When should roof sheathing be replaced, and how is it done?
Short answer. Replace roof sheathing when it is rotted, delaminated, swollen, cracked or brittle, when it sags between framing, when nails pull out easily, or when it is spaced boards that cannot properly support shingles. Damaged sections are cut out back to the centers of the rafters and replaced with panels of the same thickness, nailed on the required schedule.
Problems that call for replacement
- Rot. Soft, dark, crumbly wood from long-term leaks, usually near chimneys, valleys, eaves and pipes.
- Delamination. Plywood layers separating, often from repeated wetting.
- Swelling. OSB edges that have absorbed water and puffed up, creating ridges under the shingles.
- Sagging. Panels dipping between rafters, from undersized panels, missing clips or moisture.
- Poor nail holding. Wood too degraded to grip new fasteners.
- Spaced boards. Gapped boards from an old wood shake roof need solid sheathing over or in place of them for shingles.
- Brittle treated plywood. Some fire-retardant-treated plywood used near party walls in townhomes built in the 1980s has darkened and turned brittle with heat over the years.
How a section is replaced
- Mark the damaged area out to the nearest rafter centers on each side.
- Set a circular saw to the sheathing depth so it does not cut the rafters, and cut along the lines.
- Pull the old section and remove leftover nails.
- Check the rafters for rot and repair them if needed.
- Cut a new panel of the same thickness to fit, leaving a small expansion gap.
- Nail it to each rafter on the required schedule, with panel clips where edges fall between framing.
When to do it
Almost always during a roof replacement, after tear-off, when the deck is exposed. Ask for the per-sheet price in writing and photos of each replaced area. If you are choosing a replacement panel, read OSB vs plywood roof decking.