Why roofs fail
Flat roofs over three-deckers, parapets that have to drain sideways, porch stacks running back into the wall. Very little around here is a simple slope.
- 1 Ridge caps. The thinnest pieces up there, and the first to work loose over a hard winter.
- 2 The field. Slate on the older stock, shingle on the rest. They age on very different clocks.
- 3 Valleys. Leaf litter mats in them, and two slopes then drain through one line.
- 4 Flashing. Old masonry chimneys, and the porch roof that meets the wall three stories up.
- 5 The vent boot. Rubber that hardens and splits well ahead of the covering it sits in.
- 6 Eaves and ice dams. Where meltwater refreezes at the cold edge and backs up under the courses.
The six
What Boston stock does to a covering
Half the housing in these towns has no pitch to speak of: a three-decker carries a flat or nearly flat roof, and water leaves it only where a scupper or a drain lets it. Where the roof does pitch, the cold edge grows ice while the warm ridge melts snow. The first metal the salt reaches is the flashing, not the covering. Under a century-old roof the boards were laid with gaps, sized for slate, not for shingle. And behind almost every house a stack of porch roofs meets the wall three times. Five of the six are somewhere two things meet.