TPO vs Modified Bitumen
Both are allowed on roofs as flat as 1/4:12 (2 percent).[1] TPO is a single-ply membrane: one layer, with heat-welded seams.[2, 3] Modified bitumen is asphalt modified with polymers and is usually laid in more than one ply; the residential system we checked, GAF Liberty, is a 2-ply system: self-adhering base/ply sheet plus cap sheet.[4, 5] The white TPO we checked has a Solar Reflectance Index of 101 initial, 92 after 3 years, while the Liberty cap sheet is granule-surfaced, with a polyester mat.[3, 4]
TPO
Pick TPO if you want a white, reflective roof and one layer with heat-welded seams.[3]
Modified bitumen
Pick modified bitumen if you want a granule-surfaced, multi-ply roof, or a peel-and-stick system for a small residential roof such as a porch or garage.[4]
At a glance
| Feature | TPO | Modified bitumen | Source |
|---|---|---|---|
| What it is | Single-ply membrane: field-applied as just one layer of membrane rather than multiple layers | Polymer-modified 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 | [2, 5] |
| Material standard in the code | ASTM D6878 | ASTM D6162, D6163, D6164, D6222, D6223 or D6298 | [1] |
| Minimum slope in the code | 1/4:12 (2 percent) | 1/4:12 | [1] |
| Layers (products we checked) | One sheet (GAF EverGuard TPO, 45, 60 and 80 mil) | GAF Liberty: 2-ply system: self-adhering base/ply sheet plus cap sheet | [3, 4] |
| How seams are made | Heat-welded | GAF Liberty: self-adhering, without torches, open flames or hot asphalt | [3, 4] |
| Top surface | White sheet, Solar Reflectance Index 101 initial, 92 after 3 years | Liberty cap sheet: granule-surfaced, with a polyester mat | [3, 4] |
| Impact and fire (products we checked) | Not compared: depends on the full assembly | Liberty: UL 2218A Class 4 (complete 2-ply system); fire Class C (UL 790) | [4] |
| Longest warranty or guarantee mentioned | GAF EverGuard TPO: up to 20 years (45 mil), 25 years (60 mil) and 30 years (80 mil) | GAF Liberty: up to 20-year limited warranty | [3, 4] |
One ply or several
The basic difference is how the waterproofing layer is built. A single-ply membrane is field-applied as just one layer of membrane rather than multiple layers.[2] Modified bitumen comes in 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, and its top layer is often a cap sheet, which NRCA describes as a sheet, often granule-surfaced, used as the top ply of some built-up or modified bitumen membranes.[5, 6]
In the residential modified bitumen system we checked, GAF Liberty, the roof is a 2-ply system: self-adhering base/ply sheet plus cap sheet. GAF says the system may be appropriate for roofs of 2,500 square feet or less.[4]
Installation and seams
TPO sheets are joined by heat; Carlisle says its TPO is heat-welded, usually with an automatic heat welder.[3, 7] Modified bitumen can be installed several ways; the Liberty sheets are self-adhering, without torches, open flames or hot asphalt.[4]
Reflectance and surface
White GAF EverGuard TPO is listed with a Solar Reflectance Index of 101 initial, 92 after 3 years and a solar reflectance of 0.81 initial, 0.75 after 3 years.[3] The Liberty cap sheet is granule-surfaced, with a polyester mat; its data sheet does not list a reflectance value.[4]
Slope range
The code allows both from 1/4:12 (2 percent) up.[1] GAF lists Liberty for slopes of 1/4:12 to 3:12.[4]
Cost
We did not find a neutral, published installed-cost comparison for TPO and modified bitumen, so this page does not show one.
Common questions
Is modified bitumen the same as built-up roofing?
No. Built-up roofing is a continuous, semi-flexible membrane of multiple plies of felts, fabrics or mats, laid in place with alternate layers of bitumen. Modified bitumen uses polymer-modified sheets, usually fewer of them.[5, 8]
Which standards apply?
TPO follows ASTM D6878. Modified bitumen follows ASTM D6162, D6163, D6164, D6222, D6223 or D6298.[1]
Can I install modified bitumen without a torch?
Some systems are self-adhering. GAF Liberty is self-adhering, without torches, open flames or hot asphalt.[4]
Sources
- California Building Standards Commission 2025 California Residential Code, Section R905 Requirements for Roof Coverings (effective January 1, 2026). Accessed November 3, 2026.
- National Roofing Contractors Association (NRCA) Glossary: Single-ply membranes. Accessed November 3, 2026.
- GAF EverGuard TPO 45, 60 and 80 mil data sheet (COMEG876) (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: Polymer-modified bitumen. Accessed November 3, 2026.
- National Roofing Contractors Association (NRCA) Glossary: Cap sheet. Accessed November 3, 2026.
- Carlisle SynTec Systems Sure-Weld TPO Reinforced Membrane Product Data Sheet (reprint 600515 TPO-2012) (January 28, 2026). Accessed November 3, 2026.
- National Roofing Contractors Association (NRCA) Glossary: Built-up roof (BUR). 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].