Types of Sash Window: Box Sash, Spiral and Mock Sash Compared

"They don't make the parts for that any more" is a true sentence about some sliding windows and a plain untruth about most of them. Which one it is gets decided inside the frame, where you cannot see it, by whatever is holding the sash up. If that is wrong at the start then everything after it is wrong too: the quote, the lead time, and the verdict on whether the window is worth keeping at all.

Four quite different mechanisms sit behind windows that all read as "sash" from the pavement. Two are repairable more or less indefinitely out of materials any joiner carries in the van. One depends on a proprietary component that may have been discontinued twenty years ago. One has no mechanism in it whatsoever.

The four mechanisms behind a London sash

The names of the timbers involved are worth having straight before anybody quotes, and our illustrated glossary of sash window components labels the lot.

Cords and weights: the window that does not wear out

This is what London is largely built out of. Two sashes, each hung on a pair of cords that pass over axle pulleys at the head of the frame and drop into a hollow box formed by the pulley stile, the linings and a back lining. Inside each box hang two weights, usually cast iron, sometimes lead where more mass has to be squeezed into a shallow box.

Nothing in that arrangement has a service life the way a machine does. The pulleys are bearings, the weights are lumps of metal, the cord is rope. When a box sash stops working, a consumable has worn out or the timber has taken water, and both are renewable at the bench.

Glass sets the weights, and reglazing moves them

Each sash is carried by two weights, one a side, and the pair should come to roughly the weight of the sash. On single-glazed London windows that usually puts each weight somewhere in the 4kg to 9kg bracket, so most sashes come out between about 8kg and 18kg complete. Big plate-glass sashes over a ground-floor bay go well beyond it.

Glass is what moves the sums. Ordinary 4mm float weighs around 10kg per square metre and 6mm around 15kg, so a reglaze changes the balance whether anyone intends it to or not. Put thicker glass or a sealed unit into an old sash, leave the original weights swinging in the boxes, and the window will run beautifully and then creep downwards. Nothing you do to the catch will fix that; working out the right sash weight will.

A good joiner does not balance the two sashes identically either: the top one goes very slightly heavy so it holds up tight against the head, the bottom very slightly light so it settles onto the cill rather than creeping up in a draught.

Cord size, and what the number on the coil means

Cord is sold by diameter, and the ordinary domestic default is 6mm. It covers upper-floor and medium sashes without argument. Eight millimetre is kept for big ground-floor bays, thick glass, and sashes that have been reglazed with sealed units. Old catalogue numbers are still said on site, but suppliers never numbered the diameters consistently between them, so buy by the millimetre and read the sash weight printed on the coil. The materials and sizes are set out in our guide to choosing sash cord.

The rating is the part that gets misread. It describes the sash, and each cord is only ever asked to hold one weight, a little over half of it. A correctly sized cord therefore spends its whole working life a long way inside its own band. Cords do not snap because the sash is heavy. They snap at the pulley end, because that is the one point that bends around a wheel every time the window moves, so decades of abrasion get concentrated into a few inches while the rest of the length stays sound. The visible part tells you almost nothing.

That is also why replacing one cord is poor value. The other three are the same age and have done the same work over the same wheels, so our sash cord replacement service does all four, the pulleys and the balance in a single visit.

When only the bottom sash moves

A genuine single-hung window has a fixed top sash, built that way to save money on rear elevations and upper floors of modest terraces. Far more often what you have is a double-hung window whose top sash was painted shut, screwed through the staff bead, or lost its cords in about 1974 and got wedged. Two pulleys a side is the tell. Freeing the top sash is not a large job and it matters more than people expect, because opening top and bottom together is the whole ventilation logic of a sash window.

Spring balances, and the three mechanisms sold as one

From roughly the 1960s, and in almost every timber or uPVC vertical slider made today, the weights and the boxed frame gave way to a balance built into the jamb. Three quite different mechanisms get lumped together as "spring balances". They fail in different ways and cost different money, so it is worth knowing which one is in the groove.

Spiral balances. A tube containing a coil spring and a twisted rod. The tube sits in a groove routed into the frame, the rod hooks onto a shoe in the edge of the sash, and the fitter winds it a set number of turns before hooking it on. Tension has to suit both the weight of the sash and the length of its travel, which is why balances are ordered by load range and drop rather than bought generically, and why over-winding a tired one snaps the spring instead of curing the sag.

Constant-force balances. A coiled ribbon of spring steel that unrolls as the sash comes down, on the principle of the return spring in a tape measure. There are no pulleys and no cord in it. The name is the point: the force barely changes across the travel, so the sash holds as well near the head as it does near the cill.

Block-and-tackle balances. A coil spring working through a small pulley block on a cord inside a channel, gaining mechanical advantage the way a block and tackle does. They are not a variant of the constant-force type and share no parts with it. Because the pulleys multiply what the spring can hold, they carry more weight for their size and turn up in heavier double-glazed sliders, and when they give up it is usually the cord or the sheave rather than the spring.

The trade-off is identical for all three. A spring-balanced window is cheaper to make, needs no boxed frame, and takes far less wall thickness. It also has a finite life. Springs relax, tapes fatigue, block-and-tackle cords fray, and after fifteen or twenty years the sash starts sinking. Putting it right means matching a proprietary part to a profile that may no longer be manufactured, which is why a failed balance on a 1990s window is sometimes the moment the whole window gets condemned. A sash cord costs a few pounds and fits anything ever made.

The horizontal slider, which has no balance at all

Instead of running up and down, two sashes sit side by side in a wide opening and slide horizontally in rebated runners at head and cill. Gravity is not fighting you, so there is no counterweight and no spring anywhere in it. The sash is held by friction and a catch. The type is northern vernacular, built for the low wide openings of stone cottages, and in London you meet it mostly in later infill, mews properties and rear additions.

They are easy to keep going. The usual faults are worn or dirty runners, a dropped cill letting the sash bind, and weather driving in at the meeting stiles, which are harder to seal than the angled meeting rails of a vertical sash.

Windows that only look like sashes

Three different things get called a mock sash, and the distinction decides whether there is anything to repair.

• A fixed or dummy light. A non-opening sash made to match the ones either side, common in Georgian tripartite windows and in bathrooms. Nothing wrong with it. Do not pay anybody to "free off" a balance that was never fitted.
• A casement dressed as a sash. A side- or top-hung casement with applied glazing bars and a false meeting rail across the middle. From the pavement it reads as a six-over-six. From inside it gives itself away at once, because it swings. Our comparison of casement and sash windows sets out how differently the two behave over a lifetime.
• A modern vertical slider. A uPVC or engineered-timber unit that genuinely slides, runs on balances, and usually tilts in for cleaning. Fitting one is a replacement rather than a repair, which means it is a controlled fitting under the Building Regulations with a U-value target attached, where repairing the sashes in your existing box frame is not.

The reliable giveaway on any imitation is the glazing bar. An original is slender, often 18mm to 22mm across the face, with a fine ovolo or lamb's tongue moulding and putty holding real glass into a rebate. A bar stuck onto the face of a pane, or a spacer floating inside a sealed unit, reads as false from the other side of the street.

What each type costs to put right

The ranges below are typical London figures for a single window and they move a long way with access, condition and borough. The arrangement matters as much as the mechanism: a ground-floor bay carries the biggest glass and the heaviest weights in the house, and Edwardian margin lights multiply the puttying. Scaffold, height, or a flat above a shop can shift the number more than the work itself does.

Set that against £1,300 to £3,400 for a properly detailed replacement sash window in London and the arithmetic usually favours repair, though not on every window and not once several members need splicing. Our job-by-job breakdown of repair costs shows where each of those figures comes from, and we quote our own London sash work the same way, window by window rather than as a single whole-house number.

Telling yours apart with a torch and a knuckle

None of this needs anything taken apart. Six checks, in this order, will place almost any London window.

1. Knuckle the frame at about shoulder height, an inch or two out from the sash. A box sash sounds hollow, because that is a void with a weight swinging in it. A spring-balanced jamb sounds dead.
2. Look at the head of the pulley stile for a brass or steel wheel set into the timber. That is an axle pulley, and it is the clearest sign of a traditional box sash.
3. Follow the edge of the sash upwards inside the reveal. A rope means cords and weights. A thin metal tube, or a flat steel tape disappearing towards the head, means a balance in the jamb.
4. Push the bottom sash to head height and let go. If it stays there, something is counterbalancing it. If it drops, either the balance has failed or there never was one.
5. Push the top sash down. If it moves, the window is double-hung. If it does not, count the pulleys before assuming it never did.
6. Look along the meeting rails. Two real sashes cross on a pair of angled rails that wedge together as they close. A mock sash has a single rail applied to the face of a fixed pane.

Note the answers down window by window, along with anything soft in the timber under the glass. What you end up with is the house sorted into two piles: windows built from parts that can always be renewed, and windows that depend on a component somebody else has to still be manufacturing. Nearly every decision about money follows from which pile a window is in, and nothing you can see from the pavement will tell you.