Built-up roof (BUR) membranesClick on a topic for more information. Material and system description
Built up roof membranes, referred to by the acronym BUR, have been in use in the U.S. for more than 100 years. These roof systems are commonly referred to as "tar and gravel" roofs. BUR systems generally are composed 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 be fully adhered if applied directly to roof decks or insulation.
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.
Surfacings 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.
Photo of an aggregate-surfaced BUR
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. NRCA considers this type to be a polymer modified bitumen membrane system.
NRCA does not make any recommendations about which product or manufacturer to use; however, NRCA does recommend that bitumens, felts and mats used for BUR systems meet standards established by ASTM International.
The following commonly referenced ASTM International standards apply to materials used in the construction of BUR systems:
- ASTM D312, "Standard Specification for Asphalt Used in Roofing"
- ASTM D450, "Standard Specification for Coal Tar Pitch Used in Roofing, Dampproofing and Waterproofing"
- ASTM D1863, "Standard Specification for Mineral Aggregate Used on Built Up Roofs"
- ASTM D2178, "Standard Specification for Asphalt Glass Felt Used in Roofing and Waterproofing"
- ASTM D2824, "Standard Specification for Aluminum Pigmented Asphalt Roof Coatings, Nonfibered, Asbestos Fibered, and Fibered without Asbestos"
- ASTM D3909, "Standard Specification for Asphalt Roll Roofing (Glass Felt) Surfaced With Mineral Granules"
- ASTM D4601, "Standard Specification for Asphalt Coated Glass Fiber Base Sheet Used in Roofing"
- ASTM D4990, "Standard Specification for Coal Tar Glass Felt Used in Roofing and Waterproofing"
When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer's warranty. In general, these warranties cover defects in the manufacture of the roof membrane. Please read NRCA's consumer advisory bulletin addressing roofing warranties for more information. Once the project is complete, be sure the contractor provides you with a certificate for your records.
Second, the roofing contractor will provide you with a warranty covering his workmanship. Typically, this will cover installation and related issues. The warranty should contain what items are covered and what will void them. Many contractors offer one year or two years of coverage; however, there is no industry standard.