What is roof decking, and what is dry rot?
Sheathing is the structural layer under the roofing: plywood or OSB on most newer homes, boards on many older ones. Rot sets in when wood stays wet long enough for fungi to break it down. Spaced (skip) sheathing is gapped boards, which suited ventilated shake roofs.
What does the code require under a new roof?
The code is clear for shingles: they go on wood structural panels or solid lumber sheathing (CRC R905.2.1). That means a shake roof on spaced boards usually gets new sheathing, and any soft or delaminated sheets get cut out and replaced so fasteners hold.
What are the signs of decking problems before tear-off?
- A wavy or sagging roofline
- Water stains in the attic or on ceilings
- Moss or debris that has sat for years
- Old leaks that were patched more than once
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 Mateo County?
On the Peninsula, the fog belt is where decking rot shows up: Daly City, Pacifica, South San Francisco and San Bruno roofs stay damp much of the summer, and moss holds water against the deck. Post-war tracts from the 1950s and 60s sometimes have spaced boards left from an original shake or early roof. Low-slope mid-century roofs rot at drains and edges where water sits. Bayside cities from San Mateo to Redwood City are drier, and rot there tends to follow old leaks and failed flashing.
Related: what drives roof replacement cost, roof size estimator, repair or replace, quote checker.
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Get a free replacement quoteFrequently asked questions
Is rotten decking covered by insurance?
Rot from long-term moisture is generally treated as wear or maintenance, not a sudden covered loss. Read your policy and ask your insurer.
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.
Why do Daly City and Pacifica roofs have more rotten decking?
Fog and moss keep roofs damp for long periods, and wood that stays wet decays. Expect the roofer to probe soft spots and price replacement per sheet.
Sources
- California Residential Code R905 (roof coverings)
- Business and Professions Code 7159.6 (change orders)
- California Residential Code R908 (reroofing)
- CEC: climate zones
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.