Can a rotten window be repaired, or does it need replacing?

Most single rotten sections can be spliced out and renewed, which is why period timber windows are usually repaired rather than swapped. It becomes a replacement when several joints have gone, or the frame is no longer holding its fixings.

Is this wet rot or dry rot?

In a window frame it is nearly always wet rot: decay confined to timber that is actually wet, which stops once the timber dries out. Dry rot spreads through masonry and into dry timber and is a different, larger job — if there are strands or sheets on the surrounding wall, that needs a specialist look.

Will new windows have to meet current regulations?

Yes. A replacement window in an existing home is a controlled fitting: the glazing has to meet the current limiting U-value in the energy guidance, the unit has to be draught-proofed, and the ventilation must not be left worse than before. Where the character of a building makes that impossible, the guidance sets out alternatives such as low-emissivity secondary glazing.

Do I need permission to replace windows in a period property?

Possibly. Listed building consent and conservation area rules can restrict what may be changed, separately from the Building Regulations. Check the position before ordering, because the answer often decides whether you repair the existing frames instead.

Caution

Updated 30 August 2026

Rotten Wooden Window Frame

Timber at the bottom of the frame or sash is soft, dark and gives way under a screwdriver.

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Window and door fitting, illustrative image

What's happening?

Timber windows fail from the bottom up, because that is where water collects. The bottom rail of the sash and the cill of the frame sit in run-off, and once the paint film cracks — usually at a joint or a putty line — water gets into end grain that has no way to dry. What follows is wet rot: the fungal decay that softens and darkens wet timber, and the reason a screwdriver pushes straight into wood that looks sound under its paint. It stays confined to the wet timber, which is what separates it from dry rot, and it stops when the timber dries. That is the good news for repair: cut out the wet, fix the water path, and sound timber alongside it is still perfectly serviceable.

What usually causes it?

Cracked or blown paint at the bottom rail and cill joints, letting water into end grain
Failed putty or bedding at the glazing line, so water tracks behind the glass and into the rebate
A cill with no drip groove, or one clogged with paint, so run-off tracks back under the frame
Blocked drainage in the rebate, leaving standing water against the timber
Condensation running off single glazing onto the inside face of the bottom rail day after day
An earlier repair filled with car body filler or non-flexible filler, which traps water behind it
Overflowing gutter or a leaking cill above, wetting one window far more than the rest

Is it dangerous?

Rot in a window frame is a structural issue for the window rather than the building, but it has two edges. Decayed timber loses its grip on hinge and lock screws, so the window can become impossible to secure. And where a sash bar or a putty rebate has rotted, the glass loses its support — a pane that is only held by soft timber can fall out, which turns a maintenance job into a glass hazard.

Can I fix it myself?

Probing with a screwdriver to map how far the softness extends is genuinely useful, and repainting sound timber before it cracks is the best thing you can do for the windows that have not gone yet. What is worth avoiding is filling soft timber. Filler over rot hides it, keeps it wet, and lets the decay run on into the joint behind. If the frame is a repair rather than a replacement, the wet timber has to be cut out, not covered.

When should you call a window fitter?

Call a window fitter or joiner when the softness reaches a joint or a hinge position, when the glass is loose in a rotten rebate, or when the sash no longer holds its screws. It is also the moment to ask the repair-or-replace question properly, because a replacement window is a Building Regulations job and a splice repair to an existing frame is not — and in a listed building or a conservation area, that distinction may decide the work.

What will the window fitter actually do?

Probe and map the decay to find where sound timber starts, rather than working to what is visible
Cut out the decayed section and splice in new treated timber, matching the section and the profile
Re-form the drip groove and the drainage path so the repair is not left sitting in water
Re-bed the glass on fresh putty or a suitable sealant where the glazing rebate was the water path
Prime all end grain and rebuild the paint system, bottom rail and cill first
Advise where a frame is too far gone to splice, and what a like-for-like replacement has to meet

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Window Will Not Close ProperlyCaution Crumbling Window PuttyCaution Wet Rot In Skirting BoardsCaution

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