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
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
ASTM D4990, "Standard Specification for Coal Tar Glass Felt Used in Roofing and
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.