A built-up roof, sometimes called BUR or a tar-and-gravel roof, is the oldest and most common flat roofing system.
Components of a BUR
Asphalt – this tar-like material is the waterproofing component
Coal tar pitch – or “pitch” for short, a waterproofing component no longer used as it presented a health hazard for roof installers
Roofing felt – like thick paper impregnated with asphalt; gives strength to the roof membrane, holding the BUR together
Plies – the layers of a BUR
Building a BUR System
A roofer applies coats of hot asphalt to several layers of roofing felt, embedding the felt into the asphalt while the latter is still hot, and continuing the process until the desired number of layers or plies is achieved. A residential home can have three to five plies, four being typical. A flood coat of asphalt is spread on top of the final felt ply, with gravel then raked into this still hot asphalt coat.
Purpose of the Gravel
Direct sunlight on asphalt will cause fast deterioration. Gravel reflects the sun’s ultra-violet radiation. Gravel also helps spread rainwater across the roof, increasing surface area for faster evaporation.
Are BURs Leak Prone?
Flat roofs are not prone to leakage so much as they are prone to neglect. A neglected flat roof is more likely to leak than a conventional sloped roof. A well-maintained flat roof, however, is reliable.
Maintenance is Key
Since a flat roof tends to collect debris, check it twice a year, clearing its surface, gutters and drains of debris. Also ensure gravel covers all areas completely.
With periodic maintenance, minor issues can be addressed before becoming major problems. Repairs to BURs are not a do-it-yourself project. Call a roofing contractor familiar with BUR.
Common BUR Defects
Blisters – bubbles that form under or between plies, usually as a result of moisture trapped between he plies. Blisters occur from prolonged exposure to the sun: heat transforms moisture into water vapor, which expands into a blister. Blisters can measure a few inches or feet. They expand and contract with he heat of the sun, prying the gravel from the asphalt, eventually breaking the roof apart at the blister. A competent roofer can repair a BUR that has a couple of blisters. If many blisters appear, consider resurfacing the roof.
Exposed asphalt – asphalt exposed to the sun will quickly deteriorate. A roofing contractor can add gravel to exposed areas.
Ponding – refers to a flat roof with standing water 48 hours after a rain. A leak that occurs during ponding is usually significant. A BUR may not have a sufficient slope towards a drain, or may have low spots that do not drain. Even a flat roof should slope towards a drain or the roof gutters. A BUR roof can be re-sloped during resurfacing. Occasionally, adding a drain at the low spot can solve a ponding problem. Before calling a contractor, clear debris from the roof drains.
Old – like any other roof, a BUR becomes unreliable as it ages. Over time, heat and ultra-violet radiation degrade the asphalt, making it shrink and become brittle, eventually cracking and exposing the felts. Water gets into the seams and between the plies, causing blisters.
Leaks – leaks from a flat roof can be difficult to trace because water does not always migrate into the ome directly below the leak. Call a competent roofer to trace and repair the leak.
Flashing defects – flashings prevent water from leaking into the roof at roof joints and roof penetration points. Most roof leaks occur at the flashings because the flashing is damaged or it was never installed properly. Flashing details on flat roofs are complex. Leave analysis and repair to a competent roofing contractor.
A flat roof is reliable when properly maintained. Since good maintenance is key to all the components of your home, a flat roof should present no more of a worry than anything else.
Source: Pillar To Post Information Series