Built-Up Roof vs Modified Bitumen
Both are asphalt-based, multi-layer low-slope roofs. A built-up roof is a continuous, semi-flexible membrane of multiple plies of felts, fabrics or mats, laid in place with alternate layers of bitumen.[1] Modified bitumen uses factory-made sheets of bitumen modified with polymers such as APP or SBS, made into sheets often reinforced with mats and sometimes surfaced with films, foils or mineral granules.[2] The code sets the same minimum slope for both, 1/4:12, with an exception for coal-tar built-up roofs at 1/8:12.[3]
At a glance
| Feature | Built-up (BUR) | Modified bitumen | Source |
|---|---|---|---|
| What it is | a continuous, semi-flexible membrane of multiple plies of felts, fabrics or mats, laid in place with alternate layers of bitumen | bitumen modified with polymers such as APP or SBS, made into sheets often reinforced with mats and sometimes surfaced with films, foils or mineral granules | [1, 2] |
| Surfacing | mineral aggregate, bituminous materials, a liquid-applied coating or a granule-surfaced cap sheet | Often a cap sheet: a sheet, often granule-surfaced, used as the top ply of some built-up or modified bitumen membranes | [1, 5] |
| Minimum slope in the code | 1/4:12 (coal-tar: 1/8:12) | 1/4:12 | [3] |
| Material standards in the code | glass fiber base sheet ASTM D4601, mineral-surfaced cap sheet ASTM D3909, roofing asphalt ASTM D312 | ASTM D6162, D6163, D6164, D6222, D6223 or D6298 | [3] |
| Example system | Not checked for a specific brand | GAF Liberty: 2-ply system: self-adhering base/ply sheet plus cap sheet, self-adhering, without torches, open flames or hot asphalt | [4] |
How each one is built
NRCA defines a built-up roof as a continuous, semi-flexible membrane of multiple plies of felts, fabrics or mats, laid in place with alternate layers of bitumen, finished with mineral aggregate, bituminous materials, a liquid-applied coating or a granule-surfaced cap sheet.[1] The layers are assembled on the roof.
Modified bitumen starts as factory-made sheets of bitumen modified with polymers such as APP or SBS, made into sheets often reinforced with mats and sometimes surfaced with films, foils or mineral granules.[2] The top sheet is usually a cap sheet: a sheet, often granule-surfaced, used as the top ply of some built-up or modified bitumen membranes.[5]
Standards in the code
The residential code lists, among other materials for built-up roofs, glass fiber base sheet ASTM D4601, mineral-surfaced cap sheet ASTM D3909, roofing asphalt ASTM D312.[3] For modified bitumen membranes it lists ASTM D6162, D6163, D6164, D6222, D6223 or D6298.[3]
A residential example
Most modified bitumen and built-up roofs are designed by a contractor or manufacturer for the project. For small residential roofs there are packaged systems: GAF Liberty is a 2-ply system: self-adhering base/ply sheet plus cap sheet, self-adhering, without torches, open flames or hot asphalt, rated UL 2218A Class 4 (complete 2-ply system), with an up to 20-year limited warranty.[4]
Cost
We did not find a neutral, published installed-cost comparison for built-up and modified bitumen roofs, so this page does not show one.
Common questions
What does a built-up roof have on top?
NRCA lists mineral aggregate, bituminous materials, a liquid-applied coating or a granule-surfaced cap sheet.[1]
What is the minimum slope for a built-up roof?
1/4:12 in the residential code, or 1/8:12 for coal-tar built-up roofs.[3]
Is modified bitumen a single-ply roof?
It is usually installed in more than one ply. The GAF Liberty system we checked is a 2-ply system: self-adhering base/ply sheet plus cap sheet.[4]
Sources
- National Roofing Contractors Association (NRCA) Glossary: Built-up roof (BUR). Accessed November 3, 2026.
- National Roofing Contractors Association (NRCA) Glossary: Polymer-modified bitumen. Accessed November 3, 2026.
- California Building Standards Commission 2025 California Residential Code, Section R905 Requirements for Roof Coverings (effective January 1, 2026). Accessed November 3, 2026.
- GAF LIBERTY Self-Adhering Cap Sheet sell sheet (RESLB117) (RESLB117-PDF-0526). Accessed November 3, 2026.
- National Roofing Contractors Association (NRCA) Glossary: Cap sheet. Accessed November 3, 2026.
We did not install or test these products. Everything on this page comes from the documents listed above. If a source changes, numbers here may be out of date until our next check. Spot an error? Email [email protected].