What Is a Box Sash Window? Parts, Anatomy and Where the Weights Live
If you live in a UK period home, there’s a good chance your windows slide up and down rather than swing open — and that they’re heavier and quieter than you’d expect. That’s a box sash window doing its quiet, clever work. In this guide we’ll explain exactly what a box sash window is, name every part so you can talk to a joiner with confidence, and show you where the famous counterweights actually live.
What is a box sash window?
A box sash window (sometimes written “boxed sash” or “sash box window”) is a traditional vertically-sliding window. Instead of hinges, it has one or two panels of glass — the sashes — that glide up and down inside the frame. The “box” part is the clever bit: each side of the frame is built as a hollow timber box, and inside that box hangs a heavy counterweight. The weight is connected to the sash by a cord running over a pulley at the top, so it balances the glass and lets the sash stay put at any height you leave it.
That counterbalance is what sets a true box sash apart. Open a box sash a few inches and let go — it stays exactly where you put it, because the weight on the other end of the cord matches the sash. There’s no spring, no friction stay, no propping it open with a paintbrush. Just gravity, neatly balanced.
It’s worth drawing one clear distinction early on. Not every vertically-sliding window is a box sash. Many modern (and some 20th-century) sliding sash windows use spring or spiral balances instead of weights — a coiled mechanism set into the frame that holds the sash against gravity. These have no box, no cords and no weights at all. They look similar from the outside but work in a completely different way. If your window frame is slim and there’s no hollow box to either side of the glass, you’re almost certainly looking at a spring or spiral balance, not a box sash.
The anatomy of a box sash window
Once you know the names, a box sash is surprisingly logical. Here are the main parts and what each one does.
- Box frame (the “box”): the hollow timber casing built into each side of the window. This is the cavity that hides the weights. No other window type has it.
- Pulley stile: the inner face of the box frame that the sash slides against. The pulley wheels are mounted near its top, which is where the name comes from.
- Upper (outer) sash: the top pane, which slides down. It sits in the outer channel, nearest the street.
- Lower (inner) sash: the bottom pane, which slides up. It sits in the inner channel, nearest the room. In a single-hung window only this one moves; in a double-hung window both do.
- Meeting rails: the two horizontal rails that meet in the middle when the window is closed — the bottom rail of the upper sash and the top rail of the lower sash. This is also where the catch usually sits.
- Glazing bars (astragals): the slim timber bars that divide a sash into smaller panes, giving you the classic Georgian “six over six” look.
- Counterweights: the heavy bars hidden inside each box that balance the sash. Traditionally cast lead, sometimes cast iron or steel.
- Sash cords: the cords (or sometimes chains) that connect each sash to its weights, running up the pulley stile and over the pulleys.
- Pulley wheels: the small wheels set into the top of each pulley stile that the cords run over.
- Parting bead: the thin strip of timber that separates the upper and lower sashes so they slide past each other without touching.
- Staff bead: the inner moulding that holds the lower sash in place against the frame and stops it falling into the room. It’s removable, which is how you access the sash for repairs.
- Weight pocket: a small removable panel low down in the box frame. Take it out and you can reach the weights inside — essential for re-cording.
- Sill (cill): the sloped bottom of the frame that sheds rainwater away from the building.
Here’s the same list at a glance:
| Part | What it does |
|---|---|
| Box frame | Hollow casing on each side that hides the weights |
| Pulley stile | Inner face the sash slides against; holds the pulleys |
| Upper / lower sash | The two sliding glazed panels |
| Meeting rails | Where the two sashes meet and lock |
| Glazing bars | Divide each sash into smaller panes |
| Counterweights | Balance the sash so it stays at any height |
| Sash cords | Connect each sash to its weights over the pulleys |
| Pulley wheels | Let the cords run smoothly at the top of the stile |
| Parting bead | Separates the two sashes so they slide freely |
| Staff bead | Holds the lower sash in; removable for repairs |
| Weight pocket | Access panel to reach the weights inside the box |
| Sill | Sheds rainwater away from the wall |
Where the weights live and how it all works
So, where do the weights live? Inside the box frame, one cavity on each side of the window, hanging vertically out of sight. A double-hung window has four weights in total — two for the upper sash and two for the lower — because each sash is hung from two cords, one on each side.
The mechanism is beautifully simple. A cord is tied to the side of the sash, runs straight up the pulley stile, passes over the pulley wheel at the top, and drops down inside the box to a weight. When you push the sash up, the weight comes down; when you pull the sash down, the weight rises. Because the weight roughly matches the sash, the two cancel out, and the sash floats wherever you leave it.
The balancing is the part that matters if you ever need replacements. As a rule of thumb, the total counterweight on each side should roughly equal the weight of that sash, split between its two cords — so each weight is about half the sash’s weight. Heavier glass, such as modern double glazing or thick old plate, needs heavier weights to balance it. Always weigh the actual sash where you can rather than guessing.
If you’d like the full detail on the wheels at the top, our guide on how sash window pulleys work walks through them. And because the weight material affects how much fits in that narrow box, it’s worth reading lead vs cast iron vs steel sash weights before you buy.
Timber box sash vs uPVC “box sash”
Traditionally, a box sash window is made entirely of timber — softwood or, on grander houses, hardwood. The box frame, sashes, beads and glazing bars are all wood, and the counterbalance uses real weights inside the box. This is what you’ll find in the vast majority of UK period homes, and it’s what most people mean by a box sash.
Modern uPVC and timber-alternative windows are often sold as “box sash” or “sliding sash” because they copy the look — slim profiles, deep bottom rails, run-through horns and astragal bars. But under the surface they usually work differently. Most uPVC sliding sashes use spiral balances rather than true weights, because a hollow uPVC box would struggle to carry the load of heavy lead bars. Some premium timber-alternative ranges do recreate a proper weighted box, so it’s worth asking the manufacturer directly if authenticity matters to you. The headline difference: a true box sash has a hollow box and weights; most uPVC lookalikes have neither.
A short history
The box sash window is a genuinely British invention, appearing in the late 1600s and coming into its own in the Georgian period. Georgian sashes are known for their fine glazing bars and the “six over six” arrangement of small panes, dictated by the size of glass that could be made at the time. As glass-making improved through the Victorian era, panes grew larger — Victorian sashes often show “two over two” or even single large panes, sometimes with decorative horns at the meeting rail. Edwardian homes kept the form but added their own flourishes, such as smaller panes in the top sash only.
This long run of fashion is exactly why so many UK terraces, villas and townhouses still have box sashes today. They were the standard window for the best part of two centuries, and a well-made timber box sash can last well over a hundred years with care.
Common problems with box sash windows
Box sashes are robust, but a few faults turn up again and again:
- Snapped sash cords. This is by far the commonest fault. When a cord frays and breaks, the weight drops to the bottom of the box and the sash either jams or crashes shut. The fix is re-cording — our step-by-step on how to replace sash window cords covers it.
- Draughts. Decades-old timber and worn beads let air through the gaps around the sashes. Draught-proofing brushes and refurbishment usually sort this without replacing anything.
- Paint seizing the sash shut. Generations of repainting can glaze a sash to its frame so it won’t budge. Careful scoring and easing frees it.
- Rattles. Loose-fitting sashes rattle in the wind, often because the parting bead has worn or the sash has shrunk slightly. Adjustment or new beads cure it.
- Missing or wrong weights. If a previous owner removed weights, or fitted the wrong size after a glass change, the sash won’t balance and won’t stay put. The answer is re-weighting to match the sash.
Restoring vs replacing
When a box sash plays up, the instinct is often to rip it out and replace it — but that’s rarely necessary, and frequently the wrong call. Because they’re built from individual timber components, box sashes are designed to be taken apart and mended. Snapped cords, worn beads, rattles and draughts are all fixable, and the original timber is often denser and better-seasoned than anything you could buy new. Restoration also keeps the character of a period home intact, which matters if your property is listed or in a conservation area.
Replacement makes sense when the timber is genuinely rotten beyond repair, but that’s less common than people fear. For a sense of what the work involves and what it costs, see our sash window restoration cost guide before you commit to anything.
Frequently asked questions
What’s the difference between a box sash and a spring sash? A box sash has a hollow box on each side of the frame holding real counterweights on cords. A spring (or spiral-balance) sash has no box and no weights — a coiled spring mechanism in the frame holds the sash against gravity instead. Both slide up and down; only the box sash uses weights.
Do all sash windows have weights? No. Traditional timber box sashes do, but spring-balance and spiral-balance sashes — common in newer windows and most uPVC sliding sashes — have none. If there’s no hollow box beside the glass, there are no weights.
Are box sash windows draughty? They can be, simply because of age — worn beads and shrunken timber let air through. But that’s a maintenance issue, not a flaw in the design. Draught-proofing brushes fitted during a refurbishment make an old box sash close snugly without spoiling its looks.
How many weights does a box sash window have? A double-hung window has four: two for the upper sash and two for the lower, with one weight on each side of each sash.
In short
A box sash window is a timber vertically-sliding window with a hollow box on each side, and inside that box hang the counterweights that keep each sash balanced at any height. Knowing the parts — box frame, pulleys, cords, beads and weights — makes every repair, from re-cording to re-weighting, far less daunting.
If your sashes have lost their balance and you need new weights, you don’t have to pay the first price you find. We’ve done the legwork so you can compare live £/kg prices for lead sash weights from UK suppliers and buy the right ones at a fair price.