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The window sill is the most exposed part of a timber sash window — the first thing to rot, and often the last thing homeowners notice until the damage spreads. We repair and replace timber window sills on period properties across London.
The window sill sits at the base of the frame, sloping outward to shed rainwater away from the wall. On a timber sash window, it takes the full brunt of the British weather — wet winters, freeze-thaw cycles, and decades of repainting over cracked putty that traps moisture rather than shedding it. Most Victorian and Edwardian sills were made from softwood, which is fast to machine and paint but vulnerable to rot once the protective surface breaks down.
Rot in a window sill rarely starts from the top surface. It almost always begins underneath, where moisture that has crept past the putty or paintwork sits on the underside of the sill where it cannot evaporate. By the time the surface starts to feel soft, the rot may already extend several inches into the timber.
Left untreated, a rotten sill allows water into the wall, softens the lower frame members, and eventually compromises the structural integrity of the whole window. Sill replacement is almost always cheaper than waiting until the frame itself needs replacing.
Press a fingernail or screwdriver into the sill. If it sinks easily with little resistance, wet rot is already present in the timber below the surface.
Paint cracks and flakes when the timber beneath is moving due to moisture absorption and drying cycles. This is often the first visible warning sign before rot takes hold.
Longitudinal cracks along the grain are moisture channels that allow water to penetrate deeper into the timber and behind the frame.
Damp patches or tide marks on the plasterwork below a window are a strong indicator that the sill is no longer shedding water correctly and moisture is tracking into the wall.
Where the sill meets the frame or the masonry, any open gap or cracked sealant is a direct water entry point. Patching over rot-damaged timber with mastic is a temporary fix that invariably fails.
If the sill rot has reached the bottom rail of the outer frame, it will continue to spread upward unless both the sill and the affected frame section are treated or replaced.
Not every damaged sill requires full replacement. The right option depends on how far the rot has penetrated and whether the sill structure is still sound enough to anchor repaired sections.
Our approach: We always assess the full extent of decay before recommending a course of action. Where repair is genuinely viable and will last, we say so. Where the rot is too advanced, we explain why and quote for replacement with the correct timber specification. We do not fill structurally compromised sills with resin and present it as a long-term solution.
We inspect the sill, the lower frame, and the surrounding masonry to understand the full extent of the damage. We probe the timber to identify how far the rot has travelled and check whether the frame joints below are still sound.
The existing sill is carefully removed, taking care to avoid damage to the surrounding masonry or the lower frame. On most box sash windows the sill is not load-bearing in the structural sense, but it is mechanically connected to the frame at its horns and must be cut free cleanly.
Any rot in the lower frame members is cut back to sound timber and treated with preservative prior to fitting the new sill. Skipping this step risks new rot transferring from untreated frame timber into the fresh sill within a few years.
The replacement sill is machined to match the original profile — the correct slope, drip groove, and horn dimensions. We use hardwood, modified timber, or Accoya depending on the specification, location, and whether listed building conditions apply.
The new sill is fitted on a bed of mortar or sealant, secured to the frame, and the horn joints are made good. Flexible sealant is applied at the junction between the sill and masonry. All exposed timber surfaces are primed before fitting to seal end grain.
The completed sill is primed, undercoated, and top-coated to match the existing paintwork where possible. A correctly prepared and painted hardwood or Accoya sill will outperform the original softwood for decades without the same vulnerability to rot.
Naturally durable and significantly more rot-resistant than softwood. A good first-choice upgrade from a failed softwood sill. Works well on unlisted properties and where paint finish is required.
Lifespan: 30–50+ years with maintenance
Acetylated softwood with Class 1 durability rating. Dimensionally stable (resists swelling and shrinking), paint holds better, and rot resistance is guaranteed for 50 years in above-ground applications. As approved Accoya contractors, this is our recommended specification for exposed sills.
Lifespan: 50+ years guaranteed
Like-for-like match with original Victorian sills. Required in some listed building situations where the specification must match the original. Adequate with correct treatment and painting but more maintenance-intensive than hardwood or Accoya.
Lifespan: 15–25 years with maintenance
Cost depends on the size of the sill, the timber specification, the accessibility of the window, and whether the lower frame also requires attention. The following gives a general guide.
Sill Repair
From £150
Splice repair to localised rot. Sound surrounding timber required.
Full Sill Replacement
From £300
Hardwood or Accoya. Includes machining, fitting, priming, and sealant.
Sill + Frame Repair
From £450
Where rot has extended into the lower frame members.
All prices are indicative and subject to a site survey. For a full guide to our pricing see our sash window price guide. We provide written quotations with no obligation after the free survey.
Yes, where the rot is localised and the surrounding timber is structurally sound. We cut back to clean wood, treat the remaining timber, and splice in new matching timber. Where filler alone has been used previously, this is rarely a lasting solution — the underlying cause of moisture ingress must be addressed at the same time.
A straightforward single sill replacement on a ground or first-floor window typically takes one day. Windows on upper floors requiring scaffolding or where additional frame work is needed will take longer. We will advise on programme during the site survey.
On most properties, like-for-like sill replacement in the same material does not require planning permission or Building Regulations approval. On a listed building, listed building consent may be required if the replacement changes the specification or appearance. We advise on this as part of the survey process.
The underside of a sill is shaded from UV light and drying wind, which means moisture that accumulates there dries out much more slowly than on the top surface. Combined with the fact that the underside is rarely repainted as frequently as the visible face, this is where decay consistently begins. A properly installed sill will have a drip groove machined into the underside to prevent water tracking back along the sill toward the wall.
Accoya is a softwood (typically Radiata Pine) that has undergone an acetylation process, which permanently changes the cell structure of the timber so it can no longer absorb water or be degraded by fungi and insects. The result is a timber with Class 1 durability — the highest rating — that is also dimensionally stable, meaning it does not shrink and swell with moisture changes the way untreated softwood does. As approved Accoya contractors, we specify it for sills, frames, and any exterior joinery where longevity is a priority.
We are joiners and glaziers, not bricklayers, so we do not carry out structural masonry repairs. However, we do bed sills correctly in mortar where required and seal the junction between sill and masonry with appropriate flexible sealant. If the brickwork needs more substantial attention, we can recommend trusted trade partners.
Yes — we carry out sill repairs and replacements across all of Greater London and surrounding areas. We are based in North London and work frequently across Islington, Hackney, Camden, Haringey, Southwark, Lambeth, and most other London boroughs. Call us on 020 3488 1695 to check availability in your area.
We offer a free no-obligation site survey across London. We will assess the damage honestly and quote only for what is genuinely needed.
Full sash window repair service covering cords, weights, frames, sills, beads, and glazing.
Full restoration service for period sash windows — frames, sills, glazing, hardware, and draught proofing.
Specialist joinery repairs for listed buildings and conservation area properties across London.
Here at Golden Ratio Windows we have built up an excellent reputation in our local area by offering the most comprehensive and expert service in London. From the planning stage to the finished article; we are with you every step of the way, making sure that whatever the task, you are completely satisfied with the end result.
Get a Free QuoteWith so many years working in the trade, we are happy to answer any queries and provide quotes for any work that you are looking to have done. Rest assured that you will be receiving a professional, yet personal service. If you are in need of any work in London or the surrounding areas, get in touch.
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