The short answer
A built-up roof (BUR) is a membrane made on site from three to five alternating layers of roofing felt and hot asphalt or coal tar, usually finished with a flood coat and gravel. It is the original 'tar and gravel' commercial roof, typically lasts 15 to 30 years and costs about $4 to $10 per square foot to install.
On this page
Before single-ply membranes arrived, nearly every flat commercial roof in America was built up by hand on site. Many are still in service on schools, offices, churches and older shopping centers, and BUR is still specified where owners want a thick, multi-layer roof that shrugs off foot traffic.
How a built-up roof is made
The name says it: the waterproofing is built up in layers on the roof.
- Insulation goes down on the deck, often with a base sheet.
- Plies of roofing felt, usually fiberglass today, are laid in hot bitumen, each overlapping the one before. Three or four plies is typical.
- A flood coat of hot bitumen goes over the top layer.
- Gravel or slag is embedded in the flood coat to shield the bitumen from sun and hail. Some roofs use a mineral cap sheet or a reflective coating instead.
The bitumen is either asphalt (most common) or coal tar, which is used on very flat roofs because it self-heals and tolerates standing water better. Coal tar fumes need careful handling, which is one reason it is less common now.
BUR vs modified bitumen vs single ply
| System | What it is | Seams | Typical life |
|---|---|---|---|
| Built-up (BUR) | Felts and hot bitumen in layers | Every ply overlaps | 15 to 30 years |
| Modified bitumen | Factory-made rolls of polymer-modified asphalt | Torched, mopped, cold-glued or self-adhered | About 20 years |
| TPO and PVC | One plastic sheet | Hot-air welded | 20 to 30 years |
| EPDM | One rubber sheet | Taped | Up to 30 years |
Lifespans are from national roofing cost guides published in January 2026.
Strengths and weaknesses
Strengths: several layers of redundancy, so one puncture rarely means a leak; the gravel surface stands up to foot traffic and hail; long track record.
Weaknesses: heavy (the gravel adds weight the structure must carry); leaks are hard to trace through gravel; installation involves hot kettles and fumes; dark gravel surfaces absorb heat, though reflective coatings and white aggregate can help, as the Department of Energy’s cool roof guide explains.
What goes wrong with BUR
- Blisters, where trapped moisture or air between plies expands in the sun.
- Splits, long straight cracks from building movement or shrinkage.
- Bare spots where gravel has washed or blown away, letting UV attack the bitumen.
- Alligatoring, a crazed surface on unsurfaced or aged areas.
- Flashing failures at walls and curbs, as on any roof.
National guides put BUR repairs at roughly $3 to $7 per square foot of damaged area. See built-up roof repair for the usual fixes.
Can you put a new roof over a built-up roof?
Sometimes. The International Building Code does not allow a new covering over a roof that is water-soaked or deteriorated, or where two or more coverings are already in place. Loose gravel also has to be removed before recovering. A core sample shows how many layers are there and whether the insulation is dry.
Working on a BUR roof safely
BUR work involves hot bitumen, kettles and work near roof edges, all covered by OSHA’s roofing guidance. Building staff should stay off the roof; report leaks from inside and let a roofing crew do the walk.
If your tar and gravel roof is leaking, send us the ZIP code and we will look for a commercial roof repair specialist who covers your building.