If one of your sash cords is broken the window will not open and shut very well and may want to slide shut when open.

The two cords on either side each go into the sash box over a roller and are connected to a heavy weight which counter balances the window. This is how the window opens with little effort and stays where you want it. If you have a broken cord your window probably won't run smoothly or stay open.

How to change a window cord

This may look like a fairly daunting job as you may feel there is no obvious way to replace the broken cord with a new one, but it is fairly straight forward. You will need to remove the window so you may want to get someone to give you a hand at certain times. The lower window cord is easier to change. If you are changing the top window cord you will have to remove the lower window first.

Work from the inside.
On the side with the broken cord remove the vertical beading(this is called the staff bead). This holds the bottom window in place. It should only be nailed on however it may have been over-painted many times so might look more permanently attached than it actually is
.
Tip run a sharp Stanley knife down the join which will avoid chipping loads of paint off when you remove it. Using a hammer and chisel carefully prise off the beading.

You should now be able to pull the lower window out.

Remove they snapped cord end from the window. It will be nailed into the side groove with 2 or 3 large headed nails. If you choose, also remove the cord from the other side so you can put the window to one side for now or, if you don't want to do that, get someone to hold the window with the other cord still in place whilst you carry on.

Tip if you detach an unbroken cord from the window to put back later take care when you've removed it that it doesn't get pulled back into the box by the weight. (tie a temporary knot in it)

With the staff bead and window removed next remove the central runner(the parting bead). This can be a little tricky. Again this should only be nailed or even just wedged in place but will probably be well stuck with paint.

Use a Stanley knife to score down the paint inside and out. Then prise it out as best you can with a hammer and chisel. If you smash this in the process don't worry they are readily available at timber merchants

Now you have access to be able remove the loose wooden plate to get to the weights. Carefully prise it open with a chisel or screwdriver. Both weights for top and bottom window on that side are in this box separated by a loose piece of wood.

Lift out the weight for the broken cord then remove the old cord end from it.

Tie the new piece to the weight with a single strong knot. Pull it tight then trim off any surplus cord with a sharp knife.

Replace the middle wooden plate and replace the central runner(parting bead). If you've smashed the central runner getting it off they are readily available a good DIY stores or timber merchants. You may need to hold it in place with some extra nails. (if so drill a pilot holes first to avoid splitting the wood)

Re-nail the new cord to the window making sure the first nail is no closer than approx. 5-6 inches from the top of the window.(otherwise it may catch on the pulley wheel). Use 3 short nails with large heads.

The length of cord is also important. You need to judge the length is not too short or long. It needs to be of a length so that when the window is closed the weight doesn't bang on the top of the sash box, and when the window is fully open the weight doesn't bang on the bottom. Pull the cord with the weight on a few times to get a feel for where the weight is in the box.

Place the window back in place and check that it runs up and down smoothly. Replace the outer beading (staff bead)

Repaint chipped damaged paintwork.

NB use only proper sash cord for the job. It comes in different thicknesses for different weight windows. Match the new stuff with the old. Sash cord is made from specially waxed rope. Don't paint it as this will make it go brittle and reduce its life.