Built-up-roof
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A Built Up Roof (BUR), generally consists of two or more layers of roofing felts laminated together using hot or cold asphalt or hot coal tar pitch as an adhesive. Built Up - meaning to "build" upon each other - The layered materials which are referred to as "plies" and are often covered with a heavy coating of asphalt or tar into which a layer of gravel is spread. The gravel surface serves several purposes; Protection from the Sun's UV rays and protection from damage caused by roof traffic are the two most important benefits. Built up roofs are desirable when heavy roof traffic or spilled contaminants are to be expected.
