A cap sheet is a building material used in built-up roofing, or BUR. BUR applications are used for flat roof surfaces, and consist of several layers of material designed to create a watertight surface. Each layer includes asphalt or tar-coated layers of fiberglass mats. The more layers, or plies used in built-up roofing, the more effectively the surface will be able to resist leaks over time. A surfacing component, such as a cap sheet, is used to cover and protect these lower layers and extend the life of the roof.
Over time, constant sun exposure weakens asphalt and causes it to dry out. As it dries out in the sun, a BUR roof becomes brittle and eventually cracks. Rain seeps into these cracks and eventually penetrates the inside of the building, leading to leaks, mold, and rot. The cap sheet not only protects the asphalt from the sun, but also acts as an extra ply to further increase the integrity of the roof.
A cap sheet can be adhered to the roof surface using a number of methods, depending on the base materials. Some installers rely on heat welding to permanently bond the sheet to the lower roof layers. Others apply lap cement or add a bituminous material such as asphalt between the sheet and roof surface. Depending on the climate and wind levels, some installers may add gravel or aggregate on top of the sheet to help weigh it down and hold it securely in place.
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Most cap sheets resemble rolled asphalt roofing. They consist of fiberglass mats coated with mineral granules for strength and durability. A roof cap sheet is fairly heavy because of the mineral surfacing, and is widely referred to as 90-pound roofing in North America. A standard roofing square of this material weighs 90 pounds (40.9 kg), though some lightweight materials are used in certain regions.
Some cap sheets are used in retrofit applications to improve the energy efficiency of a building. These sheets feature a light-colored mineral surfacing that creates a white or off-white roof. This light coloring reflects sunlight to keep building's cool in hot, sunny climates. By reflecting sunlight, these roof sheets reduce the need for air conditioning to lower utility costs and help protect the environment. In this type of application, the cap sheet may be used with a standard BUR roof or with a variety of other roof finishes.