It is not actually flat
A properly built low-slope roof has a slight built-in fall so water runs to a drain or scupper rather than sitting. The City of Philadelphia's own rowhouse guidance puts the practical minimum at about a quarter inch of fall per foot. Most leaks on these roofs trace back to that fall having been lost somewhere — a sagging deck, a blocked drain, a patch that created a low spot — rather than a fault in the membrane itself.
The systems actually used here
- EPDM rubber — a single large sheet with few seams. Long-established, straightforward to repair, the usual choice for a rear addition or porch roof.
- Polyglass SBS modified bitumen — a torch- or self-adhered granulated system. Common on rowhome roofs and where a tougher, walkable surface matters.
- Roll roofing — the budget option, and honestly a shorter-term one. Offered where the budget requires it, with no pretense about what it is.
- Protective coatings — where an existing membrane is sound but tired, a silver or white coating adds UV and heat protection and buys real time. The City of Philadelphia's own guidance is explicit that a coating is maintenance, not a repair for an active leak — worth knowing, because some operators sell recoats as fixes.
What actually fails on these roofs
- Seams and laps opening as the membrane ages and the building moves.
- The joint where the flat section meets a parapet, a wall, or the main pitched roof — more failures start here than in the open field of the membrane.
- Drains and scuppers blocked with debris, so water backs up behind the flashing line.
- On a shared party wall between two rowhomes, flashing that was never carried up properly, which can create a leak that shows up on one side and not the other.
Why this needs its own trade, not a shingle mindset
Asphalt shingle technique does not transfer to a flat roof. The building code does not permit shingles at all below a 2:12 slope, and requires doubled underlayment between 2:12 and 4:12 — which is exactly why a low roof needs a membrane system and a different skill set, not a lighter version of the same job as the pitched roof above it.