Roofing Asphalt (BUR Asphalt Types)

Roofing asphalt is the hot-applied bitumen used to laminate the plies of built-up roofs and to mop down some modified bitumen sheets. In North America it is specified by ASTM D312, which divides mopping asphalt into four types by softening point and by how much the material tends to flow on a slope.

ASTM D312 types

• Type I (dead level): softening point about 57 to 66 C (135 to 151 F); for slopes of about 1/2 in per 12 in or less.
• Type II (flat): moderate flow resistance; about 1/2 to 1 1/2 in per 12 in. It is now hard to find in many markets.
• Type III (steep): relatively resistant to flow; about 1 to 3 in per 12 in.
• Type IV (special steep): softening point about 99 to 107 C (210 to 225 F); about 2 to 6 in per 12 in, and suitable for flashings.

Types III and IV make up most hot roofing asphalt sold, about 95% by one industry estimate.

Application

Asphalt is heated in a kettle or tanker and applied at its equiviscous temperature (EVT), the temperature at which it has the right viscosity: about 125 centipoise for hand mopping and 75 centipoise for mechanical spreaders. Overheating makes it brittle; underheating applies too much material, which can create a slip plane. A typical rate is about 25 lb per 100 sq ft per ply (about 1.2 kg/m2).

Other binders

Other standards cover oxidised asphalt (ASTM D7654) and SEBS polymer-modified asphalt (ASTM D6152). Wax-modified and polymer-modified low-fuming asphalts were developed from the 1980s onward to cut application temperatures and fumes; some form a skin on the kettle that traps fumes.

Health

US NIOSH identified roofing asphalt fumes as a potential occupational carcinogen in 2000 and published work-practice guidance in 2003 and 2007. In 2011 IARC classified occupational exposure to oxidised bitumen during roofing as probably carcinogenic to humans.