Flat roofs, also called low-slope roofs, are not truly flat. They must have a slope of at least 1/8 inch per foot to avoid water pooling and leaking into the building. Choosing the correct type of flat roof for the application is important to ensure proper drainage. Flat roofs that are properly built and maintained can last up to 25 years. The most common flat roofs are among one of four types.
The built-up roof is the traditional "hot tar and gravel roof." Layers of fiberglass membranes, or other waterproof material, are interspersed with layers of bitumen (hot tar). Crushed stone or smooth gravel embedded in bitumen make up the top layer (ballast). The built-up roof is attractive when seen from above. It is fire-retardant and usually the most cost-effective type of flat roof. It is also very heavy. Gravel can break off, clogging gutters and scuppers. Installation of the built-up roof is messy and foul-smelling due to the use of bitumen. And leaks are difficult to locate under the ballast.
Metal Flat-Seam Roof
Metal flat-seam roofs use sheets of metal, such as copper or stainless steel, joined together by folding adjacent edges over, then under, to make lock seams. The seam is flattened with a mallet and soldered to prevent leaks. On large areas of roof, expansion battens help the roof contract or expand in response to heat and cold without damage.
Modified Bitumin Roof
A modified bitumen flat roof uses a single-ply membrane of asphalt and polymer. The wear surface can be gravel in bitumen, a reflective paint-on surface or a mineral surface. The light, reflective surface can reduce energy consumption. Modified bitumen roofs are medium-cost flat roofs. Application is peel-and-stick or "torch-down" (heat applied to the adhesive) as the membrane is unrolled. The torch-down application is used only on unoccupied buildings because of the possibility of fire.
Rubber Membrane Roof
A modified bitumen flat roof uses a single-ply membrane of asphalt and polymer. The wear surface can be gravel in bitumen, a reflective paint-on surface or a mineral surface. The light, reflective surface can reduce energy consumption. Modified bitumen roofs are medium-cost flat roofs.
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