Lead Sash Weights: Working Out the Right Weight for a Sash

The weights hanging in the box beside your window were sized, by somebody with a set of scales in about 1885, for a pane of glass that may no longer be in the sash. Reglaze a Victorian window with modern 4mm float where 3mm cylinder glass used to be and you have quietly added a kilo and a half to a sash that was balanced to the ounce. Nobody adjusts the weights when they replace a cracked pane. So the window starts to creep, and thirty years later somebody decides the whole thing is worn out.

It is not worn out. The sum has changed and nobody redid it. Here is how to redo it.

Weigh the sash. Everything else is a check on that number

There is no shortcut worth having. Take the sash out, cut or unpin the cords, and put it on the scales.

Bathroom scales are accurate enough. Stand on them, note the reading, then pick the sash up and stand on them again. The difference is the sash. Do it twice, because a large sash held awkwardly gives a soft reading, and pencil the number on the top rail along with which window it came from.

Weigh the existing weights too, individually rather than as a pair. Plenty of London windows have two different weights on the same sash, because one broke or went missing and whatever was in the van went in instead. A mismatched pair makes the sash rack diagonally as it moves, grinding one stile and leaving the other loose.

The offset that stops a sash creeping

The single most common misunderstanding is that the weights should equal the sash. They should be close, and deliberately not equal.

• The upper sash gets a pair of weights totalling slightly more than the sash, in the region of half a kilo more across the pair. Being marginally overweighted, it pulls itself up against the head and stays shut through a winter of temperature swings.
• The lower sash gets a pair totalling slightly less than the sash, around half a kilo less. Being marginally underweighted, it settles down onto the cill instead of drifting up when you leave it open.

Split the offset evenly, so each weight on an upper sash is about 250g heavier than half the sash, and each weight on a lower sash about 250g lighter. Match the two weights on a given sash to within about 100g of each other.

What actually holds a sash where you leave it is not the weights alone. The counterweight cancels most of gravity, and a small residual friction between the sash and the beads does the rest. That is why the offset can be as small as half a kilo and still work, and why adding six runs of draught seal to a marginal window changes its behaviour. If the mechanism is not familiar, our explanation of how a box sash actually works sets out the whole system.

Working it out from the glass when you cannot weigh the sash

Sometimes you need a figure before the sash comes out, usually to decide whether a reglazing job is going to need new weights. You can get within a kilo with two constants.

Soda-lime float glass weighs 2.5kg per square metre for every millimetre of thickness. So 3mm glass is 7.5kg/m², 4mm is 10kg/m² and 6mm is 15kg/m². Baltic redwood, the timber most London sashes are made from, sits around 500kg per cubic metre at normal moisture content, so a sash frame in 45mm stuff with 50mm to 70mm stiles and rails comes out at roughly 3kg to 7kg depending on size, plus putty and paint.

That is a ready-reckoner, not a specification. Glazing bars add timber and reduce glass, and a sash with sixty coats of paint is heavier than the same sash stripped. Use it to sanity-check a weight you have taken off the window, or to see whether a proposed change of glass will move the sum enough to matter. Then weigh the thing.

Notice the range in the last column: roughly 3kg to 9kg a side covers most London domestic sashes, which is exactly the range you will find in the boxes.

Cast iron or lead, and why it is a question about volume

Both metals do the same job. The difference is how much space the mass takes up.

Cast iron is about 7.2 g/cm³. Lead is about 11.3 g/cm³. That is a little over half as much again per unit of volume, and it is the whole reason lead weights exist.

Take a cylindrical weight 44mm across and 300mm long. In cast iron that is roughly 3.3kg. Cast the identical cylinder in lead and it is roughly 5.2kg. Same hole, nearly two kilos more counterweight.

That matters because the weight box is not generous. The cavity behind each pulley stile is typically 100mm to 115mm from front to back, and it is divided down the middle by the parting slip, a loose 6mm strip hung from a nail whose only job is to stop the front weight and the back weight colliding. That leaves two channels of roughly 45mm to 50mm each, one for the upper sash's weight and one for the lower's. Weight diameters in practice run from about 32mm to 50mm, and a weight that fouls the parting slip will knock and eventually saw through it.

So the decision goes like this. If there is room, use cast iron: it is cheaper, it is what was there, and it is inert. Where the box is narrow, where the sash is unusually heavy for its size, or where new glazing has pushed the mass up, lead is the way to get the figure without a fatter cylinder.

There is a second constraint people discover the hard way. The pocket is the only door into the box and it is usually only 250mm to 350mm tall. A weight much longer than that has to be tilted in through the opening, and in a 45mm channel there is very little room to tilt. That is another argument for density over length.

What reglazing does to the arithmetic

This is where most wrong weights come from, so it is worth doing the sums explicitly for a 900mm by 800mm sash with 0.55 m² of glass.

A change from 3mm to 4mm float adds about 700g per weight. That is enough to make a lower sash creep upward, and it is the reason so many windows behave slightly wrongly for no obvious reason.

A slim double glazed unit is a different order of problem. Adding seven and a half kilos to a sash means adding nearly four kilos to each weight, and on a window that already had 5kg cast iron weights you now need 9kg in the same 45mm channel. That is a lead job, or a longer weight, or both, and sometimes it cannot be done without altering the box. Anyone quoting to put sealed units into your original sashes should be able to say what the new weights will be and where they will fit; if it has not come up, it has not been thought about. Our piece on slimline double glazing unit depths and trade-offs goes into the rest of that decision.

The cords have to be reconsidered at the same time. Waxed cotton cord is rated by sash weight, roughly 30kg for 6mm, 40kg for 7mm and 50kg for 8mm, so a substantial increase in mass may push you up a size. The differences between cord materials matter more once the loads go up.

The reasons yours are wrong

In rough order of how often we find them:

1. The sash was reglazed and the weights were left alone.
2. A previous repair swapped them. Top and bottom weights are not interchangeable, and neither are weights between two windows that look identical.
3. One weight is missing entirely, and the sash hangs on a single cord, or on a length of chain and a brick. Both exist in London.
4. The box was stuffed with insulation during a well-meant retrofit, and the weights now snag.
5. A new bottom rail was spliced in using a denser timber than the original.
6. Draught seals went in without anybody revisiting the balance, so the friction budget changed and a marginal window became a stiff one.

Getting it right, in order

1. Take the sashes out and open both pockets. Keep each weight with the side and sash it came from, marked in chalk.
2. Weigh the sash on scales. Weigh each existing weight separately.
3. Do the sum. Upper sash: target pair total = sash mass + about 0.5kg. Lower sash: target pair total = sash mass − about 0.5kg. Halve each for the per-side figure.
4. Compare with what you have. Anything within about 250g a side is fine. Beyond that, change the weights.
5. Buy to the figure, not the appearance. Old weights often carry a raised numeral, usually pounds, but paint and rust make it a rough guide at best. Trust the scales.
6. Check the weight fits the channel before you commit: diameter clear of the parting slip, length able to pass through the pocket.
7. Hang the upper sash first, run it through its full travel, listen for the weight touching the back of the box or reaching the pulley, and only then do the lower.
8. Test the balance. Push each sash to halfway and let go. It should sit still. More than about 25mm of drop means light weights or a snagging cord. If it climbs, they are heavy. The full sequence for the cord side of this job is in our guide to replacing sash cords from pocket to parting bead.

Small shortfalls can be made up with a short lead make-weight hung below the main weight on the same cord, provided the knots are tidy and the combined length still clears the pulley. Building a chain of three weights a side is not a repair, and it will knock in the box for the rest of its life.

Reading the symptoms

That third row is worth dwelling on, because an upper sash that has crept down a few millimetres opens a horizontal gap across the whole width of the window at chest height, which is where the pressure difference across a London terrace is greatest. It is a draught problem with a weights cause, which is why sealing it without correcting the balance never quite works. The same logic applies to sashes that rattle in the wind.

What to do next

If one window misbehaves and the rest are fine, weigh that sash and its weights and you will find the answer in ten minutes. If every window creeps, look for a common cause: a reglazing campaign, a previous overhaul, or seals fitted without a balance check.

If you are having any work done that takes the sashes out, whether that is cords, draught-proofing or new glazing, that is the moment to have the weights checked, because the disassembly is already paid for. Getting it done separately costs the same labour twice, and while the sashes are out it is worth looking at the axle pulleys as well, since a worn wheel adds friction that no amount of correct weight will overcome.

We check the balance as part of any sash cord replacement, which is usually where this problem surfaces, and carry cast iron and lead weights on the van. If you would rather establish what you are dealing with first, weigh the sash and give us the figure: it tells us more than a photograph does. Our sash window repair service covers the London boroughs.