Porch and low-slope roofs
Walk any block of twins in Upper Darby, Norristown or Lansdale and you will see the same shape repeated: a shallow main roof, and a nearly flat porch roof across the front. That porch is a different roof, built of different material, aging on a different clock, and it is where these houses leak.
- 1 Ridge caps. Thin, fully exposed, and usually the first course to let go.
- 2 The field. Slate on the older rows, shingle on the rest, each on its own clock.
- 3 Valleys. Two slopes emptying through one line, with litter holding the water there.
- 4 Flashing. Old masonry, and the party wall a rowhouse shares with the house next door.
- 5 The vent boot. A small rubber part with a far shorter life than the roof it sits in.
- 6 Eaves. Where the porch roof ties into the wall, and where backed-up water gets in.
Why a low slope is a different animal
Shingles work by shedding water downhill fast; below about a 3-in-12 pitch they stop working, because water moves slowly enough to find its way under the courses. A porch roof therefore needs a membrane, rolled roofing, modified bitumen, or single-ply, installed as a continuous surface rather than as overlapping pieces. When someone puts ordinary shingle on a porch because it matches the main roof, they have built a leak with a schedule.
The second problem is the junction. The porch roof meets the front wall of the house, and that wall continues upward to the main roof. Water shedding off the main roof arrives at the porch, and the flashing where the porch meets the wall is what stands between it and the front bedroom. On houses of this age that flashing has often been re-sealed rather than replaced, sometimes several times.
What to ask
When you gather quotes for a house with a porch, ask three questions before comparing any numbers. Is the porch roof included at all, or is the quote only for the main field? What material is specified for it, and why? And is the flashing where the porch meets the wall being replaced or reused? A bid noticeably below the others on a block of twins has, in most cases, simply left the porch out.