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What is modified bitumen roofing?

Asphalt that has been taught to stretch: the rolled, reinforced membrane that took over from built-up roofing on a great many low-slope roofs.

Updated September 12, 2026 · Every figure links to its source · Sources

DefinitionSystemAlso called: mod bit, modified bitumen membrane, torch-down roofing

Modified bitumen — a roof membrane manufactured from asphalt modified with polymers — atactic polypropylene (APP) or styrene-butadiene-styrene (SBS) — reinforced with a polyester or fibreglass mat and supplied in rolls, installed in one or more plies on low-slope roofs; the Asphalt Roofing Manufacturers Association covers the product class, and the 2021 IRC, Section R905.11 regulates it.

What it is

Plain asphalt is brittle in cold and soft in heat. Blending it with a polymer changes that: SBS, a synthetic rubber, makes the sheet elastic and forgiving at low temperatures; APP, a plastic, makes it tougher and more heat-resistant. The modified asphalt is coated onto a reinforcing mat, surfaced with mineral granules or a foil, and rolled up in the factory, so that most of the assembly that a built-up roof does on site has already been done. On the roof the sheet is bonded down by torch (APP), hot asphalt or cold adhesive (SBS), or by a self-adhering backing, usually as a base sheet plus a granulated cap sheet.

The IRC, Section R905.11 requires a design slope of at least one-quarter unit in twelve for drainage and lists the ASTM standards for each sheet type; the IBC, Section 1507.11 does the same for commercial buildings. The ARMA technical bulletins cover installation practice.

Why it matters

Modified bitumen kept asphalt's virtues — mass, redundancy in a two-ply system, a granulated surface that resists foot traffic and is easy to patch — and dropped the kettle and the gravel. It is the common choice for porch roofs, small commercial buildings and the flat parts of houses, and it is the system a homeowner is most likely to have quoted as "torch-down". Its risks are the torch (a fire hazard that has burned buildings down, and that many contractors now avoid in favour of cold-applied or self-adhered sheets), seams that must be rolled and lapped correctly, and, as with any low-slope roof, the flashings and drains.

Against single-ply it is heavier, darker unless a reflective cap sheet is chosen, and more tolerant of a less-than-perfect substrate. The directory's guide to flat and low-slope roofing compares it with TPO, EPDM and built-up roofing.

What it costs and who does it

Priced by the square, with the number of plies, the attachment method, the insulation, penetrations and flashing, and the tear-off as the variables; self-adhered systems trade a higher material cost for less labour and no flame. Roofing contractors who do low-slope work install it, and torch application should be done only by a crew with the training and fire watch it requires. The directory's flat-roof replacement cost guide explains how the estimate is built.

Frequently asked questions

Short answers; the sections above carry the detail and the sources.

What is the difference between APP and SBS modified bitumen?
APP (atactic polypropylene) is a plastic modifier that makes the sheet tough and heat-resistant and is usually torch-applied; SBS (styrene-butadiene-styrene) is a rubber modifier that makes the sheet elastic in cold and is applied with hot asphalt, cold adhesive or a self-adhering backing.
Is torch-down roofing safe?
It is a recognised method, but the open flame has caused building fires, and it needs a trained crew, a fire watch after the work and care near walls and vents. Many contractors now prefer cold-applied or self-adhered modified bitumen for that reason.
How many layers does a modified bitumen roof have?
Usually two — a base sheet and a granulated cap sheet — sometimes with an additional ply. Single-ply modified bitumen exists but gives up the redundancy that is the system’s main advantage.

See also

Next step

Find a roofer who does this

Ask a contractor which application method they use and why; the answer says a lot. The directory lists roofing contractors by state and city.

Find a modified bitumen roofing contractor

Sources

  1. Asphalt Roofing Manufacturers Association — ARMA
  2. 2021 International Residential Code, Chapter 9: Roof Assemblies — International Code Council (ICC)
  3. 2021 International Building Code, Chapter 15: Roof Assemblies and Rooftop Structures — International Code Council (ICC)
  4. Technical bulletins on asphalt roofing — Asphalt Roofing Manufacturers Association (ARMA)