Coal-Tar Pitch Roofing

Coal-tar pitch roofing is a form of built-up roofing in which the waterproofing bitumen is coal-tar pitch rather than petroleum asphalt. Coal tar is produced by high-temperature carbonisation of bituminous coal, for example when making coke. Both materials are black and thermoplastic and both are melted in kettles, so they are often confused, but they behave differently.

Properties

Compared with asphalt, coal-tar pitch has lower heat stability, a lower flash point and higher specific gravity. It stays slightly fluid at roof temperatures, which allows small cracks to "self-heal", and it resists water and oils well. These traits made it popular on dead-level roofs and on roofs exposed to standing water. Coal-tar BUR is normally surfaced with a flood coat and gravel to protect it from sunlight.

Slope limits

Because it flows, coal-tar pitch is limited to very flat roofs. The International Building Code permits coal-tar built-up roofs at a design slope as low as 1/8 in per 12 in (1%), half the 1/4:12 required for most other membranes. Industry guidance generally advises against using it above about 1/2 in per 12 in (4%), and many references limit it to 1/4:12 or less.

Materials

Coal-tar systems traditionally use coal-tar-saturated organic felts. Glass-fibre felts are a poor match because their porosity does not suit coal tar. Type I coal-tar pitch, used since the late 1800s, is the traditional product; a lower-fuming Type III introduced in the 1980s has had reported performance problems.

Durability and decline

Well-built coal-tar roofs have commonly lasted 50 to 70 years, and some over a century. Even so, asphalt largely replaced coal tar from the 1980s onward. Coal-tar pitch volatiles carry occupational health advisories, which complicate handling, and lower-fume cold-process coal-tar products have been developed. It remains in use mainly on specialised dead-level and reroofing projects.