History of Built-Up Roofing

A built-up roof (BUR) is made on site from several layers, or plies, of reinforcing felt bonded together with hot bitumen, usually finished with a flood coat and gravel. It is often called a "tar and gravel" roof. For much of the 20th century it was the standard covering for commercial flat roofs in North America.

Origins

Flat roofs are ancient, characteristic of the dry climates of Egypt, Persia and Arabia, where mud and plaster sufficed. Wetter climates needed a true waterproof layer. In the 1840s the Warren brothers of Cincinnati produced composition roofing from fabric coated with pine tar and sand, an early form of layered roofing. By the late 19th century built-up components were also being combined into factory-made roll roofing.

Coal tar and asphalt

Two bitumens dominated:

• Coal tar pitch, a by-product of coke making, heated and mopped between layers of tar paper, usually on slopes of 1/4 in per foot (about 2%) or less. Coal tar roofs commonly last 50-70 years, and some have performed for more than a century.
• Asphalt, a by-product of oil refining, graded into four types (I to IV) by the slope it suits.

BUR systems dating from the early 1900s are still reported in service, though that claim is not well documented.

Gravel

Gravel or slag embedded in the top coat protects the bitumen from ultraviolet light, hail and foot traffic, and originally also helped hold the roof down.

Performance

Roofers often quote 20-30 years for tar-and-gravel roofs. Studies give averages of 12-27 years, with some exceptional roofs lasting up to 120 years.

Competition

From the 1960s onward BUR faced competition from new systems:

• Modified bitumen, developed in Europe (APP from Italy and SBS from France). Sources differ on the date, giving the mid-1960s or the 1970s; US development followed in the late 1970s and early 1980s.
• Single-ply membranes such as EPDM, PVC and, from the early 1990s, TPO.

Hot-applied BUR remains in use, but its share of new low-slope roofing has fallen as single-ply and modified bitumen have grown.