A traditional tar-and-gravel roof is built from three or more plies of waterproof material alternated with hot tar and ballasted by a layer of smooth river stone. Once made of tar paper, these types of roofs gradually are using more-advanced materials such as fiberglass membranes.
Built Up roofs are very heavy. Ponding water is common and it is hard to find the source of leaks.
Modified Bitumen
A single-ply rolled roof similar to ice-and-water shield, but impregnated with a mineral-based wear surface. Torch-down systems involve heating the adhesive as the material is unrolled. Newer peel-and-stick systems are safer and easier.
EPDM Rubber Membrane
EPDM (short for ethylene propylene diene monomer) is a true rubber. The durable material resembles an inner tube, but it’s engineered to resist damage from sunlight. EPDM can be mechanically anchored with fasteners, ballasted with stone, or glued.
The standard black material absorbs heat, and light-colored coatings (recommended in warm climates) add 30% or more to the cost. Even the black version, though, costs more than BUR or modified bitumen. It’s also more vulnerable to punctures than other choices.