Flat Roofing Options

Flat roofs have special requirements. Unlike steep sloped roofs, which depend on gravity to shed water, flat roofs use drains to remove water and rely on waterproof membrane to protect the property. Actually, the term 'flat' is misleading here -- no roof should be completely flat or dead level. A dead level roof is useless, except maybe as a pond for birds. "Low slope" is a more accurate description of what is commonly called a flat roof. Low slope roofs should have at least a 1/4 inch of slope per foot (preferably more) so water can flow downhill to drains and then to the ground.

There are many things to consider when selecting a low slope roof. A variety of systems can be used, each with distinct advantages and disadvantages, but not every system is suitable for every roof. Discuss the pros and cons of each with your contractor and ask him which one he recommends for your roof and why. Here is an overview of the most common systems:

"Unlike steep sloped roofs, which depend on gravity to shed water, flat roofs use drains to remove water and rely on a waterproof membrane to protect the property." Built-up roofing, BUR, for short, is what many people call "hot tar roofing." A century old, time tested roofing method, built-up roofs are made of three or four overlapping layers of asphalt felts fused together with hot asphalt.

Built up roofs must be protected from weather, sunlight and foot traffic by some kind of surfacing material -- gravel, a mineral granule covered top layer (a cap sheet), a smooth coat of hot asphalt or a special reflective coating.

Modified bitumen's, like built up roofing, are made of reinforced asphalt impregnated felts. But, unlike BUR, the asphalt in the felts is modified with plastic or rubber polymers (hence the name) to improve its elasticity, durability, and overall performance. Because of the added polymers, modified bitumen are often applied in a single layer with overlapping seams. They can be "glued" to the roof with hot asphalt or an adhesive, or by using propane torches to melt the asphalt underside. Some modified bitumen require protective surfacing materials, usually mineral granules or a liquid coating.

Single ply membranes are made of rubber, plastic or a hybrid of the two. As the name implies, single ply membranes are applied in a single layer. Installation is simple: the membrane is rolled out, the seams between sheets are heat welded, chemically welded or glued together, and the membrane is attached to the roof with fasteners (usually screws/flat plates), ballast (usually rocks), or adhesive.

Polyurethane foam is sprayed directly onto the roof, where it "foams up" and solidifies before a protective coating is applied. Because of its flexibility, foam is especially useful for roofs with unusual shapes or configurations. Whichever roofing system you choose, workmanship is critical on low slope systems. Make sure you hire an experienced, professional contractor. And make sure he follows the roofing material manufacturer's installation specifications.