Modified bitumen roofing for low-slope roofs
Modified bitumen is a rolled roofing for flat and low-slope roofs. It comes in two main kinds, and how it is put down matters as much as what you buy.
Shingles need a steep roof so water runs off fast. Flat and nearly flat roofs need a sheet that keeps water out even when it sits. Modified bitumen, often called "mod bit", is one of those sheets.
Bitumen is the asphalt base. "Modified" means a plastic or rubber is mixed in, so the sheet stays flexible in cold and stable in heat. It comes in rolls, often with a mineral top so it stands up to sun.
SBS and APP: the two main kinds
| Kind | What is added | How it is usually put down |
|---|---|---|
| SBS | A rubber (styrene-butadiene-styrene). The sheet is stretchy, like rubber. | Self-adhered (peel and stick), cold adhesive, hot asphalt, or torch. |
| APP | A plastic (atactic polypropylene). The sheet is stiffer. | Mostly torch-applied: the back is heated with a flame and pressed down. |
Many jobs use two or more layers: a base sheet and a mineral-surfaced cap sheet.
Torch-down vs self-adhered
A torch-down roof is put down with an open flame on your roof. Done well, the seams fuse into one piece. It also puts fire next to wood, so the crew needs training and a fire watch. Some cities and insurers limit torch work on homes.
Self-adhered and cold-adhesive sheets need no flame. They are a common choice on homes, porches and additions for that reason.
What the building code asks for
The International Residential Code covers modified bitumen in section R905.11. The roof needs a slope of at least 1/4 inch per foot so water drains. The material must meet set ASTM standards: D6162, D6163 or D6164 for SBS, and D6222 or D6223 for APP. Your local building department decides which code edition applies.
What to ask a modified bitumen installer
Questions to ask
- Do you install low-slope roofs often? Flat roofs are a different skill from shingles. Ask for addresses of recent mod bit jobs.
- SBS or APP, and how many layers? The quote should name the product, the layers, and the method.
- How will water drain? Ponding water is the main enemy of a low-slope roof. Ask about drains, scuppers and slope.
- Will you pull the permit? A re-roof usually needs one. The licensed contractor should pull it.
- What warranty do I get, and from whom? Ask for the maker warranty and the labor warranty in writing. They are not the same thing.
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International Code Council, 2021 International Residential Code, Chapter 9, section R905.11 (modified bitumen roofing): codes.iccsafe.org.
Questions homeowners actually ask
What is modified bitumen roofing?
It is a rolled asphalt roofing with a rubber or plastic mixed in. It is made for flat and low-slope roofs.
Is modified bitumen the same as torch-down roofing?
Torch-down is one way to install it. Modified bitumen can also be self-adhered, put down with cold adhesive, or set in hot asphalt.
Can modified bitumen go on a steep roof?
It is made for low slopes. On a steep roof, shingles or metal are the usual choice.