Built-Up Roof membranes, referred to by the acronym BUR, have been in use in the United States for more than 100 years. These roof systems are also commonly referred to as “tar and gravel” roofs. BUR systems generally are comprised of alternating layers of bitumen and reinforcing fabrics that create a finished membrane. The number of plies in a cross-section is the number of plies on a roof. The term “four plies” denotes a four-ply roof membrane construction. Sometimes, a base sheet, used as the bottommost ply is mechanically fastened. Built-up roofs generally are considered to fully adhered if applied directly to roof decks or insulation.
Surfacing for built-up roofing systems include aggregate (such as gravel, slag or mineral granules), glass-fiber or mineral surfaced cap sheets, hot asphalt mopped over the entire surface, aluminum coatings or elastomeric coatings.
A roof system composed of a built-up roof membrane with two or more three plies and a polymer-modified bitumen membrane cap sheet is commonly referred to as a “hybrid” system. The National Roofing Contractors Association (NRCA) considers this type to be a polymer-modified bitumen membrane system.
Built-Up Roofing Lifespan
Depending on the client and the specific materials used, built-up roofing has an average life expectancy of 15 to 30 years, but some constructions can last up to 40 years. In general, built-up roofing tends to fare better in warmer climates than in cold regions. This lifespan makes built-up roofing comparable to composition (asphalt) shingles, which last between 15 to 30 years, depending on their quality.
Pros and Cons of Built-up Roofing
Built-up roofs tend to provide excellent waterproofing and ultra-violet protection. Thanks to the aggregate top layer, they are also fire-resistant. Built-up roofing is generally low maintenance and therefore costs little to maintain over its life.
On the downside, built-up roofing can be slow to install and, with the exception of cold bitumen processes, installation involves hazardous fumes. Overall, installation costs are relatively high, and some types can be susceptible to wind and water damage.
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