Classification of Roof Systems

Low-Slope Basics

• Flat roofs are usually found on warehouses and commercial buildings. Residential low-slope roofs have a problem in colder climates due to water freezing in gutters and down spouts, causing water to back up on the roof. Water tends to form pools on flat roofs.

Low-Slope Systems

• Built-up roof systems (BUR) are alternating layers of felt saturated with bitumen, commonly called asphalt. Bitumen is derived from petroleum or coal tar. Single-ply membranes are waterproof sheets typically glued to roofs. EPDM (ethylene propylene diene monomer, also called a rubber roof) is the mostly commonly applied membrane on low-slope roofs. It is easy to apply and competitively priced. Dependable PVC (polyvinyl chloride) and less expensive TPO (thermoplastic olefin) membranes may be gray, black or white and contain flame retardants and materials to absorb ultraviolet rays. Polyethylene foams are sprayed on flat roofs in two layers, a base and a top of rubber-like plastic. Waterproof metal panels are also made for flat roofs.

Steep-Slope Basics

• Steep-slope roofs, most often seen in residential houses, have the advantage of being able to shed rain and snow quickly. They are also more expensive to build and maintain. The exterior covering of shingles, shakes and tiles do shed water, but it is the underlayment between the covering and the roof deck that makes the roof waterproof. The underlayment is usually felt saturated with asphalt.

Steep-Slope Systems

• Most American houses with steep roofs are covered with asphalt shingles. They are inexpensive, come in multiple colors, and last 20 to 30 years. Wooden shingles and shakes, typically made of cedar, are considered the most attractive steep-roof covering. While they are more expensive and difficult to lay, they last from 30 to 50 years. Slate and artificially manufactured slate shingles are attractive and long-lasting, but they are expensive and must be cleaned and oiled periodically to prevent mildew buildup. Panels of inexpensive, long-lasting corrugated sheets of galvanized steel are marketed for steep roofs on barns, sheds and utility buildings. Copper roofs are elegant and long-lasting, suitable for residential roofs, but also they are also expensive. Ceramic and cement roof tiles are attractive and heavy but can last from 60 to 80 years.