A built-up roof (BUR) is the most widely used material for covering flat roof surfaces. These roofs have been used for well over a century, and are made from multiple layers of reinforced fabric joined by a binding agent, or bitumen. Each layer of fabric is known as a ply, and higher numbers of plies are associated with more long-lasting and durable roofs.
There are two main types of built-up roofs and each is categorized by the binding agent, or bitumen, that is used. Coal tar is the more durable of the two bitumen products, and is made using by-products of coal production. Asphalt is less durable, but often less expensive, and is a by-product of petroleum manufacturing. The use of these two bitumens may be limited depending on regional availability and environmental laws.
Built-up roof installers begin by adding a waterproofing membrane to the roof deck. Two layers of building paper or rubber membrane are nailed to the surface, making the roof effectively waterproof before any additional BUR plies are added. Next the installers will alternate layers of roof felt or fiberglass with mopped-on asphalt or coal tar. Finally, the entire surface is coated with a protective top coat such as gravel or clear glaze. The average built-up roof will have between three and five plies, not including the waterproofing membrane layers.
The primary benefits associated with built-up roofing is the low cost of materials and installation, especially when compared to alternative roofing products like metal or rubber. Built-up roofs are also durable and low-maintenance, and are able to withstand wind or fire damage. They can also be easily patched and repaired over time as needed. Because a double layer of waterproofing is used on these roofs, the plies themselves are redundant and help to create a reliable and watertight installation.
Despite their many benefits, homeowners should also consider the potential drawbacks of built-up roof construction. The materials used to construct a built-up roof are susceptible to both sun and water damage. Prolonged exposure to these elements can cause small cracks to form in the surface, which will eventually cause the roof to fail. While a built-up roof is known for its durability, it is unlikely to last as long as many alternative materials. The bitumens used to construct a built-up roof tend to produce a foul smell during installation, and may release toxic fumes into the atmosphere.