The most common repairs to sash windows are replacing broken sash cords and damaged or loose beads. When a cord breaks, you have to take the sash right out of the frame to replace it - so it's a good idea to replace both at the same time. You can be sure that if one breaks, the other will soon go too.
Sash cord can be made from a range of materials in different diameters, such as waxed and unwaxed hemp and synthetic fibres. Make sure your new cord is the same diameter as the old one - so take a piece to the shop, if necessary.
Most sash cord comes in a pack containing enough for all types of window. Alternatively, you can measure from the top of your window to the sill and add about two-thirds as much again for each length of cord. You'll need two lengths of cord for each sash (upper and lower), but don't cut it to length in advance.
If you take off both sashes, make sure you label the weights so you know which comes from the upper (outer) sash and the lower (inner) one. Although they might look the same, they could be slightly different and if you put them back the wrong way round, your sashes might not run smoothly.
If you're replacing a lower sash cord, you'll have to take out your lower sash window. But if it's the upper sash cord, you'll need to remove the lower sash first and then the upper one.
Take off the left- and right-hand staff beads from the inside of the frame. Try starting in the centre of each bead so you don't damage the corners and use a mallet to tap a chisel into the joint.
When you've lifted a staff bead from the middle, the rest of it should pull away from the frame and spring out from the mitred corners.
If the cords aren't broken, tie some string around them near the pulley. You can use this to pull the new cord into place. Then cut through the old cord below the string and gently lower the weight to the bottom of its compartment - running the string over the pulley. Next, lift the lower sash and rest it on a table or workbench. Remove the piece of cord from each side of the sash and pull out any securing nails with pincers.
Prise the narrow parting beads out of their grooves. You'll probably find they're wedged in place, although they could be nailed or screwed. Lift the upper sash into the room if you're replacing all the cords (otherwise just leave it in place). Then cut and remove the cords in the same way as you did for the lower sash.
Use a chisel to prise off the pocket cover from each side of the frame. You'll probably find that the covers have just been pushed in place, although they might have been secured with screws or nails that you'll need to take out.
Lift out the weights in the pockets and remove the old cords. If you've attached string to a cord, just leave it in place over the pulley with one end in the pocket.
If you've removed both sashes, replace the cords on the upper sash first and fit it back in the frame. Then do the same for the lower sash.
A figure-of-eight knot in the sash cord will stop the weight slipping off the end. First, make a loop in the cord about 75mm from the end. Then take the end round the back of the cord to form a figure-of-eight. Finally, bring it back to the front and pass it through the first loop.
Where the cord was broken, tie a 50mm nail or screw to a piece of string and push it into the hole above the pulley. Then feed the string through the hole so it drops down into the weight compartment and you can retrieve it from the pocket.
Tie lengths of new cord - each at least one and a half times the depth of the window frame - to the ends of the strings hanging from the pulleys. Pull each cord over the pulley, down into the weight compartment and out through the pocket.
Remove the string from each cord, thread the cord through the hole in the top of the weight and tie it with a stop knot. Tuck the end into the cavity at the top of the weight, put the weights back in the pockets and replace the pocket covers.
Before replacing the lower sash in the frame, you'll need to refit the parting beads - unless you've taken out the upper sash, in which case you should put that back in first.
Tap the parting beads back into their slots with a mallet (you may need to use an old screwdriver or chisel to clean out any debris first). If the parting beads were nailed rather than pushed in, they're likely to have split when you removed them - so they'll need replacing. Measure the window and cut new parting beads to the correct length. Plane or sand them if they're too thick, then tap them into place.
Rest the lower sash on the sill and get a helper to pull the cords and raise the weights to their highest position. Tie a knot in each cord level with the knot holes in the sides of the sash and cut off the extra cord. If the knot holes are damaged, secure the cord with three or four galvanised clout nails and replace the sash in the frame.
Refit the staff beads on each side of the frame. You can do this by tapping them into place with the mallet, then nail them with 25mm oval nails - but don't drive the nails all the way in. Check first that the sashes move up and down without sticking and adjust the beads in or out if you need to.
When you're happy that your window operates smoothly, hammer the nails in all the way. Use wood filler to repair any damage before you touch up or repaint the whole window.