A built-up roof is composed of three elements: felt, bitumen and surfacing.
Felt—which is made of glass, organic or polyester fibers—serves much of the same purpose as reinforcing steel in concrete. The felt is necessary as tensile reinforcement to resist the extreme pulling forces in the roofing material. Felt is installed in a layering fashion, which allows more bitumen to be applied to the whole system.
Bitumen—either coal-tar or asphalt— acts as the “glue” that holds the felt layers together. It is also the waterproofing material in the built-up roof system.
Surfacing—normally applied to built-up roofs, is composed of either smooth, gravel, slag, or a mineral-coated cap sheet. Gravel or slag may be embedded into the still-fluid flood coat. Gravel and slag serve as an excellent wearing surface to protect the membrane from mechanical damage. On some built-up roof systems, a mineral coated cap sheet is applied on top of the plies of felt.