Quick Answer
If rot is localised (bottom corners, sill edge), a joiner can splice in new timber for £150–£400 per repair. If the sill is soft throughout or the frame has structural rot, replacement is better value at £1,500–£2,500 per window — with a 60–100 year lifespan.
How to Check for Rot
You don’t need a specialist — a penknife or flat-head screwdriver is enough. Push the blade into the timber at these four critical points:
- Bottom of each sash — where water collects on the meeting rail and bottom rail
- Sill — especially the front edge and underside. Run your hand along the bottom — if paint is bubbling or crumbling, moisture is inside
- Bottom corners of the frame — where the jamb meets the sill. Water runs down the frame and pools here
- Around the glass — where putty has cracked or fallen out, water gets behind the glazing
If the blade sinks more than 3mm into soft timber, you have active rot. If it penetrates easily to 10mm or more, the timber is structurally compromised.
Repair vs Replace — Decision Table
| Condition | Repair? | Typical Cost |
|---|---|---|
| Surface decay only (paint failure, surface softness) | Yes — sand, treat, repaint | £80–£150/window |
| Localised rot (corner, rail end) | Yes — cut out and splice | £150–£400/repair |
| Sill rot (front edge only) | Yes — sill face repair or replacement | £300–£500/sill |
| Full sill rot (soft throughout) | Borderline — new sill or full replacement | £400–£600/sill |
| Frame structural rot (jamb or head) | No — replace | £1,500–£2,500/window |
| Multiple areas rotted per window | No — replace (repair cost approaches new) | £1,500–£2,500/window |
The Repair Process
For localised rot, a skilled joiner will:
- Cut back the rotten timber to at least 50mm beyond the visible damage
- Treat the remaining wood with preservative and hardener
- Shape a new piece of timber to match the profile
- Splice it in with waterproof adhesive and pin it
- Sand flush, prime, and repaint
A good splice repair is invisible once painted and can last 15–20 years. We do this regularly on windows where the main frame is sound but a few areas have deteriorated.
When Replacement Makes More Sense
The tipping point is usually £500 per window in repairs. At that point, you’re spending a third of the cost of a new window that will last 60–100 years, compared to a repair that lasts 15–20. The maths stops working.
Other signs it’s time to replace rather than repair:
- Rot in more than two locations on the same window
- The sashes are warped or twisted (won’t close square)
- The frame is single-glazed and you want thermal improvement
- You’re in a conservation area and need windows that match exactly (new windows are easier to get approved than patched originals)
Preventing Rot in New Windows
Modern timber windows are far more rot-resistant than the originals. We use:
- Engineered timber — laminated sections that resist warping and splitting
- Factory preservative treatment — vacuum-impregnated, not just surface-coated
- Microporous paint (Sikkens/Teknos) — lets moisture escape, prevents trapping
- Drainage channels in the sill — water exits before it can sit
- Accoya timber option — modified wood that is naturally rot-proof for 50+ years
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Open the 3D Configurator Book a SurveyFrequently Asked Questions
Can rotten window frames be repaired?
Yes, if the rot is localised and the main frame structure is sound. A skilled joiner can cut out the rotten section and splice in new timber. This costs £150–£400 per area repaired. If rot has spread through the frame or the sill is soft throughout, replacement is more economical.
How do I know if my window frame is rotten?
Push a screwdriver or penknife into the timber. If it sinks in more than 3mm, the timber is soft and has rot. Check the bottom corners of sashes, the sill (especially the underside), and anywhere paint has cracked or flaked.
How much does it cost to repair a rotten window frame?
Minor spliced repair: £150–£400. Sill replacement: £300–£500. Full sash rebuild: £400–£700. At around £500+ per window in repairs, new windows become better value considering their 60+ year lifespan.
What causes window frames to rot?
Water getting into the timber and staying there. The most common causes are failed paint allowing moisture in, blocked drainage channels in the sill, putty failure around the glass, and condensation trapped between the sash and frame.
How long do repaired window frames last?
A well-executed timber splice repair using preservative-treated wood and proper paint finish should last 15–20 years. A full window replacement lasts 60–100 years.