Home / Roof Replacement / Roof Decking & Sheathing Replacement
Every roofing estimate you receive contains one number that is a guess. Not the shingles — those are counted. Not the labor — that is scheduled. The guess is the decking, because until the old roof comes off, the sheathing underneath is hidden from everyone, including the contractor writing the quote.
This is why re-roof projects go over budget, why homeowners feel ambushed on day two, and why the same house can receive quotes that differ by six thousand dollars. It is also, in Seattle specifically, the most consequential technical variable on the entire job, because our housing stock and our climate combine to produce decking problems at a rate well above the national norm.
This article explains what is actually under your shingles, how it fails here, how to assess it yourself before anyone quotes, and — most usefully — how to structure an estimate so the decking is not a blank check.
The roof deck, or sheathing, is the continuous structural surface fastened across your rafters or trusses. Everything else — underlayment, shingles, flashing — is attached to it. It carries live loads, transfers wind uplift into the frame, ties the structure together laterally, and provides the nail holding on which every shingle depends.
That last function is the one that matters for material selection, and it comes down to a specific requirement: roofing fasteners must penetrate at least three quarters of an inch into the deck, or fully through its thickness where the deck is thinner than that. A nail that does not achieve this is not attaching your shingle to your house in any meaningful sense. It is holding until the first serious wind event.
One-by-four or one-by-six boards nailed across the rafters with deliberate gaps between them. Extremely common in Seattle homes built before roughly 1950, because the original roof was cedar shake and cedar needs airflow across its underside to survive. It was the right assembly for that material. For asphalt shingles it is disqualifying: there is no continuous support, and a fastener landing in a gap holds nothing whatsoever. Finding skip sheathing under an old Ballard, Wallingford, Beacon Hill, or Mount Baker roof is routine, and it changes the project.
One-by-six or one-by-eight boards laid tight or shiplapped, forming a continuous surface. Common from roughly the 1920s through the 1950s. This is workable decking — it is continuous and it usually holds nails — but it comes with age-related caveats. Century-old boards cup, split along the grain, and shrink apart at the joints. Individual boards may need replacement even where the assembly as a whole is sound, and split boards will not hold fasteners reliably.
The postwar standard, dominant from roughly the 1950s onward. Typically half-inch or five-eighths. It is dimensionally stable, holds fasteners well, and handles moisture better than most alternatives because it dries relatively evenly. Sound plywood is the best-case discovery on a tear-off: nothing to do but re-nail as needed and move on.
Standard from roughly the 1980s onward, usually 7/16 inch. Structurally strong and entirely code-compliant, but with one relevant weakness in this climate: OSB swells irreversibly at the edges when it gets and stays wet, and does not return to dimension on drying. On a roof that has been leaking quietly for a couple of seasons, OSB fails visibly at the panel edges — a raised, spongy, crumbling perimeter around otherwise intact sheets.
The classic failure. Water enters at a compromised detail — a failed vent boot, a valley full of conifer needles, chimney flashing that has separated from its mortar joint, a missing kickout — and wets the deck continuously through the wet season. Because Seattle rain is persistent rather than dramatic, the wetting is slow and the drying never quite catches up. Fungal decay needs sustained moisture above roughly 20 percent content in wood, and an eight-month wet season delivers that reliably. The deck softens, darkens, and eventually loses structural integrity in a spreading patch around the entry point.
This is the failure mode that surprises homeowners, and it is disproportionately common here. Warm, moist indoor air — from showers, cooking, laundry, and simply breathing — rises and leaks into the attic through gaps around light fixtures, plumbing penetrations, attic hatches, and top plates. In a cold attic, that moisture condenses on the coldest surface available, which is the underside of the roof deck.
In a hard-freeze climate this shows up as dramatic frost and gets noticed. In Seattle's mild, damp winter it shows up as persistent low-grade dampness that never freezes and never dries, which is arguably worse: it is the ideal condition for fungal growth and it produces no obvious symptom until the sheathing is already compromised. The roof surface above may be in excellent condition. The tell is that the damage is distributed broadly and evenly across an entire slope rather than radiating from a single point, and it is usually worst on the north side and near the ridge.
Two contributing conditions are near-universal in older Seattle homes and both are fixable. First, bathroom and dryer exhaust fans that terminate into the attic instead of through the roof or wall, dumping a continuous stream of humid air directly at the sheathing. Second, soffit vents that have been painted over, blocked by blown-in insulation, or never adequately sized, which starves the intake side of the ventilation system so that no amount of ridge venting produces actual airflow.
A thick moss mat holds water against the shingle surface permanently and lifts shingle edges, creating an entry path. The decking beneath a long-established moss colony is frequently in worse condition than the surrounding area, and this correlation is strong enough that we treat heavy moss as a decking-risk indicator when scoping a project.
Plywood that has been repeatedly wetted separates along its glue lines, losing the layered strength that defines it. It may look intact and still have no capacity to hold a nail. Similarly, old fasteners corrode — rusted nail shanks visible from the attic are a reliable signal of sustained moisture exposure, and a deck can be structurally adequate while its attachment to the rafters has quietly degraded.
You can gather most of the relevant information yourself, and doing so changes the quality of the quotes you receive, because a contractor who knows you have already looked will scope more carefully.
Go up on a bright day with the attic lights off and a flashlight. What you are looking for:
Board spacing. If you can see gaps between the sheathing boards, you have skip sheathing, and you now know the single most important fact about your project before you have paid anyone. Daylight anywhere it should not be. Dark staining trails running down rafter sides, which map water paths back toward entry points. Rusted nail shanks protruding through the deck, indicating chronic moisture. Discoloured or blackened sheathing, especially in broad even patches near the ridge or across the north slope, which points to condensation rather than a leak. Compressed, dark, or damp insulation. Exhaust ducts terminating in the attic rather than exiting through the roof or wall.
Only if you are comfortable and safe doing so, and never on a wet or steep roof. Walking a roof, you feel deck problems before you see them: a spongy or springy area underfoot means the sheathing has lost integrity. Visible dips or waves between rafters indicate sagging. From the ground, sighting along the roof plane at a low angle will reveal deflection that is invisible when looking straight on.
Seattle permit records are public. The build year tells you the probable decking type, and prior roofing permits tell you what has been done since — including whether previous work was permitted at all, which matters for both disclosure and insurance.
Individual compromised sheets or boards are cut out and replaced. This is the common case on a plywood or OSB roof with localized damage around a failed penetration. New material must match the existing thickness so the roof plane stays flat; a thickness mismatch telegraphs through the finished shingles as a visible step.
Where the existing structure is spaced boards, the correct approach is normally to leave the boards in place and sheathe over them with new panels, creating the continuous nailable surface asphalt requires. This is more economical and less disruptive than stripping the original boards, and it retains their contribution to the structure. On a 20 square roof, expect this to add meaningfully to the project — frequently several thousand dollars — which is precisely why it should be discussed before tear-off, not during.
Reserved for widespread deterioration. Uncommon, expensive, and occasionally accompanied by structural repair to rafters where decay has reached the framing rather than stopping at the sheathing.
When decay reaches structural members — typically rafter tails at the eaves, where water from failed gutters has been wetting the ends for years — this becomes structural work, requires permitting, and is priced separately. It is also the outcome most reliably prevented by dealing with gutters and drainage early.
Both are code-compliant and both are used widely. In this climate we lean toward plywood for repair and overlay work, for one specific reason: moisture behaviour. Plywood absorbs and releases water relatively evenly and recovers much of its dimension on drying. OSB swells at the edges and does not recover. On a roof that will be exposed to rain during the tear-off window — which in Seattle is most of the year — and that lives its life in a damp environment, that difference has practical value. OSB is meaningfully cheaper, and on a fully dried-in job with good detailing it performs well. It is a defensible choice; it is just not the one we default to here.
Panel thickness must suit the rafter spacing. Seven-sixteenths is common at 24 inch on-center spacing but produces noticeable flex underfoot and benefits from panel edge clips between rafters to support the unsupported edges. Half-inch or five-eighths gives a stiffer, more durable deck and better fastener holding. Where we are already opening a roof and adding sheathing, the incremental cost of stepping up a thickness is small relative to the improvement.
Wood panels expand with moisture uptake. Panels butted tight have nowhere to go and buckle, producing ridges that telegraph through the shingles. A small gap at panel ends and edges — typically an eighth of an inch — is standard practice and frequently skipped by crews working quickly.
Sheathing fastening pattern is a code requirement, not a preference, and it is what transfers wind uplift from the roof surface into the structure. On older homes we also re-nail existing sheathing during a tear-off where the original fastening is sparse or corroded, which is cheap while the deck is exposed and impossible afterward.
Any time decking is open in a Seattle home, it is the right moment to confirm that soffit intake is actually functioning and to install baffles that keep insulation from blocking the airflow path at the eaves. Given how much local decking damage originates from condensation rather than leaks, correcting ventilation while the roof is open is the difference between fixing the damage and fixing the cause.
This is the practical takeaway, and it applies regardless of which contractor you hire.
Insist on a stated per-sheet or per-board price in the written estimate, before work begins. A quote that says decking replacement will be billed as needed, without a unit price, gives the contractor unilateral pricing authority at the exact moment you have no leverage — your roof is open, the crew is on site, and rain is forecast. A quote with a defined unit rate lets you verify the arithmetic afterward.
Ask what allowance is already included. Many estimates build in a small quantity of replacement sheets. Knowing whether your quote includes zero sheets or six changes the comparison between two bids that otherwise look similar. A low bid with no allowance is not actually the low bid.
Ask for documentation. Photographs of the damaged areas before replacement, and a count. This is standard practice for contractors operating in good faith and mildly inconvenient for those who are not.
Ask what they expect to find, and why. A contractor who has been in your attic and looked at your build year should be able to tell you the probable decking type and the likely scenarios before tear-off day. If the answer is a shrug, the estimate is a guess with a number attached to it.
Decking is the one part of a re-roof that genuinely cannot be known with certainty in advance. That makes honest scoping more important, not less — the uncertainty is a reason for transparency, not a license for open-ended billing.
Spaced boards with deliberate gaps between them, common in Seattle homes built before about 1950 because the original cedar shake needed airflow underneath. Asphalt shingles need continuous support and reliable nail holding, so skip sheathing normally must be overlaid with new panels, which can add several thousand dollars to a project.
From the attic, look for daylight, dark staining trails on rafters, rusted nail shanks, blackened or discoloured sheathing, and damp insulation. On the roof surface, spongy areas underfoot or visible dips between rafters indicate deck failure. Sighting along the roof plane from the ground reveals deflection.
Almost certainly condensation. Warm moist indoor air leaks into a cold attic and condenses on the underside of the deck. The tell is damage spread evenly across a whole slope rather than radiating from one point. Common causes are bath or dryer fans terminating in the attic and soffit vents blocked by insulation or paint.
Both are code-compliant. We lean toward plywood in this climate because it absorbs and releases moisture more evenly and largely recovers its dimension after drying, while OSB swells irreversibly at panel edges when wet. OSB is cheaper and performs well on a properly dried-in job.
Require a stated per-sheet or per-board unit price in the written estimate before work starts, ask how many replacement sheets are already included in the bid, and ask for photographs and a count of what gets replaced. A quote billing decking as needed with no unit price hands over pricing authority at the moment you have no leverage.
Sheathing replacement is structural work and generally requires permitting, as does any repair reaching rafters or framing. We pull permits as part of the project rather than leaving it to the homeowner.
We walk the roof in person, check the attic, and give you a written fixed quote. Free estimates across Seattle and the greater Puget Sound region.
Why covering old shingles usually costs more across two roof cycles than it saves today.
How asphalt, cedar, metal, composite, and membrane actually perform in a wet, mild, moss-heavy climate.
