Built-up roof (BUR) – “Tar and Gravel” Roofing
Built-up roof (BUR) systems or “tar & gravel” as more commonly known in layman’s terms is a multiple ply roof system consisting of plies of roofing ply sheets or felts installed on the rooftop with inter-ply moppings of asphalt. All the components are brought to the roof and an essentially seamless membrane is manufactured in place covering the entire roof area, thus the term “built-up roof”.
Built-up roofs come with diverse specifications, depending on application, with varying number of organic or fiberglass reinforced ply sheets and different types of asphalt. The finished surface can be pea gravel, a mineral surfaced cap sheet or a reflective aluminum coating depending on the application requirements.
Built-up roofing in some form has been the most common type of membrane roofing until recent years with the advent of the newer single ply systems. It was generally believed that the single ply systems would take over the market rendering the built-up roofing system obsolete. However, this has not happened. The redundancy and durability of the multiple plies make the built-up roof system an excellent performer in a wide range of applications.
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Often, a built up roof will have a layer of pea-size gravel on top, to protect the roof from sun’s devastating UV rays.
The reinforcing fabrics also are called roofing felts or ply sheets. Roofing felts are reinforced with either glass-fiber mats or organic mats. Felts are produced in a standard width of 36 inches and metric width of about one meter.
The bitumen typically used in BUR roof systems is asphalt, coal tar or cold-applied adhesive. The asphalt or coal tar is heated in a kettle or tanker and then applied by mop or mechanical spreader. Asphalt is a petroleum product refined from crude oil; coal tar is derived from the distillation of coal. Cold-applied adhesives typically are solvent-based asphalts that don’t have to be heated in a kettle or tanker.
Surfacing for built up roof 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 three plies and a polymer-modified bitumen membrane cap sheet is commonly referred to as “hybrid” system.