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Rotten Window Sill
The timber cill under a window has gone soft, dark and spongy, and a screwdriver pushes into it.
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What's happening?
The cill is the one part of a timber window designed to get wet, so it is the first part to fail. It sheds rain away from the wall, which means it stands in water every time it rains and dries slowly because it faces the sky. Underneath it there should be a drip groove — a small channel routed along the underside — that breaks the surface tension so water drops clear instead of running back into the wall. Blocked with paint, that groove stops working and water tracks inwards to the joint between cill and frame. That joint is end grain, which drinks water, and once timber stays above roughly a fifth moisture by weight, wet rot fungi can colonise it. Soft, dark, crumbly timber at the corners is the usual result.
What usually causes it?
- Drip groove on the underside of the cill blocked with successive coats of paint, so water runs back to the wall
- Paint film cracked on the top face, letting water in while sealing it from drying out
- Failed mastic or sealant between the cill and the masonry below, holding water against the timber
- Open joint where the cill meets the frame jamb, exposing end grain that soaks up water
- Cill laid flat or back-falling instead of sloping outwards, so rain pools rather than runs off
- Trickle vents or condensation running down the inside of the glass onto the internal cill
- Sapwood or unpreserved softwood used for a replacement cill in an exposed position
Is it dangerous?
The rot itself is slow, but three things follow it. A cill is what holds the bottom of the frame in position, so once it has gone the frame can drop and the glazing can loosen — a real concern on an upstairs window. Water that has passed the cill is now inside the wall, which is how a damp patch appears on the internal reveal and how the timber behind the plaster starts to decay. And an upstairs window is often the escape window for that room, so a frame that will not open safely matters. Wet rot needs sustained wetting, so the leak is the fault and the rot is the symptom.
Can I fix it myself?
Small, shallow damage yes; structural loss no. Probe the whole cill with a bradawl, not just the obvious dark patch — sound timber resists, rotten timber gives way with no effort. If the decay is a few millimetres deep and the cill is otherwise solid, cutting back to sound wood, treating and filling with a two-part timber repair filler is a legitimate repair. Then rake out and reinstate the drip groove, because leaving it painted over guarantees a repeat. What does not work is filling over soft timber — the filler seals the moisture in and the rot carries on underneath, out of sight.
When should you call a carpenter?
Call a carpenter or joiner if the probe sinks in more than a few millimetres, if the corners where the cill meets the jambs have gone, if the frame has dropped or the glazing has loosened, or if the window is upstairs. Also call if there is already a damp patch on the inside reveal, because the repair then includes finding how water is getting past the cill rather than just replacing the wood.
What will the carpenter actually do?
- Probe the full length of the cill and the jamb feet to map how far the decay actually runs
- Cut out decayed timber back to sound wood and splice in a new section, or replace the whole cill in durable timber
- Rout or reinstate the drip groove on the underside so water is thrown clear of the wall
- Set the new cill to a positive outward slope and re-bed it on the masonry with a sound weather seal
- Treat exposed end grain and prime all faces before fixing, including the underside that never gets painted again
- Check and repair the jamb feet and lower rail, which are usually wet if the cill was
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