Sash Window Locks: Security, Insurance and the Rules Explained

Ask most homeowners how their sash windows lock and they’ll point to the small catch where the two sashes meet. Twist it shut and the window feels secure. The awkward truth is that the catch was never really designed as a lock — and if your insurance policy mentions key-operated locks on accessible windows, that little fastener alone is probably not what the underwriter had in mind.

The good news is that timber sash windows are easy to secure properly. The hardware is simple, discreet and quick to fit, made in much the same patterns for over a century, and none of it spoils the look of a period window. The confusion lies in the rules around it: what insurers actually ask for, what the building regulations do and don’t say, and what a British Standard on a door lock has to do with any of it — which, as we’ll come to, is nothing.

This guide covers the traditional fasteners and what they’re really for, the locks that genuinely secure a sash window and how each is fitted, the insurance conditions in plain English, and where the law does — and mostly doesn’t — come into it.

The catch on the meeting rails isn’t really a lock

Every traditional sash window has a fastener where the meeting rails cross. The most common is the fitch, or cam fastener: a crescent-shaped cam mounted on the inner meeting rail that rotates into a keeper on the outer one. As the cam turns, it draws the two rails tightly together. The other classic is the Brighton fastener: a hinged arm swings over a keeper and a knurled knob screws down to clamp it firm — slower to use, but it grips hard and never shakes loose.

Notice what both are doing: pulling the meeting rails into line and holding them there. Rails pulled tight stop the sashes rattling, close the gap that lets draughts pour through the middle of the window, and keep the top sash from creeping down. That — alignment and draught reduction — is the job they were designed for.

Security, though, is not what a standard fastener really offers. From inside it feels reassuring; from outside, on its own, it is not a serious obstacle. Key-locking versions exist and are worth fitting, but think of the fastener as the start of securing a sash window, not the end.

The hardware that actually secures a sash window

For timber sashes the effective options are a short list, every item small, traditional in appearance and reversible. Here is the field at a glance, then each in detail.

Dual screws: the workhorse

The dual screw — often sold simply as a sash screw — does the heavy lifting. It is a threaded barrel passing through the inner meeting rail into the outer one, plus a removable key-operated bolt that winds through the barrel and bridges both rails. With the bolt wound home the sashes are pinned together: neither can slide, and because the fixing runs through both meeting rails there is almost nothing on show to attack. Undo the bolt and the sashes run exactly as before.

Fitting is straightforward but unforgiving of shortcuts. The window must be fully closed and the rails properly aligned, because the hole is drilled through both rails in a single pass — a brad-point bit matched to the barrel diameter keeps the hole clean and stops the tip wandering. The barrel is then tapped into the inner rail, the receiver sits in the outer, and the bolt does the rest. On wider windows we fit a pair, one towards each end of the meeting rail.

The one non-negotiable is sound timber: a dual screw is only as strong as the wood its thread bites into, and a rail gone soft with damp or rot will let the barrel work loose. If a screwdriver tip sinks into the rail with hand pressure, it needs repairing before it needs locking.

Sash stops: security with the window open

If dual screws lock a window shut, sash stops solve the opposite problem: how to leave a window open for air without leaving it open to everyone. They are small stops fitted to the stiles of the upper sash, a set distance above the meeting rail. With the stops in place, the lower sash can rise roughly 100 mm — enough to air a room — then meets the stop. Turn a key and it comes out, freeing the window for cleaning or a fully open summer afternoon.

They come in two forms. Morticed stops sit in a recess bored into the stile, only a neat circular face showing — tidier, and our usual choice. Surface-fixed stops screw straight onto the stile: quicker to fit and kinder to very slim stiles, at the cost of being visible. Either way, sash stops are the only common sash hardware that gives locked-open ventilation — far more useful day-to-day than their size suggests.

Key-locking fitch fasteners

The simplest upgrade of all: a fitch fastener with a key barrel in it, so the cam cannot be rotated without the key. It is a like-for-like swap for the existing catch; where the base plate misses the old screw holes, they are plugged with glued hardwood pegs and fresh pilots drilled. You keep the traditional look and the rail-tightening function, and gain a lock where there was only a catch. On any accessible window we treat this as a companion to dual screws rather than a substitute for them.

Restrictors for children and night-time ventilation

Where small children live or visit, the widely used safety guidance is to restrict window openings to 100 mm or less — guidance worth following, though it is not law. Conveniently, that is the gap a standard sash stop allows, so one fitting covers night-time ventilation and child safety. What matters is position: stops set too high give an opening a child could get through — place them for the 100 mm gap first and convenience second.

What your insurer actually asks for

Many home insurance policies attach security conditions, and a common one reads along the lines of key-operated locks on all accessible windows. Accessible typically means every ground-floor window plus anything that can be reached from a flat roof, balcony, drainpipe or outbuilding roof. The condition has teeth: an unlocked or non-compliant window may affect a theft claim — not automatically, but it is not a risk worth running for the sake of a few small pieces of brass.

Two honest points. First, requirements genuinely differ between insurers: some spell out window locks in detail, some barely mention them. Never assume your policy matches your neighbour’s — read your own schedule and wording, and if it is ambiguous, ask your insurer to confirm what they expect. Second, on a timber sash window, meeting a key-operated-locks condition in both letter and spirit usually means dual screws on the meeting rails, ideally alongside a key-locking fastener. That combination leaves no room for argument.

BS 3621: a door standard, not a window one

A point of confusion we meet constantly. Homeowners read that their insurer requires BS 3621 locks and go looking for a BS 3621 window lock — which doesn’t exist. BS 3621 is a British Standard for thief-resistant lock assemblies on doors: the mortice deadlocks insurers ask for on final exit doors. It has nothing to do with windows.

For windows, the policy language is nearly always simply key-operated locks, with no standard number attached. If you have been hunting for certified sash window locks and coming up empty, stop: that certification belongs on your front door. On the windows, well-made key-operated hardware of the kinds described above is what the condition means. Knowing this saves money and fruitless searching.

Are window locks required by law?

This is the part most articles fudge, so let’s be precise about what the rules say — and what they don’t.

For an existing home: no

There is no law requiring you to fit locks to the windows of an existing house. Nobody will inspect them, and no regulation is breached by a sash window secured by nothing more than its original 1890s fastener. The pressures to fit locks are practical ones — your insurance conditions and common sense — not legal ones.

Approved Document Q applies to new dwellings

The building regulation people half-remember is Approved Document Q — Security: Dwellings, in England — which requires easily accessible doors and windows to resist physical intrusion, with windows built to a standard such as PAS 24. But its scope is narrow: it applies to newly built dwellings and to dwellings created by a change of use, such as an office or barn converted into a home. It is not triggered by repairing a sash window, overhauling one, or fitting locks to the windows of an existing house.

Fire escape matters more than most people think

In many homes, the upper-floor habitable rooms rely on a window as the emergency escape route. A key-locked window with its key in a kitchen drawer downstairs is a hazard dressed up as a precaution. The rule we give every customer: keep a key near each locked window — out of sight and out of reach of the glass, but somewhere everyone in the house knows about and can find in the dark. And if a window is a designated escape window, think twice before fitting anything that stops it opening fully; a keyed restrictor is a poor companion to a smoke alarm at two in the morning.

Listed buildings: ask before you drill

Fitting small, reversible hardware to a listed building is usually uncontentious — a dual screw or a morticed stop makes a modest, concealable intervention. But alterations to the windows of a listed building can require consent. Our advice is simply to have a conversation with the local conservation officer before drilling into a listed sash. It costs nothing, and it is a far easier conversation to have before the holes exist than after.

Security beyond the lock

A lock is only as good as the window it is fitted to. A rattling sash, rails gone soft with rot, or a top sash slumped on failed cords will defeat any hardware you screw to it — the meeting rails must close tight and true for a dual screw or fastener to do its job. Locks only hold if the joinery is sound, so overhaul first, lock second. If the cords have gone and the sashes no longer sit where they should, start with our guide to sash window cord replacement before adding hardware.

On genuinely vulnerable windows — a concealed rear ground-floor window, say — laminated glass is worth considering as an upgrade when a sash is being reglazed anyway. It holds together rather than falling out of the frame when broken, and looks no different from ordinary glass.

Finally, don’t underestimate visibility. Locks that can be seen from outside are part of the deterrent: a window that plainly carries key-locking hardware advertises that it will not yield quietly, and most opportunists move on.

Choosing finishes that suit a period home

Sash hardware comes in the finishes it always has: polished or lacquered brass, satin chrome, and bronze in various depths of brown-black. The right answer is nearly always to match what is already on the window — if the lifts and fasteners are aged brass, a satin chrome dual screw will shout at them from across the room. Where a window is getting all-new furniture, pick one finish and carry it through every piece.

Under the plating, materials matter more than looks. Solid brass hardware machines cleanly and its threads last; cheap plated zinc-alloy versions can look identical on day one, but on a dual screw the durability of the thread is the whole point of the fitting, and alloy threads wear and strip far sooner. Buy the solid brass version of anything with a thread in it.

Common mistakes we keep seeing

• Fitting locks into soft or rotten rails. The thread has nothing to grip and the fixing pulls out under a firm shove. Repair the timber first, then fit the lock into sound wood.
• Relying on the fitch alone on an accessible window. The catch is a draught and alignment device with a modest security bonus. On any window a person could reach, it needs company — dual screws at minimum.
• Painting over locks and keyholes. A decorator’s brushful of gloss in a key barrel or over a stop thread turns working hardware into decoration. Mask hardware before painting, or remove it and refit afterwards.
• Fitting so many locks the window never opens. A window that is a chore to unlock stays shut for years — and stiff sashes and condensation follow close behind. Fit what the window needs, not everything the catalogue offers.
• Losing the keys. Dual screw and sash stop keys are small and famously easy to mislay. Keep a spare set, labelled by room, somewhere the whole household knows about.
• Forgetting hidden security screws years later. A previous owner’s dual screws, painted over and invisible, are a classic cause of the mystery window that will not budge. Before forcing anything, read our guide to stuck sash windows — the cure may be a key, not a crowbar.

If you would like the locks on your sash windows looked at — or a window overhauled so its locks finally have something solid to hold on to — get in touch and we’ll be glad to help.

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