Plumb Cut vs. Heel Cut
The two rafter angles: the plumb cut matches the pitch angle, the heel cut is its 90-degree complement, plus the bevel when meeting an existing roof.
Run Roof Sheathing the Right Way Across the Rafters or the Sheet Will Deflect Between Them
OSB is engineered with a strength axis: the long dimension of the sheet has to run perpendicular to the rafters or joists it spans. Lay it the other way and the sheet flexes underfoot between supports even with an H-clip in the seam, and the fix is blocking the seam from below.
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The Problem
A sheet of OSB looks the same both ways round. It isn't. Oriented strand board is built from layers of wood strands laid down in alternating directions, and the outer layers run along the length of the sheet. That's what the "oriented" means, and it's why the sheet is stiffer along its 8-foot dimension than across its 4-foot one.
Cut and lay a sheet the wrong way round on a roof and you get a deck that deflects underfoot between the rafters. It is not a failure you can see from a distance — it shows up as a soft spot when you walk it, and it's still there after the shingles go on.
The Technique
The long dimension of the sheet runs perpendicular to the framing it spans. On a roof, that means the 8-foot length crosses the rafters, with the sheet bearing on several of them rather than running parallel with two. Same rule on a floor deck and on wall sheathing: the strength axis crosses the supports.
The rest follows from that:
- Stagger the end joints between courses so seams don't line up in a continuous run.
- H-clips in the unsupported edge seams keep adjacent sheets level with each other. What an H-clip does not do is stiffen a sheet laid the wrong way round — the sheet in this video deflected with a clip in place.
- If a sheet has already gone down backwards, the remedy is to get underneath and block the seam: a length of timber between the rafters under the joint, giving the edge the support the sheet's own stiffness should have provided.
Plywood is more forgiving but not exempt. Its plies alternate at right angles, so it's closer to equal in both directions — but its face grain still runs the length of the sheet, and it's still rated to span with the long dimension across supports.
Pro tip: Every structural sheet is stamped with a span rating on its face — a pair of numbers like 32/16, meaning the maximum support spacing in inches for roof use and for floor use. That rating assumes the sheet is laid with its strength axis across the supports. Laid the other way it isn't a lower rating, it's an untested one.
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