A sash weight box is the hollow cavity inside the frame of a traditional sliding sash window in which the counterweights hang and travel up and down. It is not a hole in the usual sense but a vertical shaft in each side jamb of the frame.
How it works
A traditional sash window moves vertically. To make the heavy sash stay put at any height, a lead or cast-iron counterweight hangs on each side:
- A cord or chain is fixed to the top edge of the sash.
- The cord runs over a pulley set into the top of the jamb.
- At the other end hangs the counterweight, moving freely inside the weight box.
- Together the weights roughly match the weight of the sash, so it stays balanced.
The box is closed with a removable pocket cover — a small panel in the jamb. Without that pocket, a broken cord cannot be replaced without dismantling the whole frame.
What to watch for
- Find the pocket cover — look for a vertical joint or painted-over screws in the side jamb. Cut the paint film along the joint with a utility knife before levering, or the timber will splinter.
- Check the balance when reglazing — swapping single glazing for insulated glass makes the sash heavier and throws the balance out. The counterweights must be increased to match, otherwise the sash slides shut on its own.
- Insulation — the weight box is a vertical column of air in the wall and a well-known source of draughts. It can be insulated, but never so that the weights bind; keep the travel path clear.
- Lead weights — historic weights are often lead. Handle them with gloves and never saw or file them.
In listed buildings the weight box is an original construction detail. Replacing it with a modern spiral balance almost always reduces heritage value — check with the local authority first.
Related terms
- Window frame
- Window
- Pulley wheel
- Rebate
- Transom
- Fanlight
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