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Rotten Sash Window
Timber in a sash window has gone soft — usually the bottom rail of the lower sash or the cill below it.
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What's happening?
Decay in a sash window almost always starts at the bottom rail of the lower sash and at the cill, because that is where water sits. The Property Care Association describes wet rot as affecting almost any timber allowed to become excessively wet for a reasonable period, with the spread limited to the wet area — which is why a sash can have a soft bottom rail and perfectly sound stiles above it. Look for bleached or darkened timber, cracking, softening and loss of strength. A screwdriver pushed into the rail with light pressure tells you more than any amount of looking: sound timber resists, decayed timber takes the blade.
What usually causes it?
- Failed paint film on the bottom rail letting water into the end grain of the joints
- Putty fillets shrunk or missing, so rain runs down the glass and into the rebate
- Cill drip groove blocked with paint, so water tracks back under the cill instead of dripping clear
- Mortice and tenon joints in the bottom rail opening up, exposing the most absorbent end grain in the sash
- Condensation running off single glazing and standing on the rail all winter
- Previous repairs made with filler over decayed timber, which traps moisture instead of shedding it
- Pointing or render above the opening failing and delivering water onto the frame
Is it dangerous?
Medium. A sash with a decayed bottom rail can drop its glass, and a large pane falling out of an upstairs window is a serious hazard below. Rot in the box frame or cill can spread into the surrounding wall plate and joist ends, which turns a joinery repair into a structural one. Windows also matter for escape: Approved Document B relies on certain windows opening as an escape route, and a sash seized or rotten shut is not available. Loose or broken sash cords bring their own risk, because an unbalanced sash can fall shut on hands.
Can I fix it myself?
Very limited. You can and should keep the drip groove clear, renew failed putty and repaint bare timber before decay starts. You should not fill decayed timber — filler over soft wood hides the problem and holds water against it. Repairing a sash properly means cutting the decay out back to sound wood and splicing in new timber with matching sections and joints, which is joinery work with a spindle moulder or hand planes behind it. Removing a sash from a boxed frame also means dealing with the weights and cords.
When should you call a carpenter?
Call a carpenter or joiner once the timber is soft to a screwdriver, once a joint has opened, or once glass has become loose in its rebate. Call sooner if the decay is in the cill or the box frame rather than the sash, because that timber is doing more than holding glass. In a listed building or conservation area, repair rather than replacement is usually what consent allows, and splicing is the repair.
What will the carpenter actually do?
- Probe the sash, cill and box frame to map how far the decay actually runs, rather than treating what shows
- Cut the decayed timber out to sound wood and splice in matching new sections, remaking the joints
- Rebuild or replace the cill and reinstate a working drip groove so water leaves the frame
- Re-glaze and re-putty the rebate, and prime all exposed end grain before it is painted
- Overhaul the mechanism — renew sash cords, service the pulleys and rebalance the weights
- Ease the sash to a working fit and reinstate draught seals without binding the box
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