Roofing system
What is built-up roofing?
The oldest low-slope roofing system still in common use, better known by its nickname: the tar-and-gravel roof.
Updated September 12, 2026 · Every figure links to its source · Sources
DefinitionSystemAlso called: BUR, tar and gravel roof
Built-up roofing — a continuous, semi-flexible roof membrane assembled on the roof from alternating layers (plies) of bitumen and reinforcing felts, finished with a surfacing such as aggregate or a cap sheet; the NRCA glossary defines the system this way and the 2021 IRC, Section R905.9 regulates it as "built-up roofs".
What it is
A built-up roof is made where it lies. Hot asphalt (or, historically, coal tar) is mopped onto the deck or insulation, a reinforcing felt is rolled into it, and the sequence repeats, three or four plies deep, until the layers form a single sheet. The top is flooded with bitumen and either gravel is embedded in it, or a granule-surfaced cap sheet finishes the roof. The gravel is not decoration: it shields the asphalt from ultraviolet light and holds the membrane down.
Because it is an asphalt product, the Asphalt Roofing Manufacturers Association covers built-up roofing alongside shingles and modified bitumen. The model codes set a minimum slope: the IRC, Section R905.9 requires a design slope of at least one-quarter unit vertical in twelve horizontal for drainage (one-eighth for coal-tar roofs), and the IBC, Section 1507.10 sets the same for commercial buildings.
Why it matters
Built-up roofing is why a great many flat-roofed schools, warehouses and mid-century houses are still dry. Its virtue is redundancy: a puncture through one ply meets three more, and the mass of gravel resists wind and foot traffic. Its weaknesses are its weight, the heat and fumes of hot-mopped installation, and the difficulty of finding a leak under gravel, which is why single-ply and modified bitumen have taken most new low-slope work.
For an owner the practical questions are whether the roof still has positive drainage (standing water that does not clear within a day or two is the first sign of trouble), whether the gravel has scoured off in patches and exposed bare asphalt, and whether the flashings at walls, curbs and drains — where most built-up roofs actually leak — are sound. The directory's guide to flat and low-slope roofs covers how the systems compare.
What it costs and who does it
Low-slope roofing is priced by the square, with the number of plies, the insulation beneath, the surfacing and the amount of flashing and drain work as the main variables; a replacement also carries the cost of removing the heavy old membrane and gravel. It is commercial roofing work, done by contractors with kettles, mops and the safety programme that hot bitumen requires — not every residential roofer takes it on. The directory's flat-roof replacement cost guide describes how those estimates are built.
Frequently asked questions
Short answers; the sections above carry the detail and the sources.
Is built-up roofing the same as a tar and gravel roof?
How many plies does a built-up roof have?
Can a built-up roof be repaired rather than replaced?
Next step
Find a roofer who does this
Built-up roofing is commercial work, and the contractors who do it well are usually the ones who also do modified bitumen and single-ply. The directory lists roofing contractors by state and city.
Sources
- Roofing glossary — National Roofing Contractors Association (NRCA)
- 2021 International Residential Code, Chapter 9: Roof Assemblies — International Code Council (ICC)
- 2021 International Building Code, Chapter 15: Roof Assemblies and Rooftop Structures — International Code Council (ICC)
- Asphalt Roofing Manufacturers Association — ARMA