Pittsburgh is a city of row houses and narrow attached homes, and a lot of them have a roof you can't see from the street. Behind the front wall or cornice, the roof is flat or slopes gently toward the back, where it drains into a gutter or through a scupper to a downspout. These roofs work well when they are maintained. When they aren't, they leak at the same few places on almost every street.
How a typical row-house roof works
The front of the house usually rises into a parapet or a decorative cornice that hides the roof. The roof itself slopes slightly from front to back, sometimes with a lower flat roof over a rear kitchen or ell. The side walls are party walls shared with the neighbors, sometimes rising above the roof as a small parapet and sometimes not, in which case one roof runs into the next. Water is supposed to run to the back and leave through a gutter, a scupper in the rear wall, or both.
The four places row-house roofs leak
1. The front parapet
The roof membrane turns up the back of the front wall and is covered by flashing and coping. Old mortar, loose coping stones and cracked flashing let water into the top of the wall, where it can run down inside the brick and show up on the top-floor front wall.
2. The party walls
Where your roof meets the neighbor's, there is either a parapet with flashing on both sides or a seam joining two roofs. Different materials, different ages and different roofers meet here, and it is a common entry point. Chimneys often sit on the party wall too, adding another flashing.
3. The rear edge and drainage
Rear gutters fill with leaves and grit from the roof, scuppers rust or clog, and downspouts come loose. Once water can't leave, it backs up under the roof edge or over the top of the flashing.
4. Low spots
Old joists sag, especially where a past leak softened them, and water collects in the low area. Ponding water wears the membrane faster and finds every weak seam. See ponding water on a flat roof.
Roof leaking right now?
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The layers problem
Many row-house roofs have been re-covered rather than replaced: rubber over roll roofing over tar and gravel. Each layer raises the roof, shortens the flashing height at the parapets, and holds water between the layers when it leaks. Building codes generally don't allow a new roof over two or more existing layers, so most of these roofs need a full tear-off when it is time. That is also when soft decking gets replaced and the slope gets corrected.
Which material for a row-house roof
EPDM rubber and cold-applied or self-adhered modified bitumen are the common choices. EPDM can often cover the whole roof in one sheet. Open-flame torch-applied roofing is a fire risk next to wood cornices and attached neighbors, so many roofers avoid it on row houses. White TPO or a white-surfaced modified bitumen helps if the top floor gets hot in summer. See EPDM vs. TPO vs. modified bitumen.
Working with the neighbors
- Tell them first. Work at the party wall can involve their roof edge, and crews may need to step onto it.
- Get photos of the shared seam. If the failure is on their side, photos make that conversation easier.
- Consider doing both roofs. If both are old, one crew replacing both gives a continuous roof with no joint to fail.
Access and permits
Row houses on narrow, steep streets take more planning: ladder or lift placement, hauling old roofing down three stories, and a dumpster or truck on the street, which may need a city right-of-way permit. A contractor who works in the city regularly will build that into the plan and the price.
The bottom line
Check the parapet, the party walls, the rear drainage and the low spots, in that order. Clear the rear gutter every fall. When the roof is finished, tear it off, fix the decking and slope, and re-flash every edge. See flat roof repair and flat roof replacement.