A rotten sash window sill can often be repaired with a splice, which costs £80 per window with us. If the rot runs through most of the sill, replacement at £120 per window is the better choice. A simple probe test tells you which one you need. Acting early keeps the price low.
Why sills rot
The sill sits at the bottom of the window and faces the weather. Rain runs down the glass and lands on it. If paint cracks or the sill is flat, water soaks in because it has nowhere to go.
Blocked drip grooves make things worse. Water then runs back toward the wall instead of away. Meanwhile, failed putty and open joints let more moisture in.
Sills also rot from underneath, because damp can rise from the wall. For that reason, the damage is often worse than it looks from outside.
How to test for rot
You need a screwdriver or a bradawl. Press the tip firmly into the sill in several places. Sound timber resists, while rotten timber does not. It sinks in or crumbles.
Check these spots first, since rot starts there. Look at the outer corners, the ends where the sill meets the frame, and under any flaking paint. Also check the bottom of the window frame and the bottom rail of the lower sash.
Mark each soft area with a pencil. Then measure how far it spreads. For example, a patch you can cover with a coin is a small fix. A patch the length of the sill is a different job.
You can also look for signs inside the house. Damp stains, peeling paint and a musty smell all suggest water is getting in. Our guide to the signs your sash windows need repair covers the other warning signs.
When a splice repair works
A splice repair removes the rotten section and joins in new timber. We cut back to sound wood, shape a matching piece and fix it in place. We then prime and finish it so the repair blends in, because a neat finish also protects the join.
This works when the rot is local and the rest of the sill is firm. The small splice repair is £80 per window. A bottom-rail repair, for the rail on the lower sash, is £120.
The big benefit is that you keep the original sill and its profile. For a period home, that matters because the shape is part of the look. It also costs far less than starting again.
When you need a full sill
Replacement makes sense when rot runs along most of the sill. It also makes sense when the timber is soft right through, or the sill has been patched several times. A new sill gives a clean start, since no rot is left to spread.
We replace the sill and fit it to the original profile. Sill replacement is £120 per window. The work includes sealing and finishing, so water runs off as it should.
If the rot has spread into the frame, the job grows. However, we always check this before quoting. Our sash window sill repairs page explains what we look at.
Timber or resin
Two materials are common for repairs. Timber is the traditional choice. It matches the original, takes paint well and can be worked later.
Resin fillers and repair systems are another option. They suit small areas, because they bond to sound wood and set hard. However, they do not suit large or structural damage, and they are not a fix for wet timber.
For most sills, we prefer new timber. It behaves like the old wood and moves with it, so joints stay tight. As a result, the repair lasts longer and looks right.
Why you should not wait
Rot spreads because moisture moves along the grain and into neighbouring timber. A sill you could splice this year may need replacing next year.
Damp can also reach the frame, the bottom rail and the wall below. By then, the repair is bigger and the quote higher. A rotten sill also hides cracks where water can sit, so the cycle repeats.
Looking after the paint helps. Our sash window maintenance guide shows how to keep sills sealed. Still, if you have already found soft timber, repair comes first.
What else to check at the same time
A sill job is a good time to look at the rest of the window. Still, loose sashes rattle, so see our post on why sash windows rattle. Stiff windows often have too much paint, and our guide to painted shut sash windows covers that.
If you are thinking about better glass, read our comparison of slim double glazing and vacuum glazing first. New glass adds weight, so the sill and frame should be sound.
Listed and conservation-area homes
Sills on listed homes need care. Like-for-like repair keeps the original look. We handle listed and conservation work case by case. Our article on sash windows in conservation areas across London explains how the rules work.
Noise and sills
A sill that has shrunk or split can leave gaps that let sound in. Fixing it helps a little. For bigger gains, see our guide to sash window noise reduction.
Local examples
Sills take a beating in damp, busy streets. Many Victorian terraces have original sills, so sash window repairs in Hammersmith and Fulham can include sill work. Older terraces in homes across Tower Hamlets face the same problem.
Meanwhile, sash window repairs in Lewisham cover many Victorian and Edwardian houses with original sills. Each one is checked on site before we quote.
Cost summary
Here is the short version. While prices vary by window, a small splice repair is £80 per window. Sill replacement is £120 per window. A bottom-rail repair is £120, and a full overhaul with draught proofing is £188 per window.
Our sash window repairs service covers all of these. For the full list, see our costs page.
Get a sill checked
Send us your postcode and a short description of the problem through our contact page. We arrange a free site visit and a written quote. The five-year no-quibble guarantee covers our labour.