What is roof decking, and what is dry rot?
The deck, or sheathing, is the layer of plywood, OSB or boards the roofing is nailed to. Dry rot is fungal decay in wood that has stayed damp; it leaves sheathing soft, crumbly or delaminated. Spaced sheathing, also called skip sheathing, is boards laid with gaps, common under old wood shake roofs.
What does the code require under a new roof?
Under CRC R905.2.1, asphalt shingles must be fastened to wood structural panels or solid lumber sheathing. Spaced boards get covered or replaced, and rotted wood must go, because new roofing can't be fastened securely to it.
What are the signs of decking problems before tear-off?
- Sagging between rafters seen from the street
- Soft or spongy spots underfoot (a roofer's observation)
- Dark stains or daylight on the underside of the deck in the attic
- Long-running leaks at the same spot
How is decking priced in a contract?
The contract should include a per-sheet or per-square-foot decking price and say how replaced sheets will be documented. Change orders must be in writing under B&P 7159.6. For your actual price, request a free written estimate: a licensed roofer measures the roof and prices your exact scope.
Where does decking rot show up in San Francisco?
San Francisco's housing is among the oldest in the Bay Area, and its roof decks show it. Pre-war homes often have board sheathing, sometimes spaced, under several old layers, and flat roofs behind parapets trap water at clogged drains and wall flashings, which is where rot concentrates. Fog keeps west-side roofs damp for long stretches. One city-specific step: replacing sheathing needs an over-the-counter building permit from DBI, separate from the online reroofing permit, so ask your roofer how they handle it if rot turns up.
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Get a free replacement quoteFrequently asked questions
What is skip sheathing?
Spaced boards with gaps between them, typical under old wood shake roofs. Shingles need a solid deck, so it's covered or replaced.
Plywood or OSB for a new roof deck?
Both are used. Ask which the roofer proposes and its thickness, and make sure it's named on the bid.
Do I need another permit if my San Francisco roofer replaces decking?
Replacing sheathing needs an over-the-counter building permit from DBI. Ask the roofer to include that process in the estimate.
Sources
- California Residential Code R905 (roof coverings)
- Business and Professions Code 7159.6 (change orders)
- California Residential Code R908 (reroofing)
- SF.gov: apply for a reroofing permit
Sources reviewed November 12, 2026. Figures are published estimates and official fees, not quotes. Confirm prices, permits and requirements with your contractor and building department. This page is general information, not legal or insurance advice.