Built up roofing is a generic term for a roof system that is literally built by adding layers. Built-up roofs are more commonly found on commercial buildings than homes, but they're well suited to houses with flat roofs. They are sometimes made from alternating layers of roofing felt with different waterproofing materials including coal tar pitch, asphalt, cold application adhesive, or modified asphalt.

Modified bitumen roofs are made from fiberglass-based asphalt sheeting, applied in layers with mopped-on hot bitumen between each. Built up roofing is also frequently known as "tar and gravel" because they are finished with a layer of gravel, crushed rock or other mineral substance to minimize sun damage.

Single ply roofing systems come in two main types. Thermoset membranes, compounded from rubber polymers, require the use of liquid adhesive or tape to form a watertight seal at the overlaps. Thermoplastic membranes based on plastic polymers have seams formed with heat or chemical welding. Most thermoplastic membranes include a reinforcement layer of polyester or fiberglass for increased strength and stability.

Built-up and single-ply roofs generally have low maintenance costs over the life of the system. Material warranties are available up to 12 years. However, these roofs must be removed and replaced after their usable lives.

Sprayed polyurethane foam (SPF) is sprayed onto the roof as a liquid to form a single continuous seamless structure. Because foam is flexible and lightweight it can be sprayed on irregularly shaped roofs and protrusions. For re-roofing, it can be applied directly over existing roof installations including asphalt, shingles, built-up roofs, clay tile, concrete, metal, and wood, eliminating the need for tear-off. foam roofing has an insulation value of R- 7.14 per 1 inch thickness. Foam roofing systems are easy to maintain; repairing and recoating an SPF roof could enable it to last indefinitely.

Flat (or built-up) roofs are more commonly found on commercial buildings than homes, but they're well suited to houses with flat roofs. They are sometimes made from alternating layers of roofing felt and hot tar, covered with a protective layer of gravel. They are also made from fiberglass-based asphalt sheeting, applied in layers with mopped-on hot bitumen between each. The surface is coated with bitumen and a layer of gravel or crushed rock to minimize sun damage.

-- Tips courtesy of HomeAdvisor.com