Repairing wood rot: when is it still possible and when is it not?

Wood rot starts under the paint, not on it. By the time you can see it, it has usually been there two to three years. That is why looking alone is unreliable and tapping only half reliable: a hollow sound tells you something is wrong, no hollow sound tells you nothing.

The test that does work takes two minutes. Take an ordinary screwdriver or a bradawl and push it with normal hand pressure into the bottom rail, into the corners and into the end grain. Healthy wood barely lets you in. Affected wood gives like cork and you sink in a centimetre or more without force.

Check three places on each frame: the bottom rail along its full length, the four corner joints, and the edge under the glazing bead. That is where water sits longest, and that is where it almost always starts.

Blisters in the paint, dark discolouration, open joints and paint peeling off like a skin are the warning. They do not automatically mean rot, but they do mean the wood is taking on water there.

Wood rot is always the work of fungi, and fungi need only one thing: moisture. Keep timber below roughly twenty percent moisture content and nothing happens. That is the heart of every repair: if the moisture keeps arriving, the rot comes back whatever you smear into it.

Brown rot breaks down the cellulose. The wood turns darker, shrinks and cracks into cubes, roughly like a chessboard. This is what you meet most often in frames and sills and it moves relatively fast.

White rot also breaks down the lignin. The wood becomes lighter, feels fibrous and falls apart into strands when touched. You see this more in structural timber and in damp crawl spaces.

Separately there is fungal attack that travels through masonry, such as dry rot. That is a different matter from a rotten sill: it affects the structure and it is not a DIY job. If you find fleshy, cottony or mushroom-like growth on or beside the timber, do not cut anything open and have it inspected.

Repair is three to five times cheaper than replacement, so the temptation to keep patching is strong. These boundaries keep that decision honest.

• Repair works if the rot is local and you are left with sound, dry timber on all sides after cutting it out. Rule of thumb: less than about a third of the cross section removed.
• Repair also works if the rot is in a non-structural part, such as a glazing bead, a drip strip or a section of sill carrying no load.
• Replace as soon as a corner joint has come loose, because that is where the stiffness of the whole frame sits. Filling a corner with epoxy holds the shape, not the structure.
• Replacing also makes more sense once you are putting in the third or fourth repair on the same frame. Four repairs of 300 euro is 1,200 euro spent on a frame you still have afterwards.
• Replace straight away if you want insulating glass anyway. You only pay once for the labour of taking the frame out, and for insulation measures an ISDE subsidy from RVO may come into play. Check the conditions in advance, because they change every year.

A capable amateur can handle this in dry weather above ten degrees. Allow half a day per frame, spread over two days, because there is drying time in it.

1. Wait for a dry spell. Repairing damp timber is wasted work: the filler will not bond and you seal the moisture in.
2. Cut out all affected wood with a chisel, until you reach hard, sound timber on every side. This is where it usually goes wrong, because the instinct is to remove too little. Anything left soft carries on rotting under your repair.
3. Brush the cavity out with a stiff brush and vacuum the dust away. Then let it dry for a day, longer if the wood still feels cold.
4. Apply wood rot hardener to the bare timber in and around the cavity and let it soak in for about six hours. It hardens the fibres and stops the fungus.
5. Fill in layers with wood rot filler or two-component epoxy, not in one go. Thick layers shrink and crack. Fill slightly proud so you can sand back later.
6. Let it cure fully as the packaging states, then sand flat with a sanding block. Check by hand whether it runs level; your fingers see more here than your eyes.
7. Prime the repair and then paint the whole face of the frame, not just the spot. A loose patch of paint bonds differently and will be the first to lift again.
8. Then go and find the cause: a blocked drainage hole, a failed sealant joint under the glass, a leaking gutter, a sill without a drip edge. Skip that step and you are back in the same place in three years.

There is a clear point where carrying on causes damage instead of repair. Stop as soon as one of these applies.

The frame moves or the window no longer closes properly. Then the joint is no longer carrying, and that is structure rather than decoration.

You are cutting through into the cavity or the masonry. Once your hole runs behind the frame, this is about weatherproofing the facade, which is another trade.

There is fleshy, cottony or mushroom-shaped growth on or beside the timber, or the masonry around it is damp and salty. That may be dry rot and it travels through the wall. Do not open it up yourself and do not cover it over.

It is a listed building or a property in a protected townscape. Separate rules apply there and frame work often needs a permit, even for repairs.

You would have to work from a ladder at first floor level or higher. Frame maintenance at height belongs on scaffolding or a lift, and that is exactly where most of a painter's charge sits.

Painting firms like to present wood rot repair as specialist work. For a good share of cases it is not, and you would rather know that before you request three quotes.

• A soft spot of a few centimetres in a ground floor sill. That is a tub of wood rot filler at 20 to 45 euro and an afternoon of work.
• Blisters in the paint with no soft wood underneath. Sand, prime, repaint. There is no rot yet, you are in time.
• A sealant joint under the glass that has come loose. Cut it out, clean it, reseal. This is the cheapest prevention there is and it stops the most expensive problem.
• A wooden shed door, a garden gate or an outbuilding. Even if the repair is not perfect, nothing structural is at stake.
• What you do outsource is not the repair itself but painting at height and the full maintenance round. That is where the labour, the scaffolding and the safety sit.

Doing it yourself: wood rot hardener 15 to 25 euro, filler or epoxy 20 to 45 euro per set, primer and topcoat 30 to 60 euro together. Under a hundred euro covers several frames.

What makes 2026 different from a few years ago: labour is the biggest cost item, not material. That makes it worth having several frames done in one visit. The second and third repair on the same day are noticeably cheaper than separate call-outs.

Exterior painting and frame maintenance are major maintenance and therefore the landlord's cost. What is yours is set out in the Besluit kleine herstellingen, the Dutch decree on minor repairs: keeping frames clean, tightening small fittings, interior painting. Wood rot in the facade is not on that list.

Report it in writing with photos and a date and ask for repair within six weeks. If nothing happens, you can go to the Huurcommissie or file a report with your municipality's building and housing inspectorate.

In a flat, the frames are common property in almost every deed of division, just like the roof and the facade. Maintenance then runs through the VvE, the owners association. So never replace a frame on your own initiative without a decision of the general meeting, because you rarely get that money back afterwards.

Water that cannot get away. That is the whole story. A sill without drainage, a blocked drainage hole in the glazing bead, a window sill without an overhang, a gutter running down the facade.

The exterior paint cycle is five to seven years, depending on how much the facade takes. In the Netherlands the south-west is the hardest side, because sun and driving rain come together there. Check that elevation before the rest.

Inspect twice a year, in spring and autumn, with the screwdriver test. A quarter of an hour per round, and that is the difference between a tub of filler and a new frame.

Interior paintwork counts too. A frame that absorbs water vapour on the inside never fully dries out on the outside.