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Modified bitumen roofing

Modified bitumen is asphalt roll roofing blended with a polymer, either SBS (a rubber) or APP (a plastic), and reinforced with a polyester or glass-fiber mat. It goes down on low-slope roofs in two or more plies: a base sheet, sometimes an interply sheet, and a cap sheet usually surfaced with mineral granules.

Warranty available
Up to 30 years from some manufacturers
Common build
2 or 3 plies; cap sheets about 150 to 165 mil
Seams
Lapped, sealed with heat, hot asphalt or adhesive
Surface
Mineral granules, or a coating on smooth sheets
Attachment
Torched, mopped, cold-adhered or self-adhered

How it's installed

The deck and the people in the building usually decide the method. Torching is ruled out over wood and other combustible decks, and hot asphalt and adhesives give off fumes, so many jobs mix methods, such as cold-applied flashings beside a torched field.

Torch-applied

The crew heats the underside of the roll and the lap below until the bitumen flows, then presses the sheet in. APP sheets go on this way or in cold adhesive, many SBS sheets are made for it, and the federal guide spec calls for a fire watch until at least two hours after torching ends.

Hot-mopped asphalt

Each SBS ply is rolled into hot asphalt mopped on just ahead of the roll. The asphalt kettle brings fumes and fire risk, so occupied buildings may need an odor control plan.

Cold-applied adhesive

Plies are set in an asphalt or polymer adhesive spread by squeegee or sprayer, with no flame or kettle, so it suits combustible decks. The adhesive needs time to cure: keep traffic off, and Elevate asks for 30 days before a third ply goes on.

Self-adhered

Self-adhered sheets come with factory-applied adhesive under a peel-off film, so there is no heat, kettle or wet adhesive. They need dry weather and warm rolls (JM lists at least 60°F), and some base sheets go over a primer.

Pros and cons

Where modified bitumen wins

  • Polyester-reinforced sheets hold up to foot traffic and punctures on busy roofs.
  • Flame-free methods let it go on wood decks and odor-sensitive buildings such as schools and hospitals.
  • A choice of chemistry: SBS stays flexible in cold weather, while APP resists heat and UV.
  • It bonds to asphalt, so manufacturers list it for repairing or re-covering sound, smooth asphalt roofs.

Where it falls short

  • Torch and hot-asphalt work carry fire risk and fumes: expect a fire watch, and an odor plan on occupied buildings.
  • Oils, grease and solvents attack the sheet, so keep kitchen exhaust and equipment leaks off it.
  • Standing water strips granules and cracks the surface, and warranties can exclude ponding damage.
  • Standard white granules reflect far less sunlight than cool-roof granules: JM lists 0.28 initial reflectance for a standard white cap and 0.70 for its cool-roof cap.

Modified bitumen roofs and hail

FM Approvals rates whole roof assemblies under FM 4470 for moderate (MH), severe (SH) or very severe hail (VSH); the VSH test fires 2-inch ice balls at 53 to 58 foot-pounds. FM Global's Data Sheet 1-34, written for the buildings it insures, maps those zones, and NRCA's 2018 summary of it says that in the VSH zone FM Global recommends modified bitumen roofs with a VSH rating. NRCA cautions that lab impact tests give a relative index, not a promise about real storms.

For modified bitumen, look at the reinforcement, the plies and the board underneath. The federal guide spec calls for polyester-reinforced sheets where impact resistance is needed, Tremco requires a cover board under its insulated systems partly to resist hail, and Elevate's hail coverage needs at least two adhered plies over an approved board. JM's standard guarantee excludes hail, but JM and Elevate both sell hail coverage by hailstone size, so ask what a quote includes.

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Sources

  1. National Institute of Standards and Technology, NISTIR 4972: Observations From a Field Study of the Performance of Polymer-Modified Bitumen Roofing (1993)
  2. USACE / NAVFAC / AFCEC (Whole Building Design Guide), UFGS 07 52 00 Modified Bituminous Membrane Roofing (August 2025)
  3. Johns Manville, DynaLastic 180 FR Data Sheet (RS-4107)
  4. Johns Manville, DynaGrip Cap Data Sheet (RS-4706)
  5. The Garland Company, Garland High-Performance Membranes brochure
  6. Elevate (Amrize), Asphalt Product Guide (9/2025)
  7. Elevate (Amrize), APP 160 Technical Information Sheet (TIS 401)
  8. Johns Manville, Bituminous Product Reference Guide
  9. Johns Manville, DynaWeld Cap 180 FR Data Sheet (RS-4707)
  10. Elevate (Amrize), QuickSpec QS-SBS-543: SBS Cool Applied System, Recover
  11. Tremco Roofing and Building Maintenance, Modified Bitumen roofing systems (POWERply)
  12. The Garland Company, StressPly / FR Mineral Modified Bitumen Membrane
  13. Johns Manville, Understanding Your JM Peak Advantage Guarantee (RS-7396)
  14. Johns Manville, DynaGlas FR CR G Data Sheet (RS-4946)
  15. NRCA, Professional Roofing, Understanding FM VSH (Mark S. Graham, December 2017)
  16. NRCA, Professional Roofing, Designing for hail resistance (Mark S. Graham, May 2018)
  17. Tremco Roofing and Building Maintenance, POWERply Installation Manual (Rev. 12/18)
  18. Elevate (Amrize), Technical Bulletin: New and Expanded Hail Warranty Coverage Options (December 2025)
  19. Johns Manville, JM Peak Advantage Guarantee Rider Charges, All Systems (RS-1691)

Updated October 2026.