What Is Built-Up Roof (BUR)?
A built-up roof is constructed on site as a laminate: hot-mopped asphalt, then a reinforcing felt ply, repeated three to five times, finished with a flood coat of bitumen and embedded gravel that shields the asphalt from sun and holds it down. Its waterproofing logic is redundancy — a defect must line up through multiple independent plies to leak, which is why old BURs often outlive their design life. It fails slowly: the surface alligator-cracks as oils bake out, blisters form where moisture vaporizes between plies, and edges and flashings open as the mass shrinks. Water that does enter travels laterally between plies, surfacing far from the entry point, and the gravel hides everything — leak-finding on BUR is genuinely difficult and sometimes ends in moisture scanning. Homeowners see bare spots where gravel has washed, soft blisters underfoot, and stains without an obvious source. Few residential crews still hot-mop; repairs today are usually modified-bitumen patches into the old system.
Important Distinction
Note: The gravel is ballast and UV shield, not the waterproofing — a 'gravel roof' leak is in the bitumen plies beneath, and the gravel is precisely what makes finding it hard.
Also Known As
These are the search terms and phrases that commonly refer to this topic, so you can find it however you describe it.
How Built-Up Roof (BUR) Relates to Other Topics
Modified bitumen is the built-up roof's successor: BUR's alternating plies of felt and hot asphalt, factory-laminated into a roll - so an old gravel-topped BUR is usually replaced with mod-bit or single-ply, not rebuilt in kind.
The gravel on a built-up roof is not decoration - it shields the asphalt from UV and holds the membrane down; bare patches where gravel has washed away are where a BUR ages fastest and leaks first.
Gravel hides ponding on a built-up roof, but the pond leaves a signature: a clean bare ring where standing water pushed the gravel aside - readable long after the water itself is gone.
Questions about Built-Up Roof (BUR)?
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