The Complete Guide to Re-Balancing a Double Hung Sash Window
How to Re-Balance a Double Hung Sash Window
If your sash window won't stay open, slams shut, or feels heavy to lift, the counterweights inside the box frame are likely out of balance. Here's how to weigh, adjust and re-hang them correctly.
How the weight and pulley system works
A traditional sash window uses a hidden counterweight system to open and close smoothly. Each moving glass pane (sash) is connected to a cast-iron or lead weight housed in the wall cavity, linked via a cord or chain that glides over a pulley wheel installed in the window frame.
What you'll need
Gather these before you start, so you're not chasing parts mid-job.
The step-by-step process
Remove the sash window
Start by removing the stop beads (also called staff beads) — the strips of timber that run around the inside perimeter of the window frame and hold the sash in place. Carefully prise these away from the frame, then lift the sash clear so you can access the box frame cavity behind it. This is your starting point for everything that follows.
Take out the existing weights
The weights sit inside the boxed frame, behind a removable pocket. Take out the parting bead first, then lift the pocket away to expose the cavity. Untie the cord from the weight, feed the cord back up through the pulley, and lift the old weight out.
Weigh the window — top sash
Weigh the sash (bathroom scales work). Add 1 pound (approx. 0.46kg) to the total, then divide by 2. That's the weight needed on each side.
Note: on the top sash, the weights need to be slightly heavier than the sash itself. This makes sure the sash holds up in the head of the frame and doesn't fall down by itself.
If you need to make up the difference, add-on weights are designed to stack on top of each other until you reach the right total.
Weigh the window — bottom sash
Weigh the sash and divide the total by 2. That's the weight required on each side.
Note: unlike the top sash, where the weights need to be slightly more than the sash, the weights on the bottom sash need to be exactly equal. This ensures the sash stays down against the sill and doesn't rise up.
Run the new cord and hang the weights
Tie a small weight to the end of a length of string, and feed it through the pulley and down into the box frame cavity. Use the string to pull your new sash cord through behind it, then attach the new counterweight to the end of the cord.
Pull the weight up to the top of the pulley, and temporarily fix the cord off with a nail just below the pulley. This stops the weight dropping back down into the cavity while you work. Once the sash is attached to the cord, the weights will drop down. Repeat for all four weights: two per side, on both the top and bottom sash.
Re-hang the sash
Replace the pocket and parting bead, then lift the sash back into the frame. This step goes much easier with two people — one to hold the sash in place, the other to pin the cord into the side of the sash using galvanised nails.
Bonus: draught-proof while you're at it
Now that the window is apart, it's a great time to fully draught-proof it too — you're pretty much there already. All you need to do is replace the parting bead and stop bead with draught-proofed ones instead of the standard timber.
Read our step-by-step guide to draught-proofing a double hung sash window.
Bonus: patch up any rotten timber
While you're in there, it's also a good time to check the timber for any soft or rotten patches — sills and casings are common spots. A quality epoxy-based wood filler is an easy, long-lasting fix without needing to replace the whole section.
Read our complete guide to repairing rotten timber with epoxy.
Common questions
Why does the top sash need extra weight, but not the bottom?
The top sash hangs above its own weights, so the counterweight needs to be slightly heavier than the sash itself or it will drop under gravity. The bottom sash sits below its weights, so an even balance is all it needs to hold and seal at any height.
My window still won't stay open after re-balancing — what now?
Double check your weigh-in was accurate and that both sides carry equal weight. If it's still dropping, the sash may need a touch more than the standard 0.46kg allowance — add a 0.23kg weight to each side and test again.
Can I reuse my old weights?
Yes — old weights are fine to reuse as long as the cord and knots are sound. Add-on weights simply let you top up the total without sourcing a whole new set.
How do I know what size add-on weight I need?
Work out the shortfall using the calculations in Steps 3 and 4, then mix and match 0.23kg and 0.46kg weights — they're designed to stack on top of each other inside the cavity.
Need add-on weights to finish the job?
Stackable 0.23kg and 0.46kg weights, sized to fit standard box frame cavities.
A note on lead safety: older sash windows may have been painted with lead-based paint. Always take appropriate precautions when sanding, disturbing or working around painted timber on older windows — see SafeWork NSW's guidelines on lead work before you begin.
