About 40 percent of Arizona homes have flat roofs. You have three flat-roof options:
A modified roofing system, also called a “torch-on” or “torch-down” roof. This is a single-ply, rolled roofing product that has a rubber backing and granules on the top. The installer uses a torch to melt the rubber backing so it adheres to the roof’s fiberglass base sheet. It’s the least expensive kind of flat roofing system. A square is a 10-foot by 10-foot area. The roof of a 2,400-square-foot home is about 24 squares. If your roofer has to tear off an old roof before applying the new surface, you’ll pay extra.
Hot asphalt, also called a “built-up” or “tar-and-gravel” roof. This is one of the oldest and most reliable roofing systems, and in other parts of the country where it’s not so hot, it’s the most common choice for flat roofs. A built-up roof consists of several layers of roof felt laminated together with bitumen, a sticky, oil-based product. That surface is coated with gravel or crushed rocks to protect the roof from the sun. The cost is slightly higher than the first option.
Foam roof. About 90 percent of Arizona’s flat roofs are protected by spray polyurethane foam because it’s energy-efficient, waterproof and fairly easy to maintain. It’s also among the most expensive flat-roof coverings. A roofer usually can cover an existing roof with foam, so there’s no need to tear off an old surface. A foam roof is covered with an elastomeric material that protects it from ultraviolet sunrays and can help the roof last for 25-plus years. So it’s important to recoat your foam roof every five to 10 years as that protective covering wears thin. Foam is among the most expensive for flat roofs, and has potential to have a removal fee, since your old roof will definitely have to be taken off.