Wood Rot Repair Under a Roof Window in Ajax — New Sheathing, Flashing & Shingles

Real job. Real home. Real result. A homeowner in Ajax called us about the sheet metal under a roof window — it had lifted, buckled and pulled away from the sill, and the shingles below it were past their service life. From the ground it looked like a trim problem. It wasn’t. Water had been running behind that metal for a long time, and the wood underneath had gone soft.

Here’s what the failed sheet metal was actually doing, what we found once the shingles came off, what needed rebuilding versus what only looked rough, and how the section went back together.

Location	Ajax, Ontario (Durham Region)
Area of work	Steep mansard-style slope with a window set into the roof
Problem	Failed window sheet metal letting water in behind the flashing — wood rot in the sheathing and framing below the window
Scope	Shingle tear-off in the affected section, removal of rotted wood, new plywood sheathing, new underlayment, new flashing, new window sheet metal, new asphalt shingles
Job type	Targeted repair — not a full roof replacement
Outcome	Sound wood under the window, a properly lapped drainage path, and a clean finished slope

What the Homeowner Was Seeing

The sheet metal running under the window sill had lost its shape. Instead of sitting tight and flat against the slope, it had waved and lifted away from the wall, opening a gap along the top edge — exactly where water comes off the window and needs somewhere controlled to go. The shingle courses below had also aged out: brittle, worn thin, and no longer sealing down the way they need to on a slope this steep.

On its own, each of those reads as cosmetic. Together they’re a drainage failure. Water coming off the glass was getting behind the metal instead of on top of it, then soaking into the wood on its way down.

What We Found Once the Shingles Came Off

Opening the section told the real story. The wood beneath the window was delaminated and crumbling — sheathing that had been wet repeatedly and had never been given a chance to dry. It broke apart in the hand rather than holding a fastener. The framing member below the window had gone the same way, dark and fibrous through the face, with the rot running back into the cavity rather than staying at the surface.

That’s what long-term flashing failure looks like. The leak never announced itself as a dramatic drip; it just kept the wood damp, season after season, until the wood stopped being structural.

What Actually Needed Fixing — and What Only Looked Bad

Not everything that looks bad on a roof is a problem, and not everything that looks fine is safe. Weathered, grey-faced wood is normal on a roof of this age and doesn’t need to come out. Soft, delaminated, crumbling wood does — it won’t hold a nail, it won’t carry load, and anything fastened to it is only as good as the wood behind it.

So the line we drew was simple: every piece of wood that had lost its integrity came out, and sound material stayed. The shingles in the affected section came off because they were both worn out and in the way of the repair — this was a targeted section repair, not a reason to sell the homeowner a whole new roof.

The mistake we see on this exact failure is a bead of caulking along the top of the lifted metal. It hides the gap for a season, traps moisture behind it, and leaves the rot to keep spreading out of sight. Sealant is not flashing.

How We Handled It

1. Stripped the section back to the framing. Shingles and the failed sheet metal came off so the full extent of the rot was visible, not guessed at.
2. Cut out all the compromised wood. The rotted sheathing and the deteriorated framing under the window were removed back to sound material.
3. Rebuilt with new plywood sheathing. A fresh, solid, flat nailing surface under the window — the foundation everything above it depends on.
4. Installed new underlayment. The secondary moisture barrier over the new deck, so if water ever gets past the surface again, it meets a membrane instead of bare wood.
5. Installed new flashing and new window sheet metal. New metal was formed and set under the window so water coming off the sill is carried out onto the shingles below — lapped in the right order, fastened properly, not sealed over.
6. Finished with new asphalt shingles. New courses laid back into the surrounding roof so the repaired section ties in cleanly.

Why the Sheet Metal Is the Detail That Decides

Shingles handle the open field of a roof. They don’t handle transitions — and a window set into a slope is one long transition. Everything depends on the metal: it has to carry water off the sill, out past the wall, and onto the surface of the shingles below, with each layer lapped over the one beneath it so gravity does the work.

Get that order wrong, or let the metal lift, and water goes behind the system instead of down it. The shingles can be brand new and the roof will still rot from underneath. That’s why the metal on this job was replaced outright rather than re-bedded — new material, formed and lapped correctly, is the part of this repair that determines whether it holds.

The Result

The window now sits over sound, new wood. The sheet metal is flat and tight to the slope with a proper drainage path off the sill, the underlayment gives a second line of defence, and the new shingles tie the repaired section back into the roof around it. The rot is out of the building rather than sealed inside it — and the homeowner spent on a section repair rather than a full roof.

Signs There’s Rot Under Your Roof

Wood rot is hidden by definition — it’s under the surface you can see. These are the signals worth acting on:

• Sheet metal or flashing that has lifted, waved, buckled or pulled away from a window, wall or edge
• Caulking smeared along a metal edge — usually a sign someone already patched a leak rather than fixing it
• Shingles that look worn, brittle or thin around a window, chimney or valley
• Staining, streaking or dark marks on the roof surface below a window
• A soft or spongy feel underfoot on the roof, or fasteners that no longer hold
• Interior signs near an upper window — peeling paint, a musty smell, or marks on the ceiling or wall below

Rot only moves one direction. The earlier it’s opened up, the smaller the section that has to come out.

Roof Repairs in Ajax & Durham Region

We’ve been repairing and replacing roofs across Ajax and Durham Region since 1994, with our own crews rather than subcontractors. Plenty of Ajax homes have windows set into steep mansard slopes like this one, and the sheet metal is almost always where the trouble starts. We’ll open it up, show you what’s actually under there, and scope the smallest repair that genuinely solves it.

See more of our work on the Projects page — including a structural sag repair on an Ajax wartime home and a cottage roof and skylight replacement in Claremont. You can also explore our Ajax roofing services, roof repair, roof replacement, roof inspections and soffit & fascia repair. Every job is backed by our warranty coverage.

Frequently Asked Questions

What causes wood rot under a roof window?

Almost always a flashing or sheet metal failure. When the metal under a window lifts, buckles or pulls away from the slope, water coming off the sill runs behind it instead of onto the shingles below. The wood underneath stays damp through repeated wet cycles until the sheathing and framing lose their strength and begin to crumble.

Can rotted roof sheathing be repaired without replacing the whole roof?

Yes. On this Ajax project we stripped only the affected section, cut out the rotted sheathing and framing, installed new plywood, underlayment, flashing and window sheet metal, and shingled the section back in. A full replacement is warranted when the deck is failing broadly — not when the damage is contained to one area.

Is caulking a fix for lifted flashing?

No. Sealant along the top of lifted metal hides the gap and traps moisture behind it, so the rot keeps developing out of sight. Flashing works by lapping layers so water is carried over the surface below it. When metal has lost its shape, it needs to be replaced and re-lapped, not sealed.

How do I know if the wood under my shingles has rot?

From the outside you usually can’t — which is the problem. The visible clues are lifted or patched metal, worn shingles around windows and penetrations, staining below a window, or a soft feel underfoot. Confirming it means opening a section or having a roofer inspect the transitions where leaks actually start.

Does C.D. Roofing do section repairs in Ajax?

Yes. We handle targeted repairs, flashing and sheet metal work, decking replacement, full roof replacements and emergency repairs across Ajax and Durham Region. Call (905) 430-7911 or request a free estimate.

C.D. Roofing & Construction Ltd.
8 Mildenhall Pl., Whitby, ON L1M 0C9
Serving Ajax, Pickering, Whitby, Oshawa, Bowmanville & Durham Region
Phone: (905) 430-7911
Email: info@cdroofingltd.com