Sash windows are an intrinsic part of a period house and should be repaired rather than replaced if at all possible. Fortunately, although the mechanism may look complicated, replacing a broken cord is a job you can do yourself in a few hours.
If you have a broken sash cord, the window will be difficult to open and judder as it is raised or lowered. If both cords are broken, the window will not stay open.
It's best to renew all the cords if one has rotted.
Remember that sashes can be very heavy, so take care when working with them.
Removing the sash
Prise the narrow beading away from the interior of the window frame with a wide-bladed paint scraper or an old chisel. Keep both hands behind the blade when cutting and wear work gloves and safety glasses to avoid splinters.
Start in the middle of one of the side beads and bend the moulding gently outwards until it springs out of the corner joints. Remove the three other pieces and keep to one side.
Make a note of their positions to make replacement easier.
Removing the sash cord
It's best to have a workbench of some sort for the next part.
Swing the bottom sash window carefully into the room and support it on the bench. You now need to Gently lever out any nails holding the broken sash cords to the window frame. If you are replacing old one which are not broken, remember that the weight will still be acting on it, and will shoot down inside the frame when you release the nails on the sash. So make sure that you hold the cord with one hand while prising out the nails, to prevent this happening. You can then lower the weight gently.
Lever out the narrow parting bead which divides the two opening sashes and remove the upper sash in the same way.
The small slip of wood at the bottom of each frame side may be loose fitted or screwed in place. Remove this to reveal the lead weights. Vacuum out any building dust in the bottom of the recess and remove the weights and broken cord.
Replacing the sash cord
Pass a piece of string with a small weight at the end over the first pulley and allow it to drop to the bottom of the jamb. Attach the other end of the string to a length of the new cord and pull this over the pulley. Use a non-slip knot to tie the cord to the weight.
Most weights have a hole in the top, through which the cord can be passed. Once the cord has been threaded through this, a figure-of-eight knot can be tied and pulled back into the recess in the top of the weight.
Lightly oil the pulley if it is sticking.
Pull the weight upwards until it touches the pulley, and lower it around 100mm. Rest the upper sash on the window ledge and use galvanised clout nails to fix the cord into the groove on the side of the sash. The highest nail should be fixed at a distance down the groove, at least equal to the measurement from the top of the window frame to the bottom of the pulley, otherwise it will not close fully. Once fixed, trim off the end of the cord. Replace the parting bead by tapping it into its groove. Then repeat the procedure for the lower sash.
Easing the window
Check both windows work without juddering before replacing the beading.
Rub a candle up and down the sash sides to make them run smoothly. If the sashes still stick, you may need to take a few shavings off the sides with a hand plane.
Some larger windows may use chains instead of weights. Replace these with new chains, available from larger hardware stores.