Built-Up Roofing (BUR)

Built-up roofing, abbreviated BUR and known colloquially as "tar and gravel", is one of the oldest membrane roofing methods. It builds a waterproof layer on site by alternating layers of bitumen (asphalt or coal-tar pitch) with reinforcing felts, called plies. The bitumen provides the waterproofing; the felts stabilise and reinforce it. A typical system uses three to five plies.

History

BUR has been used in the United States since the mid-to-late nineteenth century and dominated North American commercial low-slope roofing for roughly a century until about 1980. Its share then fell as modified bitumen and single-ply membranes arrived and oil prices rose. NRCA market surveys around 2000 still showed BUR at 20% or more of US low-slope sales, and an estimated 50 billion square feet was in service; later estimates put its share of new US installations far lower. Many old BUR roofs remain in place, often covered by newer systems.

Components

• Deck: steel, concrete, poured gypsum or wood.
• Vapour retarder and insulation where required.
• Base sheet: a heavier felt, often mechanically fastened, that anchors the membrane.
• Plies: organic, glass-fibre or polyester felts set in bitumen. Asphalt-impregnated glass-fibre felts are the most common today.
• Flashings: frequently made from modified bitumen.
• Surfacing: a flood coat of bitumen with embedded gravel or slag, a mineral-surfaced cap sheet, or a coating.

The membrane alone is roughly 13 mm (1/2 in) thick and weighs about 12 kg/m2 (2.5 lb/sq ft); gravel adds about 15 to 20 kg/m2 (3 to 4 lb/sq ft).

Hot and cold application

In hot-applied BUR, asphalt is heated in a kettle or tanker and applied by mop or mechanical spreader, typically about 25 lb per 100 sq ft per ply (about 1.2 kg/m2). Correct temperature is critical; the industry uses the equiviscous temperature (EVT) to target the right viscosity. Cold-process BUR uses solvent- or water-based adhesives applied by spray, brush or squeegee, avoiding kettles but needing longer cure times.

Performance and problems

Multiple plies give redundancy, and well-built systems have served for decades. Typical problems are blisters, splits (often from unanchored insulation), ridging over insulation joints, fishmouths, slippage, alligatoring of exposed bitumen and gravel scour at corners. Most leaks start at flashings and penetrations rather than in the field of the roof.

Health considerations

US NIOSH has identified roofing asphalt fumes as a potential occupational carcinogen, and IARC classified occupational exposure to oxidised bitumen during roofing as probably carcinogenic in 2011. Low-fuming asphalts and kettle controls were developed in response.