Field guide

Roof Decking Rot: When a Tear-Off Reveals Plywood That Has to Go

Rotted decking has to come out, because shingles cannot hold a nail in spongy wood and the code requires roofing over a solid, nailable surface. Expect to pay roughly $85 to $100 per sheet replaced, charged only for the sheets actually swapped, and expect the final count to be unknown until tear-off exposes the deck.

The deck is invisible until it is not

The one thing a roofer cannot tell you before the job starts is how much decking needs replacing. The deck is fully visible only after tear-off, when the shingles and underlayment are off and the wood is exposed. Fort Worth's roofing FAQ puts the rule plainly: sheathing in good condition does not need to be replaced, and the code does not allow a new layer of decking over an existing one. So bad sheets come out and new sheets go in at the same plane, and the count is whatever the tear-off reveals.

That uncertainty is the single biggest source of distrust in a reroof quote, and it is mostly unavoidable. A contractor who prices the wood before tear-off is guessing, and the guess is usually wrong in the direction that favors the contractor. The honest contract prices decking per sheet, charged only for sheets actually replaced, and states the per-sheet rate up front. In northeast Florida that typically runs $85 to $100 a sheet, with a bundle discount of about $5 to $10 a sheet when the whole roof gets redecked.

What makes a sheet come out

The test is physical. A sheet comes out when it is soft or spongy underfoot, sagging between rafters, delaminated with plies separating, swollen and crumbling at the edges, stained dark and weak from a long-term leak, or thinner than the shingle manufacturer's minimum. OSB and plywood fail differently: OSB absorbs moisture slowly but struggles to dry once saturated, so its edges swell and push shingles upward, while plywood absorbs fast and dries fast if ventilated, but its layers separate when it stays wet.

The code reason is structural. The International Residential Code, Chapter 9, requires all roofing materials to be applied to a solid, securely fastened, nailable surface, and rotted wood cannot hold a roofing nail. Owens Corning requires every shingle fastener to penetrate at least three quarters of an inch into the deck, or completely through it by at least a quarter inch. Drive a nail into spongy OSB or delaminated plywood and the fastener eventually backs out or pulls through in a high wind. The Asphalt Roofing Manufacturers Association is equally blunt: new shingles over warped or rotted decking voids the manufacturer's warranty on the spot.

The right way to handle the surprise

A good crew stops when it finds rot. The standard protocol is an immediate pause in the affected zone, photographic documentation, and a cut that reaches solid, dry wood on all sides before the new sheet goes in. New panels go down in a staggered pattern, flush and securely fastened, with a small gap, typically an eighth of an inch, between them for thermal expansion. The APA minimum schedule for roof sheathing is 8d nails at six inches on center along supported panel edges and twelve inches in the field, with heavier schedules in high-wind zones.

Two traps to watch for. First, the overlay temptation: some crews offer to lay new plywood over the old deck to save the tear-off. The code prohibits it, because layering sheathing prevents inspection of the framing underneath and traps moisture between the layers. Second, the thickness rule: the minimum is typically half an inch or five eighths, and some jurisdictions set it higher. Universal City's re-roofing bulletin requires damaged sheathing to be replaced with a minimum of five-eighths decking. If your contract does not name the replacement thickness, ask.

What to put in the contract before the first shingle comes off

Three lines protect you. The per-sheet replacement rate, the replacement panel thickness and material, and the rule that decking is photographed before it is covered. A contractor who resists any of those is telling you how the surprise will be handled. The deck is the foundation of the roof in the most literal sense, and a reroof that leaves rotted sheets in place is a new roof on a failing foundation. The tear-off is the one moment anyone can see it. Use it.

Frequently asked questions

How much does roof decking replacement cost?

Most contractors charge per sheet, only for sheets actually replaced. In northeast Florida the typical range is $85 to $100 per sheet of plywood, with a bundle discount of about $5 to $10 a sheet for a full redeck. The final count is unknown until tear-off exposes the deck.

Can you put new plywood over old rotted decking?

No. Building codes prohibit installing new sheathing over existing sheathing, because it prevents inspection of the framing and traps moisture between the layers. Rotted sheets must be cut out and replaced at the same plane.

How do I know if my roof decking is rotted?

The reliable signs are soft or spongy spots underfoot, sagging between rafters, delaminated or swollen plywood edges, and dark staining at eaves, valleys, or around penetrations. The full picture only appears after tear-off.

Does rotted decking void the shingle warranty?

Yes. Shingle manufacturers require installation over a solid, nailable surface, and the Asphalt Roofing Manufacturers Association states that installing over rotted decking voids the warranty. The code requires the same standard in IRC Chapter 9.