Roofing materials
Modified bitumen roofs for Philadelphia flat roofs
Modified bitumen is the modern, factory-made descendant of the old tar roof: heavy rolls of reinforced asphalt with a gritty mineral surface. It's tough underfoot and familiar to many Philly roofers. The big decision is how it's applied, with a torch or without one.
The material
What "mod bit" is
Modified bitumen is asphalt blended with rubber-like or plastic polymers, so it stays flexible in cold weather and resists softening in summer heat. It comes in rolls about three feet wide, reinforced with polyester or fiberglass, and usually topped with ceramic granules that shield it from the sun.
Most roofs are built as two plies: a base sheet fastened or bonded to the roof, then a granulated cap sheet on top. The rolls overlap at every seam, and those laps are fused or bonded together. Cap sheets come in dark colors and in white granules that reflect more heat.
A well-installed modified bitumen roof typically lasts 15 to 20 years, and longer with a reflective coating applied later in its life.
Signs a mod bit roof needs work
On an existing torch-down or peel-and-stick roof, watch for:
- Bald patches where the granules have washed away
- Laps lifting or opening at the seams
- Blisters or bubbles in the cap sheet
- Cracking where the roof turns up a wall
- Granules piling up in the gutter or downspout
- Soft spots underfoot near a seam or drain
Application methods
Torch-down vs self-adhered
The material is similar. The way it's stuck down is not, and on attached houses that matters.
| Compared | Torch-down | Self-adhered (peel-and-stick) |
|---|---|---|
| How it bonds | A propane torch melts the back of the roll as it's unrolled | A release film peels off a sticky back, then it's rolled firmly in place |
| Open flame | Yes, on the roof and at the walls | None |
| Weather | Works in cooler weather, since the torch supplies the heat | Bonds best in mild, dry weather |
| Seams | Fused by melting the overlap | Pressed together, sometimes with heat or adhesive at laps |
| Where it's used | Open field areas on experienced crews | Whole roofs, or wall and edge details on torch jobs |
Fire safety
Torching on attached houses: what to ask
A Philly rowhome roof sits against wood: the boards themselves, wood blocking inside parapet walls, old cornices, and your neighbors' roofs a few inches away. Heat from a torch can travel into a hidden cavity and smolder there after the crew has moved on. That's why many careful roofers avoid putting a flame near walls, edges and openings, and use self-adhered sheets for those details even on a torch-down job.
Torch work is regulated as hot work, and reputable crews follow fire-safety rules: a fire extinguisher within reach, no torching directly on wood, and a fire watch, staying on the roof for a period after the torches are shut off to check for hidden smoldering.
Before a torch-down job, ask: Will you torch at the walls or use self-adhered flashing there? Who stays for the fire watch, and for how long? Does your insurance cover torch work? A good contractor answers all three without hesitating.
The work
What installing a modified bitumen roof involves
Tear off & prep
Old roofs come off, bad boards are replaced, and insulation or a cover board goes down where needed.
Base sheet
The first ply is fastened to the boards or bonded to the insulation, overlapping at every seam.
Cap sheet
The granulated top ply is torched or self-adhered over the base, with laps staggered so seams don't line up.
Details & check
Walls, curbs and pipes get two-ply flashing, edges get metal, and every lap is checked. Torch jobs end with a fire watch.
Planning your job
What affects the price
For a modified bitumen roof, the main factors are:
- How it's applied. Torch-down, self-adhered, or a mix of both.
- Number of plies. A two-ply system uses more material than a single cap sheet.
- Cap sheet color. White, reflective granules versus standard dark ones.
- What's underneath. Old layers to remove and boards to replace.
- Walls and openings. Party walls, hatches, skylights and vents each need two-ply flashing.
- Access and setup. Heavy rolls going up three stories, and fire-safety precautions for torch work.
Check what your roof likely needs with the Roof Problem Checker.
Related pages
- EPDM rubber roofs: glued, no flame, few seams
- TPO roofing: white and heat-welded
- Silver coat roof replacement: moving off an old tar roof
- Flat roof materials compared: all three membranes side by side
- Fishtown & River Wards flat roof repair: decks and busy rooftops
Modified bitumen questions
Is torch-down safe on a rowhome?
It can be, in experienced hands with proper precautions. The risk is heat reaching hidden wood at walls and edges. Ask whether the crew will use self-adhered flashing at the walls, and whether they'll stay for a fire watch after torching.
Is peel-and-stick as good as torch-down?
Self-adhered modified bitumen performs well when it's installed in suitable weather on a clean, dry surface and firmly rolled. Many roofers like it on attached houses because it removes the flame. Torch-down can be the better fit in cooler weather.
How is this different from my old tar roof?
An old built-up roof was assembled on the roof from felt and hot tar, then painted silver. Modified bitumen is made in a factory with polymers that keep it flexible, and arrives as consistent rolls with a granule surface, so it doesn't need repainting every few years.
Can it handle foot traffic or a roof deck?
It's one of the tougher flat roof surfaces underfoot, which is why it's often chosen where people walk to a deck, condenser or hatch. Walkway pads and protection under deck supports still help it last.
Have a different question about modified bitumen? Call and ask a local roofer.
(215) 702-6130