A rotten window sill looks like a job for a builder and a fresh length of timber. Most of the time it is not. If the rot sits in the outer face or one end of the sill and the core underneath is still firm, you can cut back the soft timber, splice in a new piece and bond it in with epoxy. Done properly it holds paint and lasts for decades.
This is the same method our crews use on heritage sash windows every week. Here is how to do it yourself with a screwdriver, a chisel and a two part wood repair epoxy.
When repair beats replacement
Timber sills rot from the top down and the outside in, usually where old paint has cracked and let water sit against the wood. The good news is that the rot is often shallow. Scrape the surface with a screwdriver. If the blade only sinks a few millimetres and hits firm timber underneath, the sill is worth saving.
Replacing a sill on a sash window means pulling the frame apart, and on an older house that can disturb render, architraves and the box frame. A splice and epoxy repair leaves all of that in place. For a wider look at repairing rather than replacing timber, see our guide on fixing wood rot in a timber window.
Cut out the rot, keep the good timber
Start by removing everything soft. Use a sharp chisel to pare back the rotten fibres until you reach timber that is firm and dry. Do not stop at paint that still looks intact, because rot travels happily under a sound skin. Aim for a clean pocket with square edges, since a square shoulder gives the new timber something solid to sit against.
Brush out the dust and let the area dry fully. Damp timber will not bond. If the weather has been wet, give it a day or two under cover before you go any further.
Splice in new timber and join it with 1 hour epoxy, the method our crews use
This is the part that makes the repair last. Rather than filling the whole void with epoxy, cut a piece of seasoned timber to fit the pocket you have made. Match the grain direction to the sill. The new piece should sit slightly proud so you can shape it back flush later.
Butter both mating faces with DT Wood Repair Epoxy in the 1 hour cure, press the new timber home and clamp or wedge it while the epoxy goes off. The epoxy is the glue and the gap filler in one. It grips end grain that ordinary wood glue struggles with, and it will not shrink as it cures. Once it is set, the splice behaves like part of the original sill.
A heavy duty epoxy gun makes mixing and dispensing the cartridge much easier if you have a run of repairs ahead of you.
Stabilise any remaining soft timber
Sometimes there is timber around the repair that is not rotten enough to cut out but is softer than you would like. Soak it with epoxy wood stabiliser. It wicks into the fibres and cures hard, so the surface takes filler and paint properly instead of crumbling a year later. Give it time to soak right in before you move on.
Fill, fair and shape
With the splice set and the surrounds firm, fill any remaining hollows and the joint line with epoxy. Overfill slightly. Before it fully cures you can knock back the high spots with a filling knife, which saves a lot of sanding later. Work the epoxy so the original profile of the sill is back to shape, including the slope and the drip edge that carry water away from the wall.
If you are weighing epoxy against a hardware store filler for this kind of work, our comparison of wood filler and wood repair epoxy explains where each one belongs.
Sand, prime, paint
Once everything is hard, sand the repair flush with the surrounding timber. Prime the bare epoxy and timber, then paint. Paint is what keeps water out and makes the whole repair last, so do not skip it or rush the coats. A well painted sill will see out the next repaint cycle without any trouble.
Be wary of the quick patch and paint approach that hides soft timber under a skim of filler. It looks fine for a season and then fails from underneath. We wrote about why in beware of patch and paint.
When the sill really is gone
Epoxy has limits. If more than about half the sill is soft, or the rot runs deep into the joints where the sill meets the frame, a splice will not give you enough sound timber to anchor to. At that point a new sill is the honest answer. The same epoxy skills still earn their keep on the rest of the window, as we found when we raised a dog door and filled the gap with epoxy, but the sill itself needs replacing.
What you need
For a typical sill repair the DT Epoxy Starter Pack brings the epoxy, the stabiliser and the basics together in one kit, which is the simplest way to get everything for a first repair without hunting for parts.
FAQ
Can I repair a rotten window sill without replacing it?
Yes, if the rot is shallow and the core of the sill is still sound. Cut out the soft timber, splice in a new piece, bond it with epoxy, then fill, sand and paint. If more than about half the sill is soft or the rot runs deep into the joints, replace the sill instead.
Do I fill the whole rotten area with epoxy?
No. For anything more than a small hole, cut a piece of seasoned timber to fit the pocket and join it in with epoxy rather than filling the void with epoxy alone. The timber splice does the structural work and the epoxy bonds and seals it.
How long does epoxy take to cure on a sill?
It depends on the product and the temperature. The 1 hour cure DT epoxy is workable quickly, while cooler weather slows any epoxy down. Wait until the repair is fully hard before you sand and paint.
Will an epoxy sill repair last?
A splice and epoxy repair that is properly stabilised, shaped and painted lasts for decades. The paint film is what protects it, so keep the sill painted and repaint before the coating breaks down.
What tools and products do I need?
A sharp chisel, a filling knife, sandpaper, primer and paint, plus a two part wood repair epoxy and a wood stabiliser for any soft surrounds. The DT Epoxy Starter Pack combines the epoxy and stabiliser in one kit.