How do I repair rotten window frames?

Rot in a window frame is the visible end of a water problem, so the repair has two halves: replacing the destroyed timber, and stopping the water that destroyed it.

Safety

What you'll need

• Screwdriver or awl for probing, and a chisel
• Two-part epoxy wood repair system, or matching timber for a splice
• Wood hardener and preservative
• Primer, undercoat and exterior paint, and exterior sealant

Step by step

1. Probe the whole frame, not just the obvious bit.Push a screwdriver into the timber at intervals all round, especially the bottom corners and the sill. Soft timber that takes the point easily is rotten, and the affected area is nearly always larger than it looks.
2. Find where the water is getting in.Failed paint, open joints at the corners, cracked putty, a blocked or damaged sill drip groove, failed sealant against the masonry, or a leaking gutter above. The repair is temporary until this is fixed.
3. Decide repair against replacement.Localised rot in an otherwise sound frame is worth repairing. Rot through the structural sections, several corners gone, or a frame that moves when pushed is usually a replacement job — and repairing it repeatedly costs more.
4. Cut out all the soft timber.A chisel back to timber that is hard and sound everywhere. Leaving any soft wood means the rot continues under the repair, which is the most common reason these fail.
5. Let it dry thoroughly.Days of dry weather, or a fan heater on it. Epoxy and hardener will not bond to wet timber and the repair fails within a season.
6. Apply wood hardener to the surrounding timber.It soaks into the slightly softened wood at the edges of the cut-out and consolidates it, giving the filler something solid to bond to.
7. Use two-part epoxy repair for small areas.Not ordinary interior filler, which absorbs water and falls out. Epoxy wood repair stays flexible enough, does not shrink, and can be shaped, sanded and drilled like timber.
8. Build it up in layers on deep repairs.Following the product's stated thickness per layer. A thick single application cures unevenly and shrinks away at the edges.
9. Splice in new timber for anything larger.Cut back to a clean angled or square shoulder, make a matching piece from treated timber of the same species, glue it with an exterior-grade adhesive and pin it. Structurally far better than a large mass of filler.
10. Treat everything with preservative.The repair and the surrounding timber, before priming. Rot spores are still present in the wood around the repair.
11. Shape and sand it back to the profile.Match the moulding and the line of the frame. Rasps, sanding blocks and a scraper. It is the difference between an invisible repair and an obvious patch.
12. Prime every surface, including end grain.End grain absorbs many times more water than the face and is where the rot started. Prime it thoroughly, including any cut faces.
13. Restore the drip groove on the sill.The groove under the front edge of the sill makes water drop off instead of running back to the wall. Blocked with paint or filler, water tracks back and rots the frame again. This is a common overlooked cause.
14. Paint properly and take it onto the glass.Primer, undercoat and two topcoats, with the top coat lapping about 2mm onto the glass to seal the putty line. Paint stopping short of the glass lets water in behind the putty.
15. Reseal against the masonry.Rake out failed sealant and replace it with an exterior-grade flexible sealant. Do not seal the drainage slots in a UPVC or aluminium frame, which are there deliberately.
16. Maintain it from then on.Check the paint annually and repaint before it breaks down. Repainting on a cycle is far cheaper than repeating this repair.

Tips

• Probe the whole frame, not the visible damage. The rot is almost always more extensive than it appears, and a repair over soft timber fails.
• Clear the drip groove under the sill. A painted-over groove sends water back onto the frame and is a common repeat cause.
• Lap the top coat about 2mm onto the glass. Paint stopping at the putty line lets water straight in behind it.

Common mistakes

• Filling over timber that is still soft — The rot continues underneath and the repair lifts off within a season or two. Cut back to hard sound timber everywhere.
• Using ordinary filler on exterior joinery — It absorbs water, shrinks and falls out. Two-part epoxy wood repair is formulated for exterior timber and does not.

If it doesn't work

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