How a Sash Window Works: Cords, Weights, Pulleys and Perfect Balance

A sash that has to be shouldered open, or that slides shut the moment you let go of it, is not a mysterious old-house problem. It is a machine with four working parts, and when one of them goes out of adjustment the window tells you which one by the way it misbehaves.

Understanding it is worth half an hour even if you never touch the window yourself, because it is the difference between accepting a quote for four replacement windows and knowing that what you need is eight lengths of cord and an afternoon.

The frame is hollow, and that is the entire point

What looks like a solid timber frame around a sash window is a box, and it is empty on purpose. Each side of the opening is built up from four separate pieces of timber: the pulley stile facing into the opening, the outer lining on the street side, the inner lining on the room side, and a thin back lining closing the assembly off against the reveal. Between them runs a cavity the full height of the window, roughly 100mm to 115mm from front to back on a normal London box.

Two weights hang in that cavity on each side of the window, one behind the other. The one nearer the room carries the lower sash; the one nearer the street carries the upper sash. They slide past each other every time a window moves, and the only thing stopping them from swinging into one another is the parting slip, or wagtail: a loose strip about 6mm thick, hung from a single nail at the head, dividing the cavity down its length into a front channel and a back one. That is why cast weights are long and narrow rather than compact, between 32mm and 50mm in diameter as a rule, and a weight fatter than its channel rubs on the slip and eventually saws through it. A missing slip is the commonest absent component in a London sash window, and the usual explanation for a dull clonk inside the frame.

At the bottom of each pulley stile there is a removable pocket piece, a short length of the stile cut out and refitted with a screw or a bevelled edge, generally 250mm to 350mm above the cill. That is the only way in: everything done to the weights happens through those two hand-sized openings.

Two sashes, two tracks, and the bead that divides them

The opening carries two sliding sashes, one in front of the other, running in separate tracks formed on the face of each pulley stile. The upper sash runs in the outer track, between the outer lining and the parting bead. The lower sash runs in the inner track, between the parting bead and the staff bead.

The parting bead is a strip of about 25mm by 8mm sitting in a groove machined into the stile, held by friction alone with no nails and no glue, so it lifts out to release the sashes. The staff bead on the room side, anything from 20mm by 15mm up to 32mm by 20mm, is pinned on, and it is the only thing holding the lower sash in the building at all. There is more on both in our piece on the small timbers that keep a sash running. Clearance between a sash and its beads is a couple of millimetres when the window is new; after a century of repainting and seasonal movement it is often four or five. That gap, along with the open pulley slots that let the weight box behave as a chimney, is where the draught comes from.

The arrangement is also why the upper sash sits outside: rain crossing it sheds onto the face of the lower sash and off the cill rather than into the room. The two sashes cross at the meeting rails, cut with a slight bevel so that throwing the fastener draws them together.

The pulley is a bearing, not a lever

An axle pulley is let into each pulley stile near the head, its faceplate mortised flush with the timber. The wheel is usually between 44mm and 50mm across and turns on a plain axle, in brass, cast iron or nylon depending on age.

It gives you no mechanical advantage. A pulley with a fixed axle only changes the direction of a force: ten kilos hanging on the box side is ten kilos pulling up on the sash side. All the wheel does is turn that pull through 180 degrees and, in doing so, replace the friction of a rope dragged over a sharp edge with the much smaller friction of a bearing.

Which is why a squealing pulley is worth attending to rather than living with. The squeal is a dry axle in a worn hole. As the hole goes oval the wheel tilts, the cord starts to rub on the edge of the faceplate slot instead of sitting in the groove, and a cord that should have lasted fifty years abrades in five. Our note on how axle pulleys wear covers what is worth saving.

The cord does all the work, and it fails where it bends

Traditional sash cord is 8-plait waxed cotton spun over a synthetic core, sold in the UK in 6mm, 7mm and 8mm. Most London sashes take 6mm or 7mm; anything unusually large, or reglazed with sealed units, wants 8mm, because cord is specified by the mass it carries.

At the sash end the cord drops into a vertical groove machined into the sash stile and is held by three or four galvanised clout nails driven through it into the bottom of the groove. Where the top nail sits matters more than people expect. Stand the sash in the opening, push it right up against the head, and mark on the sash stile where the pulley falls: every nail goes below that mark. Nail above it and the window runs sweetly for most of its travel, then stops an inch or two short of the head, because the top nail head has reached the pulley wheel and cannot pass it.

Cords almost always part within an inch or two of the pulley, and that is not chance. It is the only point in the run where the rope bends around something and straightens again, every time the window moves, for a hundred years. The fibres on the outside of the bend abrade, the wax dries out of them, and the cord lets go where it has been working hardest. The other three cords in that window are the same age and have done the same work, which is why replacing a sash cord is a four-cord job.

The weights are deliberately a little bit wrong

Each sash is carried by two weights, one a side, and the pair together comes to very nearly the mass of the sash. Not exactly, and the difference is intentional, running in opposite directions for the two sashes. The upper sash is hung a shade heavy, so it pulls itself up towards the head and holds there as the timber moves with the seasons. The lower sash is hung a shade light, so it stays down where you put it rather than drifting up when the window is left open. Half a kilo across the pair is enough in each case.

That asymmetry explains a window whose top and bottom weights were swapped in a previous repair: the top sash slides open on its own, the bottom sash climbs, and neither holds anywhere, though nothing is broken. Weights in a London house run around 3kg to 9kg a side depending on the sash and the glass, most of them cast iron, with lead where the channel is too narrow to fit enough iron in. Our guide to sizing sash weights sets out the arithmetic and what reglazing does to it.

Balance is counterweight first, friction second

Cancel gravity exactly and a sash would be in neutral balance. It would sit where you left it only until something nudged it, and in a London terrace something always does: wind on the front of the house, a door slamming downstairs. What pins a sash in place is the small residual friction between the sash edges and the two beads. The weights take almost all of the load off; friction holds the remainder.

The two have to stay in proportion, which is why so much well-meant work leaves a window worse than it was. Fit draught seals with the pile a couple of millimetres too tall and a sash that ran under one hand needs a shove to start and then runs away once moving. Plane a whisker off the staff beads in August and it will not hold position in February, when the timber has shrunk back. Get the counterweight right first and set the friction afterwards, against a window that is already balanced. Anyone fitting draught-proofing to a box sash without opening the pockets to see what state the cords and weights are in is doing half the job.

Five checks, and what each one tells you

About a kettle's worth of time per window. Do both sashes.

1. Let the lower sash go at halfway. It should not move. A slow slide of an inch and then a stop points at the balance or a stretched cord; a sash that runs all the way back to the cill has lost a cord, and usually racks diagonally on the way down.
2. Run the lower sash right up to the head. If it goes suddenly heavy in the last few inches, a weight has reached the bottom of the box and its cord has gone slack, so from there on you are lifting the sash yourself.
3. Pull the upper sash down halfway and let go. The same test in reverse. An upper sash that creeps down leaves a cold slot open along the meeting rails, right at chest height.
4. Move each sash slowly through its travel and listen. A rhythmic scrape is a cord riding out of the pulley groove. A knock near the top is a weight reaching the pulley. A dull clonk inside the frame is a missing parting slip. A gritty grind right through the stroke is a worn wheel, or seals with the pile too tall.
5. Look at the cord where it passes over the pulley, then move the sash and look again. Grey, furred and thin at the bend means the end of its life, broken or not.

Fail only the first check and you have a cord and weight job, the cheapest work on a sash window and the work that gives back most. Fail the fourth as well and the sashes have to come out, which is the moment to have the beads, cords and draught seals dealt with in one visit rather than paying three times for the same disassembly. A window that will not move at all is a different problem: diagnosing a stuck sash works through paint, swelling and seized pulleys separately.

The sash windows that have no weights at all

Not every sliding sash is a box sash. From the middle of the twentieth century, and on almost all modern replacements, the counterweight was replaced by a spiral balance: a spring inside a tube, screwed into a groove in the sash stile or into the frame and tensioned on installation with a key. There is no box, so the jamb is a solid section of 50mm or 60mm rather than a hollow assembly around 150mm deep.

You can tell from the pavement. A box sash shows a wide flat band either side of the glass, usually 130mm to 175mm, because that band is the box. A spiral-balanced window has a slim frame and nothing hidden inside it. Spiral balances fail by losing tension: the sash stops holding up, and the balance has to be replaced rather than adjusted. They do not last the way a cast iron weight does, which is indefinitely. Our comparison of the types of sash window covers the spring-balanced, horizontal and mock-sash variants.

The box sash has survived in London for 350 years for an unglamorous reason: every part of it is separately replaceable with hand tools, and the parts that never wear out are the expensive ones.

Deciding what to do about it

Most misbehaving sash windows are failing on cords, weights and beads, the cheapest components to put right, while the parts that would justify replacing the window, the box frame and the pulleys and weights inside it, are generally still doing their job.

Cords, weights, pulleys and beads are repair rather than replacement, so no Building Regulations U-value target and no FENSA or Certass certificate is engaged by any of it. Those arrive when somebody proposes taking the whole window out, which in a conservation area or on a listed building is also where the local authority becomes interested.

If you are not sure which fault you have, the noise the window makes is usually enough to tell over the phone. We do sash cord replacement and full overhauls across the London boroughs, and our sash window repair service treats the diagnosis as part of the visit.