Most Philadelphia rowhouse roofs look flat but slope toward a drain, with a minimum fall of a quarter inch for every foot. Water leaves through a drain and parapet scupper, or along a rear gutter. The usual failure points are parapet flashing, drain and scupper seams, and standing water.
Key takeaways
- The city's homeowner guidance gives a minimum fall of a quarter inch for every foot on rowhouse roofs.
- Water leaves by a drain that may pass the cornice or parapet to a downspout, or along a gutter at the rear.
- Parapet flashing, drain and scupper seams, and ponding are the usual places a leak starts.
- At a party-wall parapet, the membrane termination should be sealed so water does not reach the next house.
- A roof walk after heavy rain shows ponding before it becomes a leak.
Why does a rowhouse roof look flat but still need to drain?
A rowhouse roof is flat only to the eye. The city's homeowner guidance says it is sloped toward a drain, at a minimum of a quarter inch of drop for every foot, so that water runs to one low point instead of standing.
A roof that is truly level holds water wherever it is low. The slope is what keeps the surface draining, and it is also why a small dip near the outlet can become a pond.
Checking the slope is part of any inspection. A technician looks at where the water stands after rain and follows its path back to the outlet.
Where does the water go?
Rowhouse roofs drain in one of two ways: through a roof drain that may pass through the cornice or parapet in a scupper and join a downspout on the outside, or along a gutter that runs across the facade or, more often, the rear.
The path tells a roofer where to look. A drain that passes through the parapet has two seams to check, the one at the drain body and the one where the scupper meets the wall. A rear gutter has joints along its length and at its outlet.
Confirm the path before anything is changed. A repair at the wrong outlet does not stop the leak.
What usually fails on a flat rowhouse roof?
Contractor and inspector guidance points to parapet flashing, drain and scupper seams, and ponding water as the usual failure points. Older flat roofs were often built-up tar and gravel, and replacements are generally modified bitumen, EPDM or TPO membranes.
These failure points are described as general practice rather than from one authority, so treat them as a checklist. Each one is a joint, and each joint has its own repair.
Membranes are generally repaired at the seam or the termination, not by covering the whole field. A roofer will say whether the membrane is still sound enough to repair or whether it has reached the point where replacement is the better course.
| Location | What a roofer checks | Why it matters |
|---|---|---|
| Parapet flashing | Caps, flashing lines and the membrane turned up the wall | Water that passes the parapet runs into the joint and then the building |
| Drain and scupper seams | Outlet, bowl, seam at the scupper and the downspout | A blocked or open seam sends water to the wall |
| Ponding | Edge of standing water after rain and the path to the outlet | Standing water finds the weakest joint |
| Party-wall termination | Membrane edge at the shared parapet | Water can move into the neighbor's house if the edge is open |
How common is this roof type in Philadelphia?
Multi-floor row homes make up about 70 percent of the city's housing stock, by one estimate from a Philadelphia architecture-and-design body. Roughly four in ten homes date from before 1940, by an older Census-based analysis.
Those figures are estimates from different sources and different years, so they show the scale of the rowhouse stock rather than the age of any particular roof.
Whether a given rowhouse roof is original or has been replaced is a question for the roof itself and the building's records, not for a statistic.
What does a roof visit look like?
A visit walks the surface, clears the drains and outlets, marks any ponding and checks the parapet, the scupper and the party-wall edge. The roofer then explains what was found and what each repair would involve.
The first thing done is clearing the outlet, because a blocked drain can hide other problems. The roofer then looks at each joint and marks any standing water so you can see whether it is spreading.
Ask for the findings in writing with photographs. That record is useful for the repair decision and for any insurance claim later.
Frequently asked questions
How much slope does a rowhouse roof need?
The city's homeowner guidance gives a minimum fall of a quarter inch for every foot toward the drain. A roofer checks the slope by looking at where water stands and following it back to the outlet.
Why does water come through the ceiling at the front of the house when the roof drains to the back?
Water can run along the joists and the party wall, so a stain may appear well away from the entry point. A technician traces the path from the outlet and the joints to find where the water actually gets in.
Is a rear gutter better than a roof drain?
Neither is better in general. The right answer depends on the building and on how the water is carried away. The roofer checks the existing path and the joints before recommending a change.
How often should a rowhouse roof be checked?
After heavy rain and before winter are the most useful times. A roof walk then finds ponding and blocked outlets while they are still small problems. Call (833) 751-2102 to arrange a visit.
Sources
This guide is general information for Philadelphia County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.