Bottom rails cop it worse than any other part of a sash window. They sit closest to the sill, they catch condensation running down the glass, and on an older window the paint film there is usually the first to go. Once water gets past the paint, the timber underneath doesn't dry out between rain events, and rot sets in from the inside before it's visible from the outside.
That's exactly what we found on a recent core service. Nine double hung windows were booked in for a general service and reseal, and one bottom rail turned out to be soft enough that it needed proper attention rather than a quick sand and repaint.
Rot at a bottom rail corner joint, further gone than it first looked.
Spotting it before it spreads
The giveaway is rarely a hole, it's usually paint that's cracking and lifting in a way that doesn't match the rest of the window, or timber that gives slightly when you press a thumbnail into it. Push a screwdriver gently into the corner joints of a bottom rail. Sound timber resists. Rotten timber is soft, sometimes crumbly, and often darker than the wood around it.
Left alone, rot doesn't stay put. It follows moisture along the grain, so a small soft patch this year is often a rail that needs full replacement in two or three. Caught early, it's a straightforward repair.
How we treated it
For rot that's more than surface deep, we don't just fill over the top of it. Filling straight over softened timber traps moisture in and gives the repair nothing solid to key into, so it fails again within a season or two. The approach is more deliberate.
1. Cut back to sound timber
Every trace of soft, dark or crumbling wood gets cut out, not just scraped. What's left is clean, solid timber all the way round the repair. It looks worse before it looks better, but this step is what makes the repair last.
2. Bond in solid timber where the loss is significant
Where enough timber has been lost that filler alone would be thin and unsupported, we bond a new piece of matching timber into the recess with epoxy rather than nails or screws alone. The epoxy fills every gap between the old and new timber, so there's no void left for moisture to collect in.
3. Fair the edges with epoxy filler
The join between the old rail and the new timber gets faired over with the same two part DT wood repair epoxy we use on smaller patches, a one hour cure two part filler that bonds properly to painted timber instead of just sitting on top of it.
4. Sand, prime and paint
Once cured, everything gets sanded back level and painted to match. From the front, there's no sign a repair ever happened. What's different is that the rail is sound timber again, not old rot with paint over the top.
Sound timber back in place, ready to sand, prime and paint.
The other eight windows on that job needed nothing more than a standard service, weighting and reseal. It's usually one rail, or one corner, that's taken the damage while the rest of the window is fine, which is exactly why a full sash replacement is rarely necessary.
What this looks like for your own windows
If you've got a soft spot on a sill or bottom rail, the size of the damage decides the approach. Small holes and shallow surface rot are a straightforward epoxy fill, the same as our cable hole and dog door repairs. Anything deeper, where you can flex the timber or push a screwdriver well into it, needs the rot cut out properly first.
Our guide on fixing wood rot in a timber window without replacing it walks through assessing how deep the damage goes before you start. For the repair itself, we use a DT wood repair epoxy with a one hour cure for the fill and fairing, and an epoxy wood stabiliser first on any timber around the repair that's gone soft but not rotten through, to firm it back up before filling.
Serviced and resealed, rail included.