The house was cut up, and the roof shows it
A great deal of the housing here started as one household and ended as two or three. Every one of those conversions added a stair, a dormer, a bathroom or a back entrance, and every one of those put something through a roof that was originally plain.
- 1 The ridge. One ridge over two households, and one of them pays to replace it.
- 2 The field. The part everyone looks at, and rarely the part that actually failed.
- 3 Valleys. Most of them were cut in later, when the house was divided into flats.
- 4 The chimney. Often two of them, and often neither one carries anything any more.
- 5 Penetrations. Two kitchens and two bathrooms mean twice the holes on the same roof.
- 6 The overhang. Where heat from a flat you do not control comes back down as ice.
What a conversion does to a roof
A second stair needs headroom, so a dormer goes in. A dormer is two more valleys, two cheek walls and a small roof of its own, cut into a plane that used to be continuous.
A second kitchen and bathroom need their own drains, so more vent stacks come through the covering. A second heating system needs its own flue. None of these were in the original design, and all of them were fitted to what was already there.
Why valleys are where it shows
A valley carries everything two slopes shed down a single line, several times the water of the field around it. In this climate it also holds snow long after the open slope has cleared, particularly on a north face.
A valley created by a later dormer is frequently the weakest one on the roof: fitted rather than designed, sometimes stopped short into a wall instead of run out onto the roof below, and often flashed by whoever framed the dormer rather than by a roofer.
What a replacement should correct rather than repeat
A tear-off is the one moment when all of this is open and can be put right. Step flashing along the dormer cheeks replaced rather than re-used. Valleys lined and run out properly. Penetrations grouped and re-flashed rather than sealed where they sit.
If a quote says the dormers will be re-flashed as found, ask what "as found" means. It usually means the same detail that failed.