The Short Answer
Roof deck rot is decay in the plywood, OSB or boards under the shingles, caused by wood staying wet for long periods. The usual sources are a slow flashing or shingle leak, ice dams backing water under the eaves, and attic condensation from poor ventilation or warm, moist air leaking up from the house. Repair means cutting out the soft wood back to the framing, installing new sheathing of the same thickness, re-roofing that area, and fixing whatever kept it wet.
Key Points
- Wood decays only when it stays wet; the USDA Forest Products Laboratory puts serious decay above about 30 percent moisture content.
- Most deck rot sits at the eaves, around chimneys and walls, in valleys, or under a long-running leak.
- Dark stains, soft or flaking wood and daylight at nail holes from the attic are the clearest early signs.
- Swelling and waviness from a short wetting is not the same as rot, and APA says it does not by itself cost the panel its strength.
- Replacing deck without fixing the leak, the ice dam or the ventilation just resets the clock.
What the Roof Deck Does and What It Is Made Of
Code requires asphalt shingles to be fastened to a solidly sheathed deck. On most homes built since the 1970s that deck is 4 by 8 foot panels of plywood or OSB (oriented strand board). Older West Michigan homes often have board decks: rows of 1 by 6 or 1 by 8 lumber, sometimes with gaps between them.
The deck holds the shingle nails, spreads snow and foot traffic to the framing, and gives the underlayment and ice barrier a smooth, solid surface. Shingle makers set minimums for it. GAF's installation instructions call for at least 3/8 inch plywood or OSB, or board decks of boards no wider than 6 inches with no more than 1/8 inch between them, and say the deck must be dry before shingles go on.
Plywood
Thin wood veneers glued in layers
When it stays wet, the outer plies can separate and peel, and the wood softens and darkens where water sits.
OSB
Wood strands pressed with resin
APA notes that wet OSB can swell and its surface strands can lift or flake. Long-term wetting lets decay take hold just as it does in plywood.
Board deck
Common on older homes
Individual planks. Rot tends to follow the leak down one or two boards, and gaps or wide boards may need an overlay before new shingles.
What Actually Rots a Roof Deck
Wood does not rot from a single soaking. The USDA Forest Products Laboratory's Wood Handbook says serious decay occurs only when wood moisture content is above the fiber saturation point, about 30 percent on average, and that wood kept air dry (usually 20 percent or less) will not decay. Rot means the wood has been wet again and again, or wet for a long time.
In West Michigan, three things do that to a roof deck.
- Slow leaks. A failed pipe boot, flashing at a chimney or sidewall, a worn valley or a lifted shingle can leak a little in every rain for years before a ceiling stain appears. The deck under that spot stays wet the whole time.
- Ice dams. Snow melted by a warm attic refreezes at the cold eave and backs water up under the shingles. The eave edge, often the first few feet of deck, is soaked every thaw. That is why so much rot is found right at the gutter line.
- Attic moisture. Warm, damp air leaking up from the house (around light fixtures, the attic hatch, plumbing stacks, or from bath fans vented into the attic instead of outside) condenses on the cold underside of the deck all winter. With poor ventilation it never dries out.
- Water at the roof edge. A missing or short drip edge lets water curl back onto the deck edge and fascia, and a missing kickout flashing can soak the deck and wall at the bottom of a roof-to-wall joint.
Signs of Deck Rot You Can See From the Attic
The attic is the safest place to look and usually the most telling. Go on a dry day with a good flashlight, stay on the framing, and do not step between joists. Look at the underside of the deck, especially near the eaves, around chimneys and vent pipes, and below valleys.
- Dark brown or black stains on the deck, often with water lines or rings around them.
- Wood that looks crumbly, cracked into small cubes, or stringy and bleached. The Wood Handbook describes brown rot as dark with cubical checking and white rot as bleached.
- Plywood plies separating, or OSB with swollen, flaking strands on the underside.
- Daylight showing around nails or along panel edges.
- Rusty nail tips, frost on nail tips in winter, or mold spotting across large areas of the deck, which point to condensation rather than a single leak.
- Wet or matted insulation at the eaves, often directly under rotted deck.
If you push a screwdriver into a stained spot, it should meet firm wood. If it sinks in easily, the deck there is decayed.
Signs You Can See From the Yard
Do not walk the roof to check for rot. A soft deck can give way, and a fall from a roof is far more dangerous than any leak. Most outside signs can be seen from the ground, ideally in low morning or evening light that shows the shape of the roof.
- A dip or dish between rafters, or a roof plane that looks wavy rather than flat.
- Shingles that look sunken in one area, or that will not lie flat.
- Rotted or stained fascia boards and soffits at the eaves (those are a carpentry or gutter trade's repair, but they often sit next to rotted deck).
- Nails backing up through the shingles in one area.
- A roofer reporting a spongy feel underfoot, the classic sign found during an inspection or a repair.
Wavy Roofs, Swollen Panels and When It Is Not Rot
Not every wave in a roof is rot. APA, the trade association for plywood and OSB, says panels sometimes buckle when high moisture makes them expand, which is why it recommends a 1/8 inch gap at all panel edges. APA says moisture buckling does not affect structural properties, though the waviness can affect how the roof looks.
Short wetting is also not rot. APA's guidance for wet panels says swelling and flaking do not cost a panel its structural capacity once it dries back to normal moisture levels. The difference is time: decay fungi need the wood to stay wet. A panel that has swelled, dried and stayed firm is usually serviceable; a panel that is soft, crumbling or delaminated is not.
Sagging is a related but separate problem. A dip between the rafters usually points to the deck, while a sag along the length of a rafter or at the ridge can point to the framing. That needs a closer structural look.
What a Roof Deck Repair Involves
- Step 1
Find the Edges of the Damage
Shingles and underlayment come off the affected area. The roofer probes the deck and follows soft wood out to sound wood, which is often farther than the stain on the ceiling suggested.
- Step 2
Cut Back to the Framing
Damaged sheathing is cut out so the edges of the new piece land on rafters or trusses. Board decks are replaced board by board. Any framing that is soft gets a closer look before anything goes back.
- Step 3
Install New Sheathing
New plywood or OSB matching the existing thickness goes in, nailed to the framing, with the 1/8 inch edge gap APA recommends so it can move with moisture.
- Step 4
Rebuild the Roof Layers
New underlayment, ice barrier at the eaves where code calls for it (in Michigan, from the edge to at least 24 inches inside the exterior wall line), flashing and shingles woven back into the existing roof.
- Step 5
Fix the Cause
Replace the failed flashing or boot, address the ice dam, or correct the ventilation and air leaks. The Michigan Residential Code's baseline for attic ventilation is net free vent area of 1/150 of the area of the vented space, with a 1/300 exception in some cases.
At Stellar Roofing Company, a repair with a section of decking and underlayment usually falls in the moderate tier (about $1,500 to $4,000); rotted deck over a larger area or structural work is major ($4,000 and up). Every repair starts at $500, and the real number comes after an inspection.
When Deck Rot Changes the Repair-Or-Replace Decision
A few square feet of rot around a chimney or at one eave is a normal part of a repair. Rot spread across several areas, rot at the eaves on every side of the house, or rot with mold across the whole attic usually means the problem is the ventilation or the age of the roof, and patching one spot will not keep up.
On a full replacement, all the old roofing comes off down to the deck, so every bad panel can be found and replaced at once. If your roof is near the end of its life and the deck is rotting in more than one spot, that is worth weighing before you pay for spot repairs.
Repair vs replace
How to decide when the roof is aging and the repairs are adding up.
Repair or replace?What Michigan's Reroofing Rules Say About a Rotted Deck
The 2015 Michigan Residential Code has a section just for work on existing roofs, R908 Reroofing, and three of its rules decide what happens to a rotted deck. R908.2 says the structural roof components shall be capable of supporting the roof covering system and the material and equipment loads expected during installation. A deck that is soft enough to flex underfoot fails that test before a single bundle goes up.
R908.3 says roof replacement shall include the removal of existing layers of roof coverings down to the roof deck. The one exception lets an existing ice barrier membrane that is adhered to the deck stay in place and be covered with a new one. Tearing down to the deck is what exposes rot, which is why a replacement is the natural time to find and fix it everywhere.
R908.3.1.1 then bars a roof re-cover (new shingles laid over old ones) where the existing roof or roof covering is water soaked or has deteriorated to the point that it is not adequate as a base for additional roofing. The same section bars a re-cover over two or more existing layers. In plain terms, Michigan's code does not allow shingling over a rotting deck, and an offer to do so is a reason to get a second opinion.
Rot that has reached the rafters or trusses is a different category of work. The code's ordinary-repair exemption from permits in R105.2.2 does not include removing or cutting any structural beam or load-bearing support, so sistering or replacing framing usually means a permit and an inspection. Replacing a few deck panels during a repair usually does not, but your local building department makes that call.
Michigan code rules for roof repairs
The chapter 9 sections that matter on a repair, from underlayment to reroofing.
Code rulesRoof repair permits in Michigan
When structural work takes a repair outside the ordinary-repair exemption.
PermitsWorried About Soft Spots or Stains?
We will check the deck from the attic and the roof, find what kept it wet, and tell you how big the repair really is. The inspection is free.
Related Guides
Sidewall and Dormer Leaks
Roof-to-wall joints are one of the most common places a deck rots unseen.
Learn MoreRoof Edge and Drip Edge Damage
Why so much rot shows up right at the eaves.
Learn MoreAttic Condensation or Roof Leak?
Frost on nails, attic rain and how to tell them from a leak.
Learn MoreVentilation Problems
How a damp, poorly vented attic rots decking from below.
Learn MoreQuestions We Hear
Is a rotted roof deck dangerous?
It can be. Soft decking may not hold a person's weight, so nobody should walk on an area suspected of rot, and a roofer will work from sound wood or framing. Severe, widespread rot can also weaken how the roof carries snow. A small rotted patch at an eave is not usually an emergency, but it should be repaired before the next winter, because snow load and ice dams make it worse.
Can new shingles be put over a rotted deck?
Not properly. Shingles need a solid, dry deck to hold nails and lie flat, and GAF's installation instructions say roof decks must be dry before shingles are installed. Shingling over soft wood leaves nails with nothing to grip, which leads to lifted shingles and leaks, and it hides the rot until it spreads. Damaged decking should be cut out and replaced first.
Is OSB worse than plywood for roof decking?
Both are engineered wood panels made for roof sheathing, and APA says OSB shares many of plywood's strength and performance characteristics. They show water damage differently: OSB tends to swell and flake at the edges and surface, while plywood tends to separate into its layers. Either one rots if it stays wet. The bigger factors are keeping water out and keeping the attic dry.
Does homeowners insurance pay for a rotted roof deck?
It depends on your policy and on what caused the damage. Rot from a slow leak or condensation that went on for a long time is often treated differently from damage caused by a sudden event like a fallen limb. Read your policy's exclusions and ask your agent before assuming either way. The guide's article on insurance and roof leaks explains how coverage usually works.
How much decking is usually replaced in a repair?
Only as much as is damaged, cut back so the edges land on framing. Around a chimney or pipe that may be part of one sheet; at a long-running eave leak it may be several feet along the edge. The damaged area is often larger than the ceiling stain suggests, because water spreads along the deck before it drips. A good roofer will photograph what comes off so you can see why.
What This Page Cites
- Wood Handbook, Chapter 14: Biodeterioration of Wood (FPL-GTR-282), USDA Forest Service Forest Products Laboratory (https://www.fpl.fs.usda.gov/documnts/fplgtr/fplgtr282/chapter_14_fpl_gtr282.pdf), accessed October 2026
- OSB (Oriented Strand Board), APA The Engineered Wood Association (https://www.apawood.org/engineered-wood-products/plywood-osb/osb-oriented-strand-board/), accessed October 2026
- Frequently Asked Questions (wet panels on the jobsite), APA The Engineered Wood Association (https://www.apawood.org/faq), accessed October 2026
- 2015 Michigan Residential Code, Chapter 9 Roof Assemblies, Sections R905.1.2, R905.2.1, R908.2, R908.3 and R908.3.1.1, UpCodes (https://up.codes/viewer/michigan/mi-residential-code-2015/chapter/9/roof-assemblies), accessed October 2026
- 2015 Michigan Residential Code, Chapter 8 Roof-Ceiling Construction, Sections R803.2 and R806.1 to R806.3, UpCodes (https://up.codes/viewer/michigan/mi-residential-code-2015/chapter/8/roof-ceiling-construction), accessed October 2026
- Timberline HDZ, UHDZ, AS II, CS, HDZ RS and RS+ Installation Instructions (RESTL622, 2023), GAF (https://images.thdstatic.com/catalog/pdfImages/ce/ce744b21-6c3d-4c1e-9396-919c6ffa7fe8.pdf), accessed October 2026
- 2015 Michigan Residential Code, Chapter 1 Scope and Administration, Section R105.2.2 Repairs, International Code Council (https://codes.iccsafe.org/content/MIRC2015P2/chapter-1-scope-and-administration), accessed October 2026