <br /> may seal within the roof system entrapped moisture that can <br /> eventually destroy the insulation,help split or wrinkle the <br /> membrane,or,in gaseous form,blister it. <br /> —In the event of a roof leak through the membrane,the vapor <br /> retarder will trap the water in the insulation and only appear <br /> as a leak when there is a disruption in the vapor retarder.A <br /> large area of insulation may be saturated before the punctured <br /> roof membrane is discovered and repaired. <br /> —A vapor retarder is a disadvantage in the summer,when vapor <br /> migration is generally downward through the roof.Hot,humid <br /> air can infiltrate the roofing system through any interruption in <br /> the membrane and moisture can condense and then collect on <br /> the vapor retarder. <br /> The following resources may provide useful information regarding <br /> vapor retarders: <br /> —NRCA Guidelines for Condensation and Air Leakage Control <br /> —ASHRAE Fundamentals Handbook <br /> AIR BARRIERS <br /> 1.Designers should consider requiring an air barrier: <br /> a.On all air porous decks,with openings in the walls or <br /> area directly below the roof deck that exceeds 10%of the <br /> total wall area. <br /> b.When the internal pressurization of the building is in <br /> excess of 5 lbs.per sq.ft. (239 Pa). <br /> c.When the building height exceeds 50 ft. (30.5 m). <br /> d.When buildings have large openings or overhangs. <br /> e.In conditions where positive internal pressure is applied <br /> suddenly,as may be the case at aircraft hangers or <br /> distribution centers,where the roofing system may fail <br /> due to pressure impact. <br /> 2.Refer to FM Global Loss Prevention Data Sheets 1-28 and <br /> 1-29 for specific installation procedures for all roofs with <br /> large openings. <br /> 3.For roofs to be guaranteed by GAF: <br /> a.Air barriers are required for all extended-length guarantees <br /> on buildings where large wall openings greater than 10% <br /> of the total wall area can be opened during a windstorm, <br /> including opening due to storm damage. <br /> b.All 10'(3 m)wide or greater membranes with side laps <br /> mechanically attached with a spacing greater than 6,,(152 <br /> mm)O.C. <br /> 18 <br />