A built-up roof (BUR) membrane consists of several plies of felt, where each successive felt is laid over a mopping of asphalt. The felt consists of glass fibers, woven into a mat, which is porous. Although the BUR membrane is meant to waterproof the roof, the porosity of felts is considered an advantage. Why?
What will be an ideal response?
The temperature of asphalt when applied on the roof is approximately 400°F - much greater than the boiling point temperature of water. Consequently, when asphalt is applied, any water on (or in) the substrate converts to steam which causes blisters in the overlying felt if it is nonporous, resulting in the loss of adhesion. Multiple felts provide the required waterproofing and membrane strength.
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Fill in the blank(s) with the appropriate word(s).
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