The asphalt which acts both as an adhesive as well as a water-proofing coating, is applied as the roof is being layed down. Because the oxidised asphalt for built up roofing must retain adhesive properties, it is less cross linked or mildly oxidized compared to asphalt prepared for roofing shingles. Roofing stone aggregate is spread over the entire built up roof to provide weather resistance to the built up roof. Roofing stone aggregate can be stone or slag. Its purpose is to prevent ultraviolet rays from degrading the roof coating. The stone allows walking over the roof without disturbing the asphalt.
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