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Built-up roof membranes are composed of alternating layers of bitumen (either asphalt or rarely coal tar) and reinforcement sheets (felts). Fiberglass felts are typically used for asphalt BURs Historically, ply sheets have been either fiberglass-mat or organic-mat reinforced. Currently organic-mat reinforced ply sheets have largely disappeared from the U.S. market. The asphalt is typically hot applied, however, cold-applied asphalt is available (cold-applied asphalt incorporates solvent). The membrane is either adhered to the substrate in bitumen or a base sheet (i.e., a heavy felt) is mechanically attached.
When a BUR is installed over polyisocyanurate, a suitable cover board should be installed over the polyisocyanurate. Four plies of felt are recommended (if a nailed base sheet is installed, four plies are recommended in addition to the base sheet). “Heavy duty” fiberglass felts are available (ASTM E2178 Type VI), but because of their stiffness, it is easier to construct unwanted voids in the membrane. Therefore, Type IV felts are recommended. Polyisocyanurate is by far the most common insulation used under built-up roof assemblies. Occasionally, mineral wool will be used.
The bitumen provides the waterproofing characteristics; the felts provide improvements to physical characteristics. Complete and full embedment of the felts into the bitumen is crucial.
Exposed asphalt is susceptible to ultra-violet degradation. Therefore, BURs are surfaced with aggregate, a field-applied coating or a mineral surface cap sheet. If aggregate is specified, wind blow-off should be considered, see Wind Safety—Roof Systems. Coatings include aluminum-pigmented asphalt, asphalt emulsion (reflective or non-reflective), urethane, and acrylic. Coatings can enhance fire resistance. However, if coatings are specified, periodic recoating will be required. Because of future maintenance demands, coatings are not recommended. If a cap sheet is specified, it should be in addition to the 4 plies of felt.
Asphalt tends to get brittle with age, making it unable to accommodate normal building movements. And, application of hot asphalt requires open flames and working with extremely hot liquids. Some clients and designers do not use BUR roofs to avoid these safety issues. Fumes are particularly noxious to many people and should be avoided on occupied buildings.
ASTM standard D312 is the product standard for asphalt used in roofing. There are four types of asphalt. Type I is much more susceptible to flow than Type IV. ASTM D6510 provides guidance for selection of asphalt Type in BURs. Caution should be used when heating asphalt to prevent changing its physical properties and thereby making it unsuitable for roofing.
Base flashings of BURs are typically constructed with modified bitumen sheets.