Rotten roof decking in Marin: what happens at tear-off, and what does it add?

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?

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?

• 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?

Protect yourself with a stated decking unit price, photo documentation of replaced sheets, and a written change order for anything beyond the contract, as California law requires (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 Marin?

Marin's combination of shade, redwood and oak litter and winter rain makes it one of the Bay Area's harder places on roof decks. Needles pack valleys and gutters, moss lifts shingles, and water sits where the deck can't dry. Older homes in Mill Valley, the Ross Valley and San Anselmo may also have spaced boards under an old shake roof. Expect your roofer to plan for some sheathing replacement on shaded slopes and to price it per sheet in the contract.

Related: what drives roof replacement cost, roof size estimator, repair or replace, quote checker.