Spaced sheathing under an old roof
On a great many houses in this region, the boards under the covering are not a continuous deck. They are individual planks with gaps between them, spaced sheathing, laid that way on purpose a century ago, and modern shingle cannot be nailed to it.
- 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.
Why the gaps are there
Slate and wood shingle were fastened to battens or spaced boards, and the gaps served a purpose: they let the underside of the covering breathe and dry. It was sound practice for those materials. It becomes a problem the day someone wants to put down asphalt shingle, which needs continuous support and a nail that holds where it lands, not one that happens to find air.
So the roof has to be sheeted over first: a layer of plywood or oriented strand board fastened across the old planks to create a continuous deck. That is material, labor and disposal weight, and it is a real number rather than an incidental.
The line that goes missing
A quote produced from the driveway cannot know the sheathing is spaced, because the only way to know is to look: from the attic, or by lifting a course. So the lightest bid on an older house is frequently the one that has not discovered the condition yet, and the difference surfaces once the roof is open and the crew is standing on it.
The way to neutralise it is to ask directly: what is the per-sheet decking price, and what happens if the sheathing turns out to be spaced? Two questions, asked before signing, that turn the largest unknown on an old roof into a rate you agreed to in advance.