How to Replace a Sash Cord: The Full Sequence, Start to Finish

A parting sash cord announces itself twice: the rope lets go at the pulley, then a dull thump inside the wall as the weight lands. Nothing is lost. The weight is still in the box and the opening that gets it out is still there.

Cords part within an inch or two of the pulley nearly every time, and that is not bad luck. It is the one point on the run where the rope bends around something and straightens again on every stroke, for a century. The fibres on the outside of that bend abrade, the wax works out of them, and the cord fails where it has worked hardest. The other three are the same age, at the same wheel, with the same wear. You are not replacing one cord. You are replacing four.

Whether this is your job or a joiner's

This is one of the genuinely doable jobs on a box sash, and also the one most often half-done. A confident DIYer with a chisel and patience will take two to three hours over the first window, less over the second. Do it yourself if the sashes come out without a fight, the beads are in reasonable order, the whole window is reachable from inside on the ground or first floor, and the timber is sound.

Get someone in if the sashes are painted solid into the frame, you cannot find a pocket, the weights are missing or wrong, the box has been boarded over or plastered across, the bottom rails feel soft under a bradawl, or the window is a large bay above head height. Also if the property is listed and beads or sashes will need remaking rather than refitting, because that shifts the job from maintenance to alteration.

One warning before you put a blade near the paint. In anything built before about 1970, the older layers may contain lead. HSE's advice is plain: do not dry-sand it, and do not burn it off with a blow lamp or gas torch. Work mechanically, or with a chemical stripper and wet abrasive, and keep the dust down.

Cord, nails, and a mouse

• Cord. Traditional waxed cotton, braided, in 6mm, 7mm or 8mm. Size it on the maximum sash weight the maker quotes, not on breaking strain, which varies so wildly between suppliers it is no use as a buying figure. Buy pre-stretched and buy generously — reckon roughly 4m per sash. The differences between waxed cotton, nylon and jute matter more than people expect.
• 25mm galvanised clout nails for the cords, and 40mm oval or lost-head nails for the beads.
• A stiff wide chisel, a bolster or bead prise, and a thin painter's knife.
• A mouse: a small lead weight or a length of light chain on about 2m of thin string, to carry the new cord over the pulley and down into the box.
• Bathroom scales, and somewhere clean to lay the sashes down.

Those are Mighton's figures; other makers rate theirs a few kilos either side. They describe the sash, not the individual weight. Most London domestic sashes sit happily on 6mm or 7mm.

Taking the window apart, in order

1. Score the paint. Run a sharp knife firmly down every joint you are about to open: staff bead to inner lining, staff bead to pulley stile, and around the pocket if you can make it out. Paint film is stronger than the timber under it and will tear the face off a bead if you do not cut it first.
2. Remove the staff beads. Start in the middle of a long one, get a wide chisel or bead prise behind it and lever gently against a scrap of ply so you do not bruise the lining. Work outwards to the mitres, and pull the old pins through from the back rather than punching them out through the face. Number the beads in pencil as they come off — they were cut to this window and will not fit another.
3. Take the lower sash out. Swing the bottom of it into the room. Both cords are still attached. Cut them, or if sound, prise the nails out and keep hold of the ends.
4. Never let go of a live cord. The instant a cord comes off the sash, the weight runs away and the end disappears over the pulley. To keep an intact cord, tie a nail across it above the pulley or grip it in a bulldog clip.
5. Pull the parting beads. These sit in a groove in each pulley stile as a friction fit — no nails, no glue. Grip the middle with pincers, wiggle, and draw the bead straight out towards you. If it snaps, which it often does on a heavily painted window, buy new. Sections run roughly 8mm to 10mm thick by 20mm to 25mm deep and vary, so measure the groove rather than ordering blind. There is more on both in our article on staff beads and parting beads.
6. Take the upper sash out the same way.
7. Open the pockets. Look for a fine vertical joint in each pulley stile, usually 250mm to 350mm above the cill. There is often a single screw buried in paint, sometimes only a bevelled edge. Prise the pocket piece out and mark it left and right.
8. Get the weights out and label them. Note which came from which side and which sash, and keep the pairs together. Behind them you should find a parting slip, a loose 6mm strip of timber hanging on a nail that keeps the front and back weights from colliding. If it is missing, that is why the window clonks, and now is the moment to fit one.

Where the weights actually sit

The weight travels exactly as far as its sash does, and the box is barely taller than the sash, so the margins at both ends are tight. Each cord must be long enough that the weight can drop close to the floor of the box when the sash is at that end of its travel, and short enough that it never reaches the pulley at the other end.

The trap is thinking in terms of open and closed rather than travel. The two sashes sit at opposite ends of the frame when the window is shut, so their weights sit at opposite ends of the box:

So a closed bottom sash has its weights right up at the top, hanging just below the wheels, and they only run down to the floor of the box when you raise the sash. A closed top sash is the reverse: its weights are already down near the floor and climb as you pull it open. Get that backwards and you will cut a cord that lets the weight ground out a few inches into the stroke, with the sash stopping dead there. If the geometry does not sit right in your head yet, the piece on how a box sash balances itself draws it out.

Measuring and cutting the cords

Work on one cord, prove it, then match the other three to it. Cutting all four before you have tested one is how people end up with four cords 40mm short.

Drop the mouse through the pulley slot, fish it out of the pocket, tie on the cord and pull it through. Always the mouse first: pushing a stiff waxed cord over a pulley from above is a fifteen-minute job the mouse does in thirty seconds.

For the lower sash. Rest the sash on the cill in its closed position. Run the new cord over the pulley and down the face of the pulley stile, and mark it level with the top of the cord groove in the sash stile. Add about 50mm past the mark for the fixing. Now the weight end: with the sash still down on the cill, the weight belongs at the top of its travel, hanging clear of the wheel by roughly 50mm to 75mm and no more. Tie it on there. Then prove it before you cut anything else — lift the sash to the top of its travel and the weight should come down and stop about 75mm above the floor of the box, still hanging free.

For the upper sash. The positions swap, because this sash is shut when it is at the top of the frame. With it closed up against the head, its weights should be hanging about 75mm above the floor of the box, not resting on it. Set the sash in that closed position, mark the sash end the same way, then check by hand that pulling the sash fully down brings the weight up to within no closer than about 50mm of the pulley.

Both errors give you a symptom you can name:

• Cord too long — the weight touches down before the sash reaches the far end of its travel. A bottom sash rises a foot and stops; a top sash goes slack at the head and creeps down.
• Cord too short — the weight jams up against the pulley first. A bottom sash will not seat properly on the cill; a top sash will not pull fully open.

Knots, nails and the 40mm rule

At the weight end, tie a figure-of-eight about 75mm from the end of the cord, through the eye of the weight. It holds in waxed cotton far better than a plain overhand knot and will not jam solid when you undo it later. Tuck the tail so it cannot foul inside the box.

At the sash end, lay the cord into the groove on the outer edge of the stile and fix it with three or four 25mm clout nails at roughly 75mm centres. Two rules:

• Keep the topmost nail at least 40mm down from the top of the groove. Nailed too high, the head meets the pulley wheel at the end of the sash's travel, and either the sash stops short or you damage the pulley.
• The cord must lie flat. Any bunching or twist makes the sash run hard against the pulley stile and wear a shine into the paint within a season.

If the old knot hole at the top of the groove is sound, a knotted cord seated in it is the traditional fixing and easier to adjust later. Worn out, nail it.

Getting the balance right

Sash weights are not simply equal to the sash. The deliberate offset is why a properly hung window stays exactly where you put it. Weigh each sash on bathroom scales:

• The upper sash takes a pair of weights totalling slightly more than the sash, half a kilo or so over. Marginally overweighted, it holds itself up at the head and does not creep down.
• The lower sash takes a pair totalling slightly less, half a kilo or so under. Marginally underweighted, it sits down on the cill rather than riding up.

If the window has been reglazed, or a previous repair swapped weights about, they will very likely be wrong. Cast iron runs around 7.2 g/cm³ and lead around 11.3 g/cm³, so where a box is too narrow for a longer iron weight, lead is how you get the mass in. Our guide to working out the right sash weights takes you from a set of scales to the weights you need.

Rebuilding, and the tests that catch mistakes

1. Hang the upper sash first, on both cords, and run it through its full travel by hand. Listen for weights knocking the back of the box, for one grounding out, and for a nail head clipping the pulley.
2. Refit the parting beads. They should push home under hand pressure. A bead that has to be hammered in is too tight and the sashes will bind.
3. Hang the lower sash and test the same way, watching the top of the stroke. A bottom sash that runs sweetly for a foot then stops is a weight touching down, so that cord is too long.
4. Refit the pocket pieces and their screws.
5. Refit the staff beads in the positions you numbered, pinned not screwed, set so the lower sash runs without slack. With the window shut you should not be able to rattle it more than a millimetre or two front to back.
6. Test the balance. Push the bottom sash halfway up and let go. It should sit still. If it drops more than about 25mm the weights are light or a cord is snagging; if it climbs, they are heavy.
7. Oil the pulley spindles with light machine oil. Not grease, and not WD-40, which washes the old lubricant out and leaves nothing behind. If a wheel is seized rather than dry, change it while the window is open — our piece on how sash pulleys wear and squeal explains when one is past saving.

If it stops going to plan

Three things bring this job to a halt. The sashes will not come out because they are painted in, which is a stuck-sash diagnosis before it is a cord job. There is no pocket, because the frame has been altered or it was never a box sash and runs on spiral balances. Or the timber under a bottom rail crumbles under a bradawl, a repair to book before any new cord goes back in.

If that is your window, or the job is a bay above ground-floor level, our sash cord replacement service does the four cords, pulleys, parting slip and balance in one visit, and the wider sash window repair service covers every London borough.