Sash Window Cord Replacement: A Complete Guide

A snapped sash cord is the single most common fault we find in traditional timber sash windows. One day the window glides; the next there’s a dull thump inside the frame and the sash suddenly weighs a ton, sits crooked, or won’t stay open without a prop. The good news: the mechanism behind it is beautifully simple, and the cord is the one part that was always designed to be replaced.

This guide explains how the box-sash counterweight system works, how to tell when a cord is failing, which cord to use, and how a professional re-cord is done step by step. We also cover UK costs, listed buildings, and the botched repairs we’re most often called in to put right.

We re-cord sash windows week in, week out across Cambridge, so everything below comes from the bench rather than theory.

How the counterweight system works

A traditional sliding sash window is properly called a box sash, and the name is literal. Down each side of the window runs a hollow timber box, concealed behind the linings, and inside each box hang two counterweights — usually cast iron, sometimes lead where space is tight. Each sash is carried by two cords, one per side. The cord leaves a groove in the edge of the sash, runs up and over a small wheel called an axle pulley near the top of the pulley stile, then drops inside the box to its weight.

When the weights roughly match the sash, the window is balanced: a heavy sash lifts with a couple of fingers and sits wherever you leave it, with no springs involved — just gravity pulling both ways at once.

A few other parts matter for this job:

When a cord parts, its weight drops to the floor of the box and that side of the sash loses all support. The sash immediately feels far heavier, cocks to one side and binds in its channels. On a lower sash that’s a nuisance. On an upper sash it’s a hazard: a heavy top sash with a failed cord can drop without warning onto fingers on the meeting rail, or slam hard enough to crack the glass. Wedge it and leave it alone until it’s repaired.

Signs a cord has failed — or is about to

A fully broken cord announces itself: the thump of the weight hitting the bottom of the box, then a sash that suddenly needs real effort to lift. The earlier warnings are easier to miss:

The SPAB — the Society for the Protection of Ancient Buildings — suggests a simple test we use ourselves. Take hold of the cord and bend it sharply. A sound cord flexes and springs back; a finished cord cracks along the surface or snaps outright. If it fails the bend test, replace it.

A sash that won’t move at all isn’t always a cord problem — paint-sealed joints and swollen timber cause the same symptom. Our guide to freeing stuck sash windows covers how to tell the difference before you force anything.

One broken cord means four new cords

It’s tempting to replace only the cord that snapped, but it’s a false economy — and not just our view: the SPAB advises that if one cord breaks, it is worth replacing all those on a window. All four cords went in on the same day and have flexed over their pulleys the same number of times; when one fails, the other three are on borrowed time.

Nearly all the cost of a re-cord is labour — dismantling, fishing the weights, rebuilding — while the cord itself costs pennies per metre. Renewing one cord today and another next winter means paying for the same dismantling twice. Doing all four in one visit also leaves time to service the pulleys and ease the channels while the sashes are out.

Choosing the right cord

Proper sash cord is braided — a woven cotton or synthetic jacket around a core — which keeps it round, consistent in diameter and smooth-running over a small pulley wheel. Twisted rope, washing line and general-purpose nylon cord have no place in a sash box: they stretch, flatten, and chew themselves to pieces against the pulley.

Two families of cord dominate the trade:

Sash cord is sold in numbered sizes, and the number that matters most is No. 6, which is 8 mm — the standard size for typical domestic sashes. The load figures show how differently the two materials perform at that same diameter: 8 mm waxed cotton suits a sash of around 50 kg, while 8 mm pre-stretched synthetic is rated to roughly 99.7 kg — close to double. For a very heavy Victorian sash glazed with thick or replacement glass, that margin is the difference between a cord that lasts and one that doesn’t.

Cord type Common size Suits a sash of about Best suited to
Waxed cotton No. 6 (8 mm) 50 kg Heritage and listed work; like-for-like renewal of original cotton cords
Pre-stretched synthetic (nylon or polyester) 8 mm 99.7 kg Heavy sashes and hard-worked windows where strength per millimetre matters

Whichever material you choose, the diameter must match the groove of the axle pulley. Cord that’s too fat binds in the groove and rides up over the flange; cord that’s too thin wears quickly and can jump the wheel altogether. Check the pulley before ordering.

The professional re-cord, step by step

Here is the sequence we follow on a standard double-hung box sash. An experienced pair of hands completes a window in a morning.

Opening up

  1. Remove the staff bead. Score the paint line with a knife, then spring the bead out from the middle of its length with a wide chisel — scoring first saves the surrounding paintwork.
  2. Swing out the lower sash. Intact cords are still under tension, so we ease each weight down inside the box before cutting — a free-falling weight can crack the pocket piece. Mark the old cord length before it disappears over the pulley.
  3. Remove the parting bead. It sits in a plough groove — in theory a friction fit, in practice often pinned and painted solid. It usually breaks coming out, which is no loss; we fit new parting bead as a matter of course.
  4. Remove the upper sash the same way, easing and cutting its cords in turn.
  5. Open the pockets and retrieve the weights. The pocket pieces come out of the lower pulley stiles — sometimes screwed, usually paint-bound. We label which box and sash each weight came from: upper and lower weights are often different, and swapping them wrecks the balance.

Fitting the new cords

  1. Service the pulleys. Each axle pulley gets spun, its fixing screws snugged up, and a drop of light oil on the axle. A seized or badly grooved wheel gets replaced now — new cord over a rough pulley is new cord ruined.
  2. Thread the cord with a mouse. A mouse is a small lead weight or a few links of chain tied to fine string. It drops over the pulley, down inside the box — on the correct side of the wagtail — and is hooked out through the pocket opening. Tie the string to the new cord and pull it up and over.
  3. Tie on and set the length. The cord is knotted securely to the eye of the weight, then cut so the weight gets full travel without over-running: never striking the pulley at one end nor settling on the box floor at the other. Get it wrong and the sash stops short of fully open or fully closed.
  4. Fix the cord to the sash. The cord lies in the groove down the sash edge, fixed with three or four clout nails placed low in the groove. The rule is absolute: every nail sits below the highest point the cord reaches, so no nail head ever crosses the pulley wheel. Some sashes instead take a stopper knot in a bored cord pocket — a tidy alternative that needs no nails.
  5. Ease the runs while everything is apart. The channels get cleaned back, eased where they bind and dressed with wax so the sashes run sweetly. The cords themselves stay bare — no paint, no varnish.

Rebalancing and closing up

  1. Check the balance. If the glazing has ever been changed, the sash is weighed and the weights adjusted to the rule described below.
  2. Rebuild in reverse. Pockets closed, upper sash re-hung and tested, new parting bead in, lower sash re-hung, staff bead refitted — set snug so the sash runs freely without rattling.
  3. Final test. Both sashes should travel their full height with light pressure and hold position anywhere in the frame. If a sash creeps or slams, the balance isn’t right and it goes back apart until it is.

Getting the balance right

Perfect balance is not the goal — a deliberate imbalance is what keeps a sash window shut. The SPAB rule of thumb is that the upper sash should be roughly 0.5–0.9 kg lighter than its combined weights, so the weights win and hold it closed up against the head of the frame. The lower sash should be roughly 0.5–0.9 kg heavier than its weights, so it rests firmly down on the sill.

When the bias runs the wrong way you get the classic complaints: a top sash that creeps open and leaves a draughty gap at the head, or a lower sash that flies up when unlatched or drops shut with a bang. The usual culprit is glass — thicker replacement panes add kilos, and a re-glazed window that was never re-weighted will not behave. The fix is made through the pockets, adding or exchanging weight until the bias is back where it belongs.

Sash cords in listed buildings

If your house is listed, cord renewal is one of the least complicated jobs you can commission: replacing sash cords like for like — with new cords of the same material — needs no listed building consent. It’s straightforward maintenance, not alteration.

That principle is also a good argument for waxed cotton on listed work: if the window was corded in cotton, renewing in cotton keeps the repair squarely like for like. Anything beyond cords — altered glazing, new joinery — is a different conversation, and worth a call to the conservation officer first.

What sash cord replacement costs in the UK

Published UK price lists for 2025–26 split into two quite different markets, and it helps to know which you’re looking at.

Fixed-price repair firms list cord replacement at roughly £70–£230 per window, with all four cords on a window typically £150–£230. This buys a defined job: the window is opened up in place, re-corded, eased and rebuilt, usually within a couple of hours.

Specialist sash-out joinery firms quote in the region of £280–£780 per sash, often under a minimum call-out charge. That looks steep until you read the scope: these are full overhauls, the sashes coming right out for timber repairs, pulley and bead renewal and thorough rebalancing, with the cords almost a footnote — you’re buying a restored window, not a cord.

Two notes. Repair work carries VAT at 20%, so check whether a quote includes it — £150 and £150 plus VAT are £30 apart. And these are market ranges, not our price list: every window differs in size, condition and access. For a figure for your own windows, get in touch and we’ll take a look.

Common mistakes we’re called in to put right

Most of the failed repairs we see come down to a handful of repeat offences:

Frequently asked questions

The questions homeowners put to us most often about sash cords. For anything broader, see our FAQ page.

How do you replace a sash window cord?

You dismantle the window from the inside: prise off the staff bead, swing out the lower sash, remove the parting bead, take out the upper sash, then open the pocket pieces to reach the weights. New cord is threaded over each pulley with a small mouse weight, tied to the sash weight, cut to length and nailed low in the sash groove. Both sashes come out even if only a top-sash cord has gone.

How much does it cost to repair sash window cords in the UK?

Published UK price lists for 2025–26 fall into two markets. Fixed-price repair firms charge roughly £70–£230 per window, with all four cords typically £150–£230. Specialist joinery firms that take the sashes away quote about £280–£780 per sash, often under a minimum call-out, because the visit includes a fuller overhaul. VAT at 20% applies to repair work, so check whether a quote includes it.

How does a sash window actually work?

Each sash hangs on two cords that run over small pulleys at the top of the frame and down into hollow boxes on either side, where they tie to iron or lead counterweights. When the weights roughly match the sash, the window is balanced: it lifts with light pressure and stays wherever you leave it. No springs — just gravity pulling both ways at once.

How long do sash cords last?

There is no fixed lifespan, and we would be wary of anyone who quotes one — it depends on the cord, the weight of the sash and how often the window is used. Instead, test the cord: bend it sharply between your fingers. If it flexes and springs back it has life left; if the surface cracks or the cord snaps, it is due for replacement.

Can you replace just one sash cord?

You can, but it is rarely worth it. All four cords on a window are usually the same age with the same wear, and the SPAB advises that if one cord breaks it is worth replacing all of them. Nearly all the cost is in dismantling and rebuilding the window, so doing one cord now usually means paying the same labour again within a year or two.

How do I stop my sash windows rattling?

Rattling means there is a gap between the sash and the beads that hold it, so the sash can move in the wind. The fix is to refit the staff and parting beads closer to the sash, or to fit discreet draught seals into the beads so the sash runs against a brush rather than bare timber. A re-cord is the ideal time to do this, because the beads are already off.

Can sash window weights be replaced or adjusted?

Yes. The weights sit behind removable pocket pieces low in the frame, so they can be taken out, swapped or supplemented without major joinery. This is how a window is rebalanced after heavier glass is fitted: extra weight is added, or cast iron is exchanged for denser lead where the box is too tight. The cords are usually renewed at the same time, since the window is already apart.

Get a quote