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Roof Decking: Plywood vs. OSB, Thickness, and When It Gets Replaced

  • Published November 7, 2026
  • Also under Replacing your roof step by step

Your roof deck is the layer of plywood, OSB or boards that the shingles are nailed to. During a re-roof it is replaced sheet by sheet where tear-off exposes rot, delamination or wood that will no longer hold nails, unless the whole deck is too thin or too open for the new covering. Because nobody sees the whole deck until the old roof is off, the number that protects you is the price per sheet and proof of every sheet replaced, not a guessed total.

Plywood versus OSB is a smaller decision than it looks. What decides the outcome is the condition of what you have, its thickness for the rafter spacing, and how the contract counts replacements.

Plywood, OSB, or Boards: Identifying Your Deck From the Attic

If the insulation you see from the hatch is loose, pebble-like vermiculite, EPA’s page on vermiculite attic insulation advises treating it as possibly containing asbestos and leaving it undisturbed, so read the deck’s underside from the hatch only and let the roofer judge the rest at tear-off. Otherwise, go into the attic with a flashlight, step only on the framing, and look up at the underside of the roof between the rafters or trusses. Three things are possible, and each looks different.

  • Plywood shows a wood-grain face and, at the panel edges, thin layers of veneer glued in alternating directions.
  • OSB (oriented strand board) shows large flat wood strands pressed together in a flaky pattern.
  • Board decks are individual planks running across the rafters, sometimes tight together and sometimes with gaps between them.

Panels carry a grade stamp, which you can read from the attic if it faces down. APA, the Engineered Wood Association, says in its guide to roof construction that “the span rating in the trademark applies when the long panel dimension or strength axis is across supports unless the strength axis is otherwise identified.” In plain terms, panels should run with their long side across the rafters, and the span rating printed in the stamp is the figure APA’s roof table matches to support spacing.

If you find boards with gaps, ask the roofer whether the new covering needs solid sheathing installed over them. The answer depends on the shingle maker’s installation instructions and your local code, so ask which of the two the roofer is following.

For the order in which the deck, underlayment and shingles go on, and the names of the other layers, see where each layer sits, from deck to ridge.

Thickness: Why Older Decks Can Be Too Thin

FEMA’s Wind Retrofit Guide for Residential Buildings (FEMA P-804, Second Edition, April 2023) observes that “many existing roof deck panels are less than 15/32-inch thick.” FEMA raises the point in a wind context. A thin panel gives each nail less wood to grip and flexes more between supports.

The right thickness depends on how far apart the rafters or trusses are. APA’s roof table, for supports 24 inches apart, lists a panel with a 24/16 span rating (7/16 Performance Category) as working at that spacing with or without edge support, while a 24/0 panel (3/8 Performance Category) needs edge support, such as panel clips, to span 24 inches. When clips are required, APA calls for one per span, and two for spans of 48 inches or longer.

That table is organized by span rating and Performance Category, so the choice between plywood and OSB for replacement sheets matters less than the rating and thickness. If you prefer one panel type, name it in the quote’s material line.

From the attic, measure the rafter or truss spacing center to center and note any stamp you can read. Then ask the roofer two things in writing: what thickness and rating the replacement sheets will be, and whether they will match the existing deck so the roof surface stays flat where old and new panels meet.

Any code minimum thickness depends on your adopted code and the panel rating, so ask the building department whether it sets one for re-roofs. Voluntary programs can set their own floor: the IBHS re-roofing checklist for the 2025 FORTIFIED Home Standard requires “7/16 in. minimum deck thickness and a maximum rafter spacing of 24 in. o.c.” when re-roofing over an existing deck.

Rot and Damage Signs You Can Check Before Tear-Off

An attic check before you sign tells you whether deck replacement is likely to be a few sheets or a large share of the roof. Do it on a dry day, step only on framing or a walkway, and follow the attic flashlight checks for safe access.

Look for:

  • Dark stains, water lines or rings on the underside of the deck, especially around chimneys, vent pipes, skylights and valleys.
  • Soft or crumbling spots; press gently with a screwdriver from below, and treat any spot where it sinks in as suspect.
  • Plywood layers separating at the edges, or OSB swollen and fuzzy at its seams.
  • Daylight through the deck or between boards.
  • Rusty nail tips, frost in winter, or mold on the sheathing.
  • Panels that dip between rafters.

Read the results by pattern. A stain around one vent pipe points to a local repair and a sheet or two. Widespread delamination or soft wood across a whole slope points to a larger replacement, and that is worth knowing before you compare quotes.

A dip that runs along a rafter or the ridge is different from a soft panel between supports. Deck sag can be fixed with new sheathing; sag in the framing cannot, and rafter or ridge sag explains how to tell them apart.

How Deck Replacement Shows Up in a Quote

The FTC’s consumer guide How To Avoid a Home Improvement Scam expects materials and price to be in the written estimate. For decking, that means the unknown quantity still needs a known price and a known rule.

A usable deck line item has five parts:

  1. The unit: per full sheet, per square foot, or, for board decks, per linear foot.
  2. The unit price, in dollars, for each unit.
  3. The replacement criteria: what condition makes a sheet count as damaged.
  4. The material: plywood or OSB, thickness and rating, plus how it will be nailed.
  5. The proof: dated photos of damaged areas before covering and a replaced-unit count on the final invoice.

Hypothetical example: if the quote states P dollars per sheet, the base price includes I sheets, and the crew replaces N sheets, the deck charge is P times (N minus I). With no included sheets, it is P times N. The only open variable is N, and the photos and invoice count verify it.

Watch for the line that says “decking as needed” with no unit price. That leaves both the count and the price open. The clause that controls this in the contract is covered in the deck allowance clause, and comparing allowances shows how to line up quotes that handle decking differently.

For new sheets, APA’s guide recommends 1/8-inch spacing “at all edge and end joints unless otherwise indicated by panel manufacturer,” which lets panels move with moisture without buckling. Its nailing table calls for 8d nails at 6 inches along supported panel edges and 12 inches at intermediate supports. Both details can be checked in photos of the new sheets taken before underlayment covers them, so add those to your photo request.

When the Re-Roof Is the Time to Upgrade the Deck

A tear-off is the one time the deck is fully exposed, so upgrades that live at deck level are easiest to add then. FEMA P-804 notes that a roof with five or more years of remaining life “is a candidate for retrofit options that do not require roof covering replacement.” The deck-level options are the ones that do require it, so they belong with the re-roof.

Sealing the deck is the main one. IBHS states that “FORTIFIED requires roof decks to be sealed to prevent this type of water damage” in its FORTIFIED Roof overview, and DOE’s Building America guide on roof deck sheathing and sealing for sloped roofs describes the three methods FORTIFIED allows, including taping the panel joints with self-adhering flashing tape and covering the deck with two layers of 30# felt. In hurricane-prone regions, FEMA’s guide includes deck sealing in its retrofit packages.

Sealed-deck details and deck nailing schedules for high wind are a separate specification; see deck sealing and nailing for high wind if you live where that applies.

Should You Approve Deck Replacement Sight Unseen?

Approve the price and the rule in advance, not the quantity. Sign only when the quote states the deck unit, the unit price, the material and thickness, and the condition that triggers replacement. Add a cap in writing: the crew calls you with photos before replacing more than a number of sheets you choose.

If your attic check found widespread damage, ask the roofer to estimate the likely sheet count in writing before you sign, so the final bill has a reference point. If the attic looked sound, a small included allowance plus a unit price for anything beyond it is reasonable.

Do the attic check first, then hold the quote to the unit, the unit price and the replacement criteria, and ask for dated photos and a replaced-sheet count on the final invoice.

This guide is general information for U.S. homeowners, not legal, insurance, tax, or engineering advice. Codes, laws, and policy terms vary by location and change over time; confirm details with your local building department, your insurer, or a licensed professional.

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