You repair wood rot in a window frame in five steps: rout out the rotten wood until you hit sound, hard timber, let it dry to below 18% moisture, apply wood hardener, fill the hole with two-part epoxy wood filler, then sand, prime and paint.
Expect half a day to a full day of work per spot and 30 to 80 euros in materials if you do it yourself. A painter or carpenter charges 80 to 250 euros per affected part.
The repair itself isn’t hard. It comes down to getting one thing right: removing the moisture that caused the rot. If you don’t, the fungus simply keeps spreading under your fresh epoxy and you’re back outside next year.
Below I’ll walk you through everything. From spotting it to the finish, with the numbers and the mistakes you need to avoid.
What exactly is wood rot?
Wood rot is the decay of wood by fungi. These fungi live off the fibres in the wood and make it soft, brittle and discoloured.
They need three things to grow: moisture, oxygen and a temperature between 5 and 40 degrees. There’s little you can do about oxygen and temperature outdoors. Moisture you can. That’s why keeping things dry is always the heart of the solution.
There are two types you’ll come across on window frames.
Wet rot. By far the most common. Occurs in wood that stays wet for a long time, above 21% moisture. The wood turns dark, soft and spongy. You can push a screwdriver straight into it.
Dry rot (fungus). Rarer, but more dangerous. This fungus spreads through white strands and can even travel through masonry. The wood turns brown, crumbly and breaks into cubes. Dry rot often calls for a specialist, because the fungus may have eaten further into the structure.
Not sure whether it’s dry rot? See white, cotton-like strands or mushroom-like growth? Then call a specialist first, before you start repairing.
How do you spot wood rot in a window frame?
You don’t need to be an expert. Run through this checklist.
- Flaking or cracked paint. Often the first sign. There’s usually moisture under a blister in the paint.
- Dark or black discolouration in the wood, especially on the bottom sill and the corners.
- Soft wood. Poke the suspect spot with a screwdriver or awl. If it sinks in easily 5 millimetres or deeper, it’s rot.
- Sticking windows or doors. The wood has swollen from moisture.
- Loose or cracked sealant joints. The classic entry point for water.
Always check the bottom of the frame and the joints between the parts. That’s where water lingers and where rot almost always starts.
Where does the moisture come from? (fix this first)
Repairing without tackling the cause is like mopping with the tap running. First find out where the water is getting in.
The four usual culprits:
- Cracked or missing sealant joints around the glass or between frame and wall. Water slowly seeps behind them.
- Broken paint layer. As soon as the paint splits open, the wood can take on water.
- Standing water on the bottom sill because the drainage isn’t working properly or the drip cap is broken.
- Condensation and poor ventilation on the inside, especially with single glazing.
Fix this before or during the repair. New sealant, a clean paint layer, making sure water drains away. Otherwise the rot simply comes back.
Repair or replace the frame?
Not every rotten frame can be saved. Use this rule of thumb.
| Situation | Advice |
|---|---|
| Rot limited to one spot, rest of the wood hard | Repair |
| Damage less than a third of the part | Repair |
| Large section soft or rotten | Replace part or frame |
| Rot in the load-bearing corner joints | Often replace |
| Dry rot / fungus in the structure | Specialist, usually replace |
| Frame older than 40 years, single glazing | Consider replacing (insulation) |
Is the rot only on the outside and the core still hard? Then repairing with epoxy is perfectly fine and the repaired spot often lasts 15 to 20 years. If more than a third of the wood is soft, replacing is cheaper in the long run.
Wood filler or epoxy: what’s the difference?
This is where a lot of people go wrong, so let me be clear.
Ordinary wood filler or putty is meant for small imperfections and scratches indoors. It isn’t flexible and isn’t really water-resistant. Outdoors on a frame it cracks loose as soon as the wood expands and contracts. Don’t use this for wood rot.
Two-part epoxy wood filler is what you need. You mix a paste with a hardener. It sets rock-hard, moves with the wood, and can’t rot itself. This is the durable solution.
Epoxy putty is a thicker variant you can shape like clay. Handy when you need to recreate a profile or a corner.
In short: for small damage indoors, ordinary wood filler is fine. For wood rot in a window frame outdoors, you always reach for the two-part epoxy system.
What do you need?
Tools:
- Multi-tool or rot cutter to rout out the rot
- Chisel and screwdriver
- Moisture meter (important, see below)
- Filling knife or spatula
- Sandpaper grit 80 and 120, or a sander
- Brush for hardener, primer and paint
- Gloves and dust mask
Materials:
- Wood hardener (also called consolidant or primer)
- Two-part epoxy wood filler
- Primer for outdoor use
- Paint (opaque, for outdoor use)
- Sealant if needed for the joints
The hardener and the epoxy are often sold together as a repair kit. Brands like Repair Care, Alabastine and Bison have good systems.
Repairing wood rot in a window frame: the step-by-step
Step 1: Remove all the rotten wood
Rout or chisel out all the soft, discoloured wood until you hit hard, sound timber. Be strict here. Leave even a small bit of rot behind and the fungus simply keeps growing under your repair.
Afterwards, poke around once more with a screwdriver to check that everything left really is hard.
Step 2: Let the wood dry and measure the moisture
This is the step everyone wants to skip and the one that most often goes wrong.
Measure with a moisture meter. The wood must be below 18% moisture before you carry on. If it’s above that, you’re sealing in moisture and working against your own repair.
In dry weather, drying usually takes 3 to 5 days. For a wet spot it can take longer. No moisture meter? Then wait at least a week of dry weather and never work in the rain or right after.
Step 3: Apply the wood hardener
Brush the hardener generously onto the bare, dry wood and onto the edges of the hole. The hardener soaks into the porous wood and makes the fibres firm again. At the same time it helps the epoxy bond well later.
Let the hardener soak in according to the packaging. For most brands that’s around 25 minutes to an hour. The surface should still be tacky when you apply the filler.
Step 4: Fill the hole with epoxy wood filler
Mix the two components in the ratio on the packaging. Mix it through well, otherwise it won’t cure evenly hard everywhere.
Press the filler firmly into the hole so no air pockets are left behind. Work it slightly proud of the surface, that way you keep margin to sand flush.
Big holes? Build up in layers of no more than 2 to 3 centimetres. If you fill a deep hole in one go, heat and stress build up and the epoxy can crack. The maximum layer thickness on many products is 20 millimetres per pass, the minimum about 5 millimetres.
Step 5: Cure, sand, prime and paint
Let the epoxy cure. That’s usually 4 hours at around 20 degrees. In the cold it takes longer, in the heat it’s faster. Below 10 degrees epoxy cures poorly, so don’t work in the winter cold.
Sand the filled spot smooth, first with grit 80, then with 120. Bring it flush in line with the existing wood.
Then apply primer to all the bare spots, including the repaired part. Let it dry and paint in one or two coats. Make sure the paint covers everywhere, because that layer is what keeps the water out from now on.
Don’t forget to check the sealant joints and reseal where needed. That’s probably where it all started.
Curing times and timeline at a glance
| Step | Time (at ~20°C) |
|---|---|
| Routing out the rot | 30-60 min per spot |
| Drying to < 18% moisture | 3-5 days of dry weather |
| Letting the hardener soak in | 25-60 min |
| Epoxy curing | 4 hours |
| Primer drying | 4-16 hours |
| Paint drying between coats | 8-24 hours |
The repair itself takes half a day of active work. The overall lead time is longer because drying and curing take time. So plan the job for a dry, mild spell.
What does repairing wood rot cost?
The numbers differ quite a bit between doing it yourself and outsourcing.
| Approach | Cost |
|---|---|
| DIY, materials per spot | 30-80 euros |
| Painter/carpenter, small repair | 80-250 euros per part |
| Partially replace a part | 200-500 euros per frame |
| Replace whole frame | 600-1,500 euros per frame |
A tradesperson charges roughly 45 to 65 euros per hour. If you have several spots done at once, it works out cheaper per spot, because you share the preparation and scaffolding.
Smart tip: combine the wood rot repair with a paint job. The painter is there anyway, already up the ladder, and puts a neat coat of paint over your repair straight away. That saves you double costs.
The 6 most common mistakes
- Leaving moisture in the wood. The biggest mistake. The fungus keeps spreading under the epoxy and the rot comes back within a year. Always measure to below 18%.
- Not removing all the rot. A leftover bit of soft wood is enough to make everything fail again.
- Skipping the hardener. Without hardener the epoxy bonds poorly and the fibres stay weak.
- Using ordinary wood filler or putty. It cracks loose outdoors. Use the two-part epoxy system.
- Filling deep holes in one go. Build up in layers of 2 to 3 centimetres to prevent cracking.
- Not tackling the cause. New sealant, a full paint layer, good drainage. Otherwise your work was for nothing.
Preventing wood rot
Prevention is a lot cheaper than repair. Do this.
- Check your window frames twice a year, in spring and autumn. Watch the bottom sills, the corners and the sealant joints.
- Keep the paint layer intact. As soon as you see a crack or blister, sand it and touch it up. Don’t wait.
- Renew sealant joints that crack or come loose straight away.
- Make sure water drains away from the bottom sill and that the drip caps are intact.
- Ventilate well indoors to prevent condensation.
- Always treat bare wood with primer and paint before you leave it outdoors.
A window frame you maintain well lasts for decades. A frame you let slide is due for repair within a few seasons.
Frequently asked questions
Can I repair wood rot myself or does a professional need to do it?
Small, surface damage where the core of the wood is still hard, you can repair yourself just fine with a wood rot kit. It’s not difficult work. If the rot is in the load-bearing corner joints, more than a third of the wood is soft, or you suspect dry rot (fungus), then bring in a carpenter or specialist.
How long does an epoxy wood rot repair last?
If you do it properly, so all the rot gone, wood dry and the cause tackled, an epoxy repair often lasts 15 to 20 years. The epoxy itself doesn’t rot and moves with the wood. The weak point stays the finish, so keep the paint layer intact.
What’s the difference between wood filler and epoxy?
Ordinary wood filler or putty is for small damage indoors and isn’t water-resistant or flexible. Outdoors it cracks loose. Two-part epoxy sets rock-hard, is water-resistant, moves with the wood and can’t rot. For wood rot in an outdoor frame you always use epoxy.
How dry does the wood need to be before I apply epoxy?
The moisture content must be below 18%, measured with a moisture meter. In dry weather, drying takes 3 to 5 days after routing out. If you seal in moisture, the fungus keeps growing and the repair fails.
Why does wood rot sometimes come back after a repair?
Almost always because the cause wasn’t tackled or there was still moisture in the wood. Reseal the joints, keep the paint layer intact and make sure water drains away from the frame. Repairing without fixing the cause is mopping with the tap running.
Can I repair wood rot in winter?
Better not. Epoxy and hardener cure poorly below 10 degrees, and you won’t get the wood dry enough in a wet spell. Plan the job for a dry, mild period, ideally in spring or late summer.
Getting started
Repairing wood rot in a window frame is very doable if you respect the steps: all the rot out, wood dry, hardener on, epoxy in, finish it neatly and tackle the cause. Take your time with the drying and don’t skimp on the finish.
Want to give your woodwork or your whole home a crisp coat of paint after the repair, and learn how to paint, finish and tackle jobs around the house like a pro? Then take a look at Koning van Je Woning. There you’ll find everything to make your home look great and maintenance-free yourself, without an expensive professional.