How do I replace a broken sash cord?

A box sash window is a counterweight system with four cords and four weights. Replacing one is a satisfying afternoon's work, and doing all four while it is apart is barely longer than doing one.

Safety

What you'll need

• Waxed sash cord, 6mm for most windows
• A chisel and a bolster
• A sharp knife
• Pliers and a small mallet
• String and a small weight for threading
• Wax candle stub

Step by step

1. Score the paint and remove the staff bead.The moulded strip on the room side that holds the bottom sash in. Score along both paint lines first, then ease it out from the middle rather than the ends.
2. Take out the bottom sash.Swing it into the room, and cut or untie the cords, holding each one so the weight does not crash down the box. Tie the cut end to a nail or wedge it in the pulley so it cannot vanish.
3. Remove the parting bead if the top sash needs doing.The thin strip in the groove between the two sashes. It usually pulls out with pliers, though decades of paint may need scoring first.
4. Open the pockets.Small removable covers near the bottom of each stile, sometimes screwed, sometimes just wedged and painted in. Behind them are the weights.
5. Note which weight goes with which sash.The outer weights serve the top sash and the inner weights the bottom sash — or the reverse, depending on the window. Label them before they come out.
6. Weigh the sash if you are unsure.Bathroom scales. The pair of weights for a sash should together roughly equal that sash's weight, or a fraction under for the bottom sash so it stays down.
7. Thread the new cord.A length of string with a small weight — a 'mouse' — dropped over the pulley and fished out of the pocket, then used to pull the sash cord through.
8. Use waxed cord, not nylon.Waxed cotton sash cord grips the knot, runs quietly over the pulley and does not stretch. Nylon slips, stretches and squeaks.
9. Knot into the weight properly.Thread through the eye and finish with a figure-of-eight or a double knot, then singe or tape the tail. A knot that pulls through drops the weight to the bottom of the box.
10. Set the cord length carefully.With the sash fully down, the weight should hang about 50mm above the bottom of the box. With the sash fully up, the weight should not touch the pulley. Get this right before nailing the cord in.
11. Fix the cord into the sash groove.Clout nails through the cord into the groove in the sash stile, three or four of them, with the top nail well below the level of the pulley so it never fouls.
12. Reassemble in reverse.Top sash, parting bead, bottom sash, staff bead. Rub a candle stub in the runs before closing up — it transforms how the window slides.

Tips

• Replace all four cords while the window is apart. The other three are the same age as the one that broke.
• Photograph each stage. Sash windows go back together in exactly one order and it is easy to forget which bead came from where.
• A candle stub rubbed into the pulley stiles is the traditional lubricant and works better than anything modern, because it does not attract dirt.

Common mistakes

• Letting the cut cord go into the box — The weight drops to the bottom and the cord end disappears over the pulley. Retrieving it means a much more awkward fishing job through the pocket, and on some windows the pocket does not reach.
• Using nylon cord — Nylon stretches under load, so the sash position drifts over the following months, and its slippery surface means knots creep. Waxed cotton sash cord costs very little and is what the system was designed around.

If it doesn't work

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