A rotten sill does not have to mean a new window. We splice out the decayed timber and fit sound wood where that works. However, if the damage is too widespread, we replace the whole sill. Sill replacement costs £120 per window, and every job carries our five-year guarantee.
Why sash window sills rot
The sill is the lowest and most exposed part of the frame. Because rain runs down the glass and sits on the timber, any gap in the paint lets water in. Over the years, that moisture softens the wood and the rot spreads.
However, many London homes make this worse. Victorian and Edwardian terraces often have sills with many layers of paint, and those layers hide small cracks. As a result, the first sign is usually a soft spot that your finger or a screwdriver sinks into.
Blocked drip grooves and failed paint are the other common causes. Because of this, a quick check each spring, before the damp season, saves a lot of money later. Our maintenance guide explains what to look for.
Repair first: splicing
First, we look at how far the decay goes. If it stops at one end or one corner, we cut it back to sound timber. Then we glue and pin in a new section, and shape it to match the original profile, because a visible patch looks poor.
In addition, a splice keeps the original joinery in place. It is cheaper than a new sill, and it disturbs less of your plaster and paintwork. That said, a splice only holds if the wood beside it is healthy, so we probe the surrounding timber before we promise anything.
We price other splices separately because they vary. For example, a small splice repair is £80, plus prime and undercoat. A bottom-rail repair, the most common splice, is £120. You can read more about these jobs on our sash window repairs page.
When we replace the whole sill
For example, a full replacement makes sense when a minor splice is no longer economical. Rot may run along most of the length, or the sill may have lost its slope so that water sits on it. In those cases a patch would fail again within a few seasons.
We cut sill sections in hardwood or softwood and fit them to the existing frame. The cost is £120 per window, and that includes splicing the styles where the rot has crept upwards. Bay and Venetian sills cost extra, because they are longer and harder to fit.
If the frame itself has moved, we also seal and point the gap. We use silicone or traditional mortar, because the right choice depends on how wide the gap is. Our price page lists every figure we quote.
Repair or replace the sill
Our rule is simple, so you know where you stand. While we repair first, we replace once repair stops being cost effective. Our longer article on rotten sill repair or replacement walks through the signs for each choice. It also helps you judge how urgent the problem is.
However, delay is the real enemy. Rot in a sill can travel into the lower sash and the frame. Then a £120 job can become a much bigger one. For a sense of how prices build up, see our guide to sash window repair costs in London.
What happens on the day
First, we protect your floors and furniture, and we remove the lower sash if we need access. Then we cut out the rot, since sound timber needs bare wood, fit new timber and shape it to match. After that, we prime and undercoat all the new wood, so the weather cannot get in.
Finally, we check that the window opens and closes smoothly and that water will drain away from the sill. However, if the sashes are tired too, we may suggest an overhaul at the same visit. That saves a second trip and often gives better value.
Where cords have also failed, sash cord replacement can be done at the same time because the sash is already out. We tell you the cost of each step before we start.
Listed buildings and conservation areas
Sills on listed homes need care because consent matters. English law makes it an offence to alter a listed building in a way that affects its character without authorisation. Like-for-like timber repair is the usual approach, yet the council decides what counts.
Our page on listed building sash windows explains how we work in these cases. We take each home as it comes, so for example tell us at the start if yours is listed or in a conservation area.
Get a free quote for your sill
We visit, look at the damage and give you a written quote at no charge. Because the visit covers the whole window, you will know if other repairs are coming. Our guarantee lasts five years on labour.
To start, send us a few photos and your postcode. We cover all Greater London postcodes, and often a little beyond the M25. We are open Monday to Friday, 08:00 to 18:00.