Not every rotten corner can be sorted with a fill. On a recent job, six double hung sashes came back to the workshop for a full service, and one bottom rail corner had gone well past the point where epoxy alone would hold anything. The paint had lifted, the timber underneath was dark and crumbling, and pressing a thumbnail into it left a mark. That's a job for a timber splice, not a patch.
It's a step up from a straightforward hole or a shallow surface patch, but it's still a repair, not a replacement. Here's how we rebuilt that corner and got the sash back in the window.
The corner joint before we touched it: paint lifted clean off, timber underneath dark and soft.
Why we don't fill over rot this far gone
Epoxy is genuinely good at filling gaps, holes and shallow surface damage, but it isn't a substitute for sound timber. Once rot has eaten into a corner joint like this, there's nothing solid left for a fill to key into. Pack epoxy over the top and you've got a hard shell sitting over soft wood, which looks fine for a season and then cracks and lifts the moment the timber underneath moves or keeps absorbing moisture. The only fix that actually lasts is cutting the rot out and putting sound timber back in its place.
How we rebuilt the corner
1. Strip back to sound timber
Every trace of soft, dark and crumbling wood gets cut away until nothing's left but clean, solid timber. On a corner joint that usually means taking out more than you'd like to, but a splice only holds as well as what it's glued to.
2. Cut and fit a new timber splice
Back at the workshop, a new piece of matching timber is cut to size and fitted into the cleaned-out corner. The glass gets masked off with tape to keep it clear of glue and sawdust while the new piece is trimmed and dry-fitted until it sits flush.
The new timber splice fitted and glued into the corner on the workshop bench.
3. Bond the splice with epoxy
The splice gets bonded in with the same two part DT wood repair epoxy we use across our smaller repairs, a one hour cure filler that also seals every gap between the old and new timber so there's no path left for moisture to creep back in behind the joint.
4. Fair, sand, prime and paint
Once the epoxy's cured, the join between old and new timber gets faired flush, sanded back, then primed and painted to match the rest of the sash. From the front it reads as one continuous corner again, not two different pieces of timber joined together.
New timber in place at both corners, ready for priming and paint before the sash goes back in.
Of the six sashes in that job, most needed nothing more than a standard service and reseal. It was this one corner, on one rail, that had gone far enough to need a splice rather than a fill, which is fairly typical: rot rarely spreads evenly across a whole window, it eats into whichever corner or joint has been catching the most water.
What this looks like for your own windows
The size of the damage decides the approach. If you can still see solid timber around a soft patch and the loss is shallow, a straightforward epoxy fill is enough, the same as our cable hole and dog door repairs or the bottom rail rot repair we've covered before. Once a corner joint has rotted through like this one, cutting it out and splicing in new timber is the only repair that holds.
Our guide on fixing wood rot in a timber window without replacing it walks through how to tell the difference before you start cutting anything out. For the bond itself, we use a DT wood repair epoxy with a one hour cure, and an epoxy wood stabiliser on any timber nearby that's gone soft but isn't rotten through yet, to firm it back up before it gets to this stage.