Modified bitumen roofing is a flat roof system built from factory-made asphalt sheets that are blended with a polymer (SBS rubber or APP plastic) and reinforced with fiberglass or polyester, laid in two or more plies with a surfaced cap sheet on top. It is allowed on roofs sloped as little as 1/4 inch per foot, handles foot traffic well, and is repaired with straightforward, well-known methods. For a modified bitumen inspection or estimate, call NEMA Roofing at 866-631-3366.
What is modified bitumen made of?
Plain roofing asphalt gets brittle in the cold and soft in the heat. Adding a polymer fixes both: the U.S. Department of Defense roofing criteria (UFC 3-110-03) note that the modifiers raise low-temperature flexibility and high-temperature stability, and that the sheets are reinforced with fiberglass, polyester or both. The two families behave differently:
| SBS (styrene butadiene styrene) | APP (atactic polypropylene) | |
|---|---|---|
| Character | Elastomeric: rubbery and stretchy | Plastomeric: stiffer, plastic-like |
| How it is applied | Hot-mopped asphalt, cold adhesive, torch, or self-adhered sheets | Typically torch-applied |
| Typical use | Roofs that move or flex; base and cap plies of multi-ply systems | Cap sheets and torch-applied systems |
A modified bitumen roof (roofers often shorten it to “mod bit roofing”) is a layered system, not one sheet: a base sheet fastened or bonded to the deck (or over insulation), one or more interply sheets on some systems, and a cap sheet whose top surface faces the weather. The 2025 California Residential Code lists the ASTM standards the membrane must meet (D6162, D6163, D6164, D6222, D6223 and D6298) and requires installation to the manufacturer’s instructions (R905.11). The DoD criteria call for at least two plies.
How is modified bitumen installed?
- Torch-applied: the underside of the sheet is heated with a propane torch until it bonds. Fast, but it puts an open flame on the roof; the DoD criteria prohibit it over combustible decks and require a two-hour fire watch after the last torch goes out. On wood-framed houses, ask how the crew will handle edges, parapets and wall framing near the flame, or choose a flame-free method (see our guide to the best flat roof material for a house).
- Hot-mopped: sheets are set in hot asphalt, a traditional method for SBS.
- Cold-applied: sheets are set in a solvent- or water-based adhesive, no heat.
- Self-adhered: the sheet’s sticky underside bonds when its release film is pulled, no heat; seams are pressed or heat-welded depending on the product.
Whatever the method, the finished roof must meet the wind rule for the covering: the residential code requires modified bitumen to be tested under FM 4474, UL 580 or UL 1897 and installed to resist the design wind loads (R905.11.4).
Which cap sheet surface should you choose?
The cap sheet’s surface decides how the roof handles sun, which is the main thing that ages it in Southern California. The DoD criteria describe three common surfaces:
- Granule-surfaced: ceramic granules shade the asphalt and cut the heat it absorbs. As granules wear off, ultraviolet light starts breaking down the exposed bitumen, so bare patches are the first thing to watch.
- Metal foil-faced: used for solar reflectivity or better fire resistance.
- Smooth, factory-coated: the criteria do not allow uncoated smooth sheets; the factory coating supplies the heat reflection and UV protection.
Color matters on a replacement. Where California’s cool-roof rule for low-sloped roofs applies, a standard dark granule cap will not meet it; a white granule cap sheet with a rated reflectance, or a rated coating over the cap, is the usual answer. Which roofs the rule covers is explained in our guide to low slope roofing.
How does modified bitumen compare with single-ply membranes?
| Modified bitumen | TPO or PVC | EPDM | |
|---|---|---|---|
| Build | Two or more reinforced plies | One sheet, seams hot-air welded | One rubber sheet, seams taped |
| Foot traffic and punctures | Thick multi-ply build takes traffic well | Needs walkway pads on service routes | Needs walkway pads; soft surface |
| Surface | Granules, foil or factory coating | White or light membrane | Usually black |
| Grease and oil | Check the product | PVC is often chosen near kitchen exhaust | DoD criteria: not resistant to grease and oil |
| Repairs | Patch plies with compatible sheet; recoat when surface ages | Weld a patch of the same membrane | Tape or adhere a patch |
The DoD criteria describe modified bitumen systems as having low maintenance cost and say they must be considered when long service life is required. How many years each system typically gives is covered in our flat roof lifespan guide.
What wears out a modified bitumen roof?
- Granule loss from sun, foot traffic and water flowing to drains, leaving bitumen exposed to UV.
- Blisters where air or moisture was trapped between plies and expands in the heat.
- Open laps at seams and at the edges of patches, where the bond was weak or has aged.
- Ponding that keeps the same area wet and speeds surface wear.
- Cracks at flashings where the roof meets walls and curbs and the materials move differently.
What each of these means for your roof, and how urgent it is, is in our list of common flat roof problems.
How is a modified bitumen roof maintained and repaired?
- Keep drains and scuppers clear and note where water stands after rain.
- Walk the laps and flashings once or twice a year and after storms; mark open seams and blisters.
- Repair open laps and damaged spots with compatible modified bitumen sheet and the method the system uses, not with tube sealant.
- When the cap sheet surface is worn but the roof is dry and sound, a compatible coating can extend its life; California’s codes permit a new coating over modified bitumen without tear-off.
- When the roof reaches the end of its life, the codes also allow one recover over a dry single-layer roof; if the roof is wet or already has two layers, it has to come off.
Modified bitumen from NEMA Roofing
We work on flat roofs of every common type, including modified bitumen, EPDM and TPO, and we repair leaks, blisters and drainage problems on them. NEMA Roofing is a licensed, bonded and insured roofing contractor (CSLB license #1083283) and a Polyglass Preferred Contractor. Our flat roofing service covers installation, repair and replacement. Five NEMA locations (Santa Barbara, Oxnard, Woodland Hills, Tarzana and Downey) let us serve all of Santa Barbara, Ventura, Los Angeles, Riverside and San Bernardino counties; the Ventura County page lists towns there. For a modified bitumen estimate, call 866-631-3366.
