Modified bitumen
Two plies of asphalt, and the redundancy that goes with them
Modified bitumen is the descendant of built-up roofing: asphalt sheets, reinforced and modified with polymer, installed in two or more plies. On a small, complicated roof with a lot of walls and penetrations it is often the most practical thing to install and the easiest thing to maintain.
Where it is the right choice
- Small, cut-up roofs with parapets, changes of level, and details every few feet — the kind of roof where rolling a large single-ply sheet is mostly cutting it up again.
- Roofs that get walked regularly, because the granulated surface is far more tolerant of boots and dropped tools.
- Buildings where redundancy is worth paying for, since a two-ply system is two layers of waterproofing rather than one.
- Low-slope sections on residential and mixed-use buildings, where it has been the standard for a long time — see low-slope residential.
- Recovers over an existing asphalt roof, where compatibility with what is already there is genuinely simpler.
How it is applied, and why that matters to you
| Method | What it involves | Building considerations |
|---|---|---|
| Self-adhered | Peel-and-stick plies, no flame, no kettle | Least disruptive; usually the right answer on occupied and sensitive buildings |
| Cold-applied adhesive | Plies set in adhesive rather than heat | No flame; adhesive odor can migrate into fresh-air intakes and needs planning |
| Torch-applied | The ply is heated and rolled into place | Open flame on the roof; requires a fire watch and is inappropriate on some buildings |
| Hot asphalt | Traditional mopping from a kettle | Odor and logistics; increasingly uncommon on occupied sites |
Application method is not just a contractor preference. Flame and odor decide whether a roof can be installed over an occupied medical suite, a school in session, or a restaurant with fresh-air intakes on the roof. It belongs in the scope discussion early.
What to watch on an existing mod-bit roof
- Granule loss in the field, particularly in the drainage paths and under equipment.
- Open laps and fishmouths at the edges of sheets, which is where water gets between plies.
- Blisters, which are trapped moisture or air between plies — worth assessing rather than automatically cutting open.
- Base flashings at parapets and curbs pulling away, which is the most common leak source on this kind of roof.
- Alligatoring on an older surface, which tells you the asphalt has aged and the surfacing is due attention.
Have a small, complicated flat roof that has been patched repeatedly?
Talk to us about the buildingIs modified bitumen obsolete?
No. It has lost ground to single-ply on large simple roofs where welded seams and reflectivity win, and it remains the sensible choice on small, detailed roofs and on anything that gets walked.
Is a torch-applied roof a fire risk?
Open flame on a building is a real risk and it is managed with procedure: a fire watch during and after the work, protection of combustible substrates, and simply not using torch application where the building does not suit it. Self-adhered systems avoid the question entirely.
Can it be repaired?
Readily, and that is one of its strengths. Compatible repairs to a mod-bit roof are straightforward and durable, which is why it suits buildings that expect to maintain a roof rather than replace it.
How does it handle ponding?
Better than some systems and no system handles it well. Standing water finds laps and terminations eventually. If a roof ponds, the drainage is the problem to solve, not the membrane.
All work is performed by licensed and insured local roofing professionals.