In time, the sash cords from which the sashes are suspended will wear and eventually break. Replace both cords even if only one has broken.
Waxed sash cording is normally sold in standard hanks, although some suppliers sell it by the metre. Each sash requires two lengths of cord, measuring about three-quarters the height of the window. Do not cut it to length beforehand.
Lower both sashes and cut through the cords with a knife to release the weights. Hold on to the cords and lower the weights as far as possible before allowing them to drop.
Use a wide-bladed paint scraper to prise off the side staff beads from inside the frame, bending them in the middle until their mitred ends spring out.
Lean the inner sash towards you and mark the ends of the cord grooves on the face of the sash stiles (1).
Reposition the sash and carry the marks on to the pulley stiles. The sash can now be pulled clear of the frame.
Carefully prise out the two parting beads from their grooves in the stiles. You can then remove the top sash, after marking the ends of the grooves as before. Place the sashes safely aside.
To gain access to the weights, take out the pocket pieces which were trapped by the parting bead and lift the weights out through the openings.
Pieces of thin wood known as parting strips are usually suspended inside the box stiles to separate each pair of weights. Push the strips aside to reach the outer weights.
Remove the old sash cords from the weights and sashes, and clean up the wood ready for the new cords.
The top sash is fitted first, but not before all the sash cords and weights are in place. Clean away any build-up of paint from the pulleys. Tie a length of fine string to one end of the hank of sash cording. Weight the other end of the string with small nuts or a piece of chain. Thread the weight, known as a mouse, over a pulley (2) and pull the string through the pocket opening until the cord is pulled through. Attach the end of the cord to the weight with a special knot.
Pull on the cord to hoist the weight up to the pulley, then let it drop back about 100mm (4in). Hold it temporarily in this position with a nail just below the pulley. Cut the cord level with the mark on the pulley stile (3). Repeat this procedure for the cord on the other side, and similarly for the bottom sash.
Replace the top sash on the sill, lean it towards you and locate its cords in the grooves in the stiles. Nail the cords in place, using three or four 25mm (1 in) round wire nails. Nail only the bottom 150mm (6in), not all the way up (4). Lift the sash to check that the weights do not touch bottom.
Replace the pocket pieces and pin the parting beads in their grooves. Fit the bottom sash in the same way. Finally replace the staff beads, taking care to position them accurately or you may trap the bottom sash.