Modified Bitumen Roofs for Philadelphia Rowhomes
Modified bitumen is the classic Philadelphia rowhome roof, usually finished in silver. It takes foot traffic well and holds up to the city's weather when the laps are sealed properly and the roof drains.
Modified bitumen is the standard membrane on Philadelphia rowhome flat roofs, with a typical 15 to 20 year service life. Published local figures put it at about $3.50 to $4 per square foot installed. It goes down as a base sheet and a cap sheet, either torched or self-adhered, and is usually finished with a reflective coating or granules.
How a Mod Bit Roof Is Built
Modified bitumen is asphalt reinforced with polymers and a fabric mat, sold in rolls about a meter wide. A two-ply system puts a base sheet down first and a cap sheet over it, with laps offset so no seam sits directly on another.
It is thicker and tougher underfoot than single-ply membranes, which suits roofs people walk on to reach a deck, a hatch or a condenser. The tradeoff is more seams than a wide EPDM or TPO sheet, so lap work decides how long it lasts. Modified bitumen systems from CertainTeed, Johns Manville and Polyglass are among those we install.
What Sets the Price of a Mod Bit Roof
- Tear-offOld layers have to come off before a new two-ply system.
- Torch or self-adheredPeel-and-stick materials cost more but need no flame.
- FinishSmooth coated and granulated caps price differently.
- DrainageTapered board or drain resets to stop ponding.
- Edges and wallsParapet flashing, coping and party-wall seams.
A Philadelphia rowhome roofing source puts installed modified bitumen at $3.50 to $4 per square foot. That is a market planning range, not our quote.
Why Torch-Down Is Not a Weekend Project
Rolls and torches are sold to anyone. The risk is fire, not just leaks.
What a homeowner can safely do
- Keep scuppers and drains clear of leaves
- Recoat a sound silver roof on a dry day
- Watch for blisters and granule loss
Where it needs a roofer
- Any open flame on a roof over living space
- Laps and seams, which decide the roof's life
- Work beside a neighbor's roof or wood trim
Torch-Down Modified Bitumen
Torch-down rolls have a layer of asphalt on the underside that is melted with a propane torch as the roll is laid, fusing it to the sheet below. Done well, the laps flow together into a solid bond. Done carelessly, the torch can scorch wood at parapets or ignite debris, which is why torch work calls for an extinguisher on the roof, no open flame at combustible edges, and a fire watch after the last torch goes out. GAF, Polyglass and Tamko make the systems we install most.
Self-Adhered Modified Bitumen
Self-adhered, or peel-and-stick, modified bitumen bonds without flame. The release film comes off and the sheet is rolled into place. It is the safer choice next to wood trim, under windows, and against a neighbor's roof, and it works well on small rowhome roofs and additions. It needs a clean, primed surface and warmer temperatures to bond fully to the sheet below, so late-fall and winter jobs are scheduled with the forecast in mind or switched to torch-down where it is safe.
Granulated Cap Sheet Replacement
The top sheet of a mod bit roof is either smooth and coated or covered in mineral granules that protect the asphalt from sunlight. When granules wash off and the black asphalt shows, the cap is drying out and starting to crack. If the base sheet is still sound and dry, a new cap sheet can be installed over it, a smaller job than a full replacement. A core cut tells us whether the base underneath is good enough to keep.
Brands we install and service
Silver roof cracking and blistering?
Call and we tell you whether it needs a cap sheet or a full replacement.
What We See on Philadelphia Roofs
Modified bitumen is the default rowhome membrane across Philadelphia, typically lasting 15 to 20 years, so many blocks have neighbors on roofs of different ages sharing one party wall. Water under a failing mod bit roof next door can reach your side through that joint. When we replace a mod bit roof we seal the party-wall edge on our side and tell you if the neighbor's roof looks likely to send water across.
How a Mod Bit Roof Gets Installed
- 1
Inspect and core
Layers counted, deck checked, drainage noted.
- 2
Choose the system
Torch-down or self-adhered, coated or granulated.
- 3
Tear-off and deck
Old roof removed and soft boards replaced.
- 4
Base sheet
Laid and bonded with laps offset from the cap.
- 5
Cap sheet and flashing
Cap applied, parapets and pipes flashed.
- 6
Fire watch and cleanup
Roof checked after torching, debris cleared.
Modified Bitumen Questions
The aluminum coating reflects sun and protects smooth mod bit from drying out. Recoating a sound roof every few years extends its life.
In trained hands with fire precautions, yes. Near wood or a neighbor's roof we switch to self-adhered sheets.
Only if the base underneath is dry and sound. A core cut tells us.
Mod bit handles foot traffic better; EPDM has fewer seams. Both suit a well-drained roof.
When the coating chalks off or bare black shows through. Check it every couple of years.
More Replacement & Installation Work in Philadelphia
From our guides: TPO, EPDM or Modified Bitumen for a Rowhome Roof?
Get a Mod Bit Roof Built the Philadelphia Way
Call to have the old roof cored and checked. We tell you whether it needs a cap sheet or a new roof.