Flashing on attached houses
On a detached house, a roof is a private matter. On a row or a twin, which is most of the older housing in this region, the covering runs to the property line and past it, and the single most failure-prone detail on the whole roof is the joint where your house meets the one attached to it.
- 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 the junction fails
Where two attached roofs meet, there is either a shared parapet, a low wall, or simply a seam, and all three depend on metal flashing and sealant to stay watertight. That joint moves: the two houses settle at slightly different rates, the metal expands and contracts through every mid-Atlantic freeze-thaw cycle, and the sealant hardens with age. When it opens, the water does not respect the deed. It enters at the seam and shows up in whichever house has the lower ceiling or the more open wall cavity.
This is why a stain on an upstairs wall of an attached house is a flashing story far more often than a field story, and why the neighbor’s roof condition is genuinely your business.
Replacing one side of a pair
It happens constantly and it can be done well, but it has to be planned. The new covering has to tie into the neighbor’s existing one, which means new flashing at the seam and a decision about how far onto the adjoining roof the work extends.
Some contractors handle this as a matter of routine; others quote the field and improvise at the wall. Ask directly how the tie-in is being handled and whether it is in the price. And if the neighbor is anywhere near their own replacement, doing both at once is cheaper and produces a better joint than doing them five years apart.