Philadelphia is a rowhouse city in the Census Bureau's own numbers: of 351,905 owner-occupied homes in Philadelphia County, 269,897, or 76.7%, are single-family attached houses, the category that holds rowhomes and twins (ACS 2020–2024). The roof most of those owners replace is not the steep shingled roof of a suburban house.
A flat roof that is not flat
The City's Rowhouse Manual describes it this way:
Most rowhouse roofs appear "flat", although they are actually gently sloped toward a drain. A slope of 1/4" vertical height over a distance of 12" is the minimum for water to move.
The building code says the same about every low-slope membrane. Built-up, modified bitumen, thermoset (EPDM) and thermoplastic (TPO, PVC) single-ply roofs all need a design slope of at least one-quarter inch per foot, a 2% slope (IRC 2021 R905.9.1, R905.11.1, R905.12.1, R905.13.1). A replacement is the one time the slope can be corrected: low spots where water stands are built up with tapered insulation or new framing before the membrane goes down.
Water leaves the roof one of two ways, the manual explains. At the front, and sometimes at the rear, it runs to a roof drain that may pass through the cornice or parapet in a scupper and into the downspout. At the rear it often runs into a gutter along the facade. Either way it must end up on your own property: the manual notes that it is illegal to send storm water onto the sidewalk, the street or a neighbor's lot.
Parapets and party walls
What makes a rowhouse roof its own job is the edges. On a rowhome the roof meets a parapet at the front, often at the rear, and the neighbors' roofs along both party walls. A new membrane is turned up the parapet and over or under its cap, and the seams with the next roofs have to be closed so water cannot run between the houses. Ask each contractor how they will handle:
- the parapet: how high the membrane runs, and whether the coping or cap comes off;
- the party walls: whether the neighbors' roofs are tied in, and how;
- the drain, scupper or gutter, and the downspout connection;
- anything on the roof: a hatch, skylight, vent stacks, a condenser or a roof deck.
Who owns which part of a shared parapet, and what a neighbor has to agree to, depends on the deeds and is not something this site can answer. A contractor will ask about the roofs next door, because a new roof tied into a failing one is a weak spot.
Which membrane
Most rowhouse replacements use one of four systems; the flat roof systems guide compares them. The Rowhouse Manual names two:
Modified bitumen is a low tech, lower cost, reliable roofing system. Rolls of flexible material are adhered to the roof and to one another, and the joints are sealed.
EPDM (rubber) and TPO are the other common choices; built-up roofs with gravel are the older system. HUD's useful-life table, used to plan capital spending, gives EPDM and TPO 15 years and a built-up roof 20 years with gravel, 10 without. A silver or white surface, the manual adds, protects the materials from sunlight and reduces heat in the rooms below.
Tear-off, sheathing and insulation
The City's EZ re-roofing standard requires every existing layer to come off first where the existing roof is water-damaged or deteriorated, where it has two or more coverings, or where it is slate, clay, cement or asbestos-cement tile. On an old rowhouse that has been covered more than once, the job is a full tear-off to the deck. See tear-off or roof-over.
Once the deck is open, damaged boards are found. The standard says planned replacement of more than 10% of the roof sheathing is outside the EZ process, and that "if damaged sheathing is discovered during tear-off inspector must be notified." The contractor's certificate at the end (form TP_019_F) asks the same question: "Less than 10% of sheathing was replaced: Yes / No."
If insulation is changed, the standard requires it to meet the 2021 energy code, R-49 or more. The Rowhouse Manual points out that some rowhouses "have almost flat roofs and no attics," and that a new roof is the moment rigid insulation can go between the deck and the new covering.
The permit
The contractor files an EZ permit for a roof covering replacement: $69 on a one- or two-family house, plus a $3 City and a $4.50 state surcharge, with the manufacturer's installation instructions attached. Adding skylights, moving rooftop equipment or switching to a ballasted system takes the job out of the EZ process. Details on the permits page; if the house is historic, read historic districts first.
How many rowhouse roofs are replaced
The City issued 2,334 EZ roof covering replacement permits in 2025, in 46 ZIP codes, from 3 in 19127 to 191 in 19143. Each ZIP code page shows its own count by year next to the share of rowhomes there.