Only one cord has snapped — can I just replace that one?

You can, but it is a wasted strip-down. The other cord on that sash has done identical work over the same pulley for the same number of years and typically fails within a season or two. Renew them in pairs, and do both sashes while the window is already open.

Why does the sash creep down slowly instead of staying put?

The weights are lighter than the sash. That usually means a weight has been removed, or heavier glazing has been fitted into an original frame without rebalancing. The cure is matching the weights to the sash, not tighter beads or friction pads.

Should I replace the whole window instead?

Rarely, if the timber is sound. The cords are a consumable in a design that was built to be serviced, and the joinery in an original box sash is often better than its replacement. Where the cills or stiles have rotted through, a repair or a spliced section is still usually less work than a new frame.

Can I use nylon cord or chain instead?

Waxed cotton sash line is the traditional choice and grips a knot well. Some joiners use chain on very heavy sashes. Thin general-purpose nylon is the wrong answer — it is slippery in a knot and cuts into the pulley wheel.

Caution

Updated 30 August 2026

Sash Window Won't Stay Open

A sliding sash drops shut on its own, or will not lift at all, because a cord has parted inside the box frame.

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What's happening?

A traditional box sash window is a counterbalance. Each sliding sash hangs on two cords that run over pulleys at the head of the frame and down hidden pockets on either side, where they are tied to cast iron weights matched to that sash. Break one cord and the sash goes lopsided and jams in the runner. Break both and it becomes dead weight that slams down as soon as you let go. The cords are the wear item — cotton or hemp sash line abrades over the pulley wheel for decades and eventually gives up, usually with a thump and a rattle inside the frame. The window itself is normally fine; what has failed is a consumable that was designed to be replaced.

What usually causes it?

Cotton or hemp sash cord worn through where it passes over the pulley wheel, which is the usual point of failure
Pulley wheel seized with paint or rust so the cord drags across a static edge instead of rolling
Cord knot pulled out of the weight or out of the mortice in the side of the sash stile
A previous repair using cord too thin or too weak for the weight it carries
Weights disconnected or removed during earlier draught-proofing or secondary glazing work
Parting bead or staff bead nailed or painted in so hard that a sash was forced, snapping a cord
A heavier sash fitted later — double glazed units in an original frame — that the original weights were never sized for

Is it dangerous?

An unbalanced sash is a genuine hazard rather than an inconvenience. With both cords gone the sash is unrestrained glazed weight above an open reveal, and it will drop hard enough to break glass or trap a hand — a real concern where children can reach the window. Approved Document K covers the risk of falling from windows and expects windows to open and close safely; a sash propped open on a book or a screwdriver is defeating that. An upstairs sash that cannot be opened is also worth attention because it may be the escape window for that room.

Can I fix it myself?

This is a repair you can do, but it is a full strip-down rather than a patch. The staff bead has to come off, the inner sash out, the parting bead out, the outer sash out, then the pocket covers in the frame lining removed to reach the weights. Both cords on a sash are replaced together, because a cord that has lasted as long as the broken one is next. Renew the cords on both sashes while the window is apart. The pitfalls are cutting the new cord to the wrong length so the sash will not travel fully, and splitting brittle painted beads on removal. Do not prop a sash open in the meantime — take it out or wedge it shut.

When should you call a carpenter?

Call a carpenter or joiner if the window is upstairs, if the sashes are large or heavy, if the beads are painted solid, or if the frame is in a listed building or conservation area where the original joinery matters. Also call if the weights are missing or plainly wrong for the sash, if the pulleys are seized, or if you find rot in the cill or the pulley stile while the window is apart — that changes the job from a cord renewal to a timber repair.

What will the carpenter actually do?

Remove staff and parting beads carefully so the original mouldings survive, and take both sashes out
Open the weight pockets, weigh or identify the weights and check they match the sashes actually fitted
Renew all cords in the correct size, free or replace seized pulleys and re-knot into the sash mortices
Set the cord lengths so each sash travels its full height and balances at any position without creeping
Ease the runners, overhaul the meeting rails and refit a working sash fastener and safety catch
Repair or splice any rot found in the cill, pulley stile or sash rails while the window is stripped

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Rotten Sash WindowCaution Rotten Window SillCaution Rotten Door Frame At The BottomCaution

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