uPVC window won't lock — causes, fixes and costs

A door that fails is annoying. A window that fails is a security problem you cannot see from the pavement, which is exactly why it gets ignored. If your uPVC window won't lock, the sash is sitting in the frame held by nothing but friction and a bit of weatherseal, and anyone who leans on it from outside will find that out before you do. This guide walks through what has actually gone wrong, which faults are a ten-minute part swap and which are not, and what the repair should cost in Manchester.

The good news first: in nearly every case the window itself is fine. The glass, the frame and the sash are almost never the problem. What fails is the hardware inside the sash — a handle, a spindle, a gearbox, a strip of steel — and every one of those parts is replaceable.

What is actually inside a uPVC window

It helps to know what you are talking about before you ring anyone, if only so you can tell whether the person on the other end knows either.

Turn the handle on a modern uPVC window and you are driving a gearbox, a small geared block sitting behind the handle. The gearbox pushes a long steel strip called an espagnolette, or espag, which runs up and down inside the edge of the sash. Bolted along that strip are mushroom cams — small steel studs with a lipped head. As the strip slides, the cams roll into keeps, the shaped metal plates screwed into the frame, and the lip pulls the sash tight against the seal.

That last detail matters. A mushroom cam does not just block the sash from moving outwards, it wedges it inwards as it turns. It is why a properly adjusted uPVC window feels like it clamps shut rather than simply latches. It is also why a sash that has dropped a few millimetres stops locking altogether: the cams and the keeps no longer meet.

Holding the whole thing up are the friction stays — the scissor hinges along the top and side that let the window swing open and hold it against the wind.

Six components. One of them has failed. Here is how to work out which.

The handle spins freely and nothing happens

You turn the handle, it goes round, possibly further than it used to, and there is no resistance at all. Nothing inside the sash moves.

This is a broken connection between the handle and the gearbox. Either the square spindle has sheared — they are usually zinc alloy and they fatigue — or the follower inside the gearbox, the square hole the spindle sits in, has worn round.

Handles fail this way for a mundane reason: people push windows shut by the handle. Every time you swing a heavy sash closed by pulling on the handle, you are twisting a 7mm spindle sideways. Do it for a decade and it gives way.

The fix: take the handle off first. Prise off the two screw covers, undo the fixings and pull it away from the sash. If the spindle is snapped or visibly chewed, a like-for-like handle usually solves it outright. If the spindle is intact and the gearbox still will not drive, the gearbox is the failure and the mechanism needs replacing.

Typical cost in Manchester: a handle replacement runs about £60–£110 fitted, depending on the pattern. An espagnolette or gearbox replacement is more like £90–£180 fitted.

The handle turns properly but the window still won't lock

Here the mechanism is working — you can hear and feel the strip moving — but the handle will not reach its final position, or it does and the sash still rattles.

Nine times out of ten the sash has dropped, and the cams are no longer meeting the keeps. Look at the gap between sash and frame all the way round with the window closed. If it is wider at the top than the bottom, or the seal is crushed on one corner and open on the other, you have your answer. This is the same fault we see on doors, and if you have read why a uPVC door won't lock the pattern will look familiar.

Worn friction stays are the usual cause of the drop. They carry the weight of a double-glazed sash on a small pivot, and after fifteen years or so they wear enough to let the sash sag at the leading edge.

The fix: on a small misalignment, the keeps can be slackened off and shifted a couple of millimetres, and many mushroom cams can be rotated a notch towards the frame to pull the sash in harder. On a genuine drop, the friction stays need replacing in pairs so the sash sits square again.

Typical cost: keep adjustment and cam setting, roughly £60–£120 for a visit covering several windows. Friction stays are usually £70–£140 a pair fitted, and the size has to be matched to the sash weight — the wrong stay will fail again within a year.

The handle is stiff and won't complete its travel

If you have to lean on the handle to get it round, stop leaning on it. That is the point at which a cheap repair becomes an expensive one.

Stiffness comes from the strip binding inside the sash: original factory grease that has dried to a varnish, grit from years of open windows, a cam catching the edge of its keep, or a sash that has expanded slightly in a warm spell. Manchester's summer heat is hardly Mediterranean, but a south-facing sash behind glass gets hot enough to move.

Forcing a binding espagnolette bends or shears one of the small components on the strip. At that point you no longer need a gearbox, you need the whole mechanism — and on some older window systems the exact strip is discontinued, which turns a routine job into a hunt for a compatible part.

The fix: clean the visible track and the cams, then use a proper silicone or PTFE lubricant along the strip and on each cam. Never use general-purpose penetrating oil or WD-40 as a lubricant here — it flushes the remaining grease out and leaves a residue that dust sticks to, and in a few months the mechanism is worse than when you started. The same principle applies to a stiff door lock, and for the same reason.

If it is still stiff after cleaning and lubrication, something inside is damaged and it needs opening up.

Older windows: cockspur handles and wedge blocks

Plenty of uPVC in Manchester dates from the late eighties and nineties, particularly on the terraces and semis around Old Trafford and the older stock in Salford. Those windows often have a cockspur handle — the type with a curved arm that hooks over a plastic or metal wedge block on the frame, rather than a multipoint strip.

Cockspur handles have two common faults. The arm wears until it no longer engages the wedge, so the window closes but does not pull tight; or the wedge block itself, if it is plastic, cracks and falls off.

Both are cheap parts, but the handle has to match the step height — the offset between the handle base and the arm, usually 9mm, 21mm or somewhere between. Buy the wrong step and it will not close on the wedge. If you are replacing one, take the old handle to the counter with you.

Worth saying plainly: a cockspur handle without a key is a weak point, and a single-point catch on a ground-floor window is not what any insurer has in mind when they ask for key-operated window locks. If your ground floor still runs on unkeyed cockspurs, upgrading is a small job with a real security return.

The lock is keyed and the key is lost

Many uPVC handles have a small key barrel that locks the handle in the shut position. Lose the key and the window is stuck locked, which is safe but inconvenient — and on a bedroom, potentially a fire safety problem, because emergency escape windows need to be openable from inside without hunting for a key.

Most window handle barrels are low-security and keyed to a small number of common patterns, so a replacement key is often available by pattern. If not, the handle is replaced. Either way it is a modest job, and it is worth doing all the handles in the house keyed alike at the same time so one key opens every window — the same logic we set out in our guide to keyed alike locks.

What to do tonight if the window won't lock

If you cannot get it repaired the same day, you are looking for a temporary measure that does not create a different hazard.

A sash jammer — a small steel arm that screws to the frame and swings across the sash — is the sensible short-term fix on a ground-floor window. It takes ten minutes and it survives a lot more force than a broken handle.

What not to do: do not screw the sash permanently shut, and do not fit anything that needs a tool to release on a bedroom window or any window designated as a means of escape. Building regulations expect an escape window to open from inside without a key, and the fire risk of a sealed bedroom window is a good deal more real than the burglary risk you are trying to solve.

If the window is on the ground floor and facing a rear alley or garden, treat it as urgent rather than something for next week. Rear elevations are where opportunist entry happens, which is the recurring theme in our piece on how burglars actually open doors.

What this looks like across Manchester

The pattern of failure is fairly predictable by area.

Around Withington and the student streets nearby, window hardware takes a hammering. Shared houses have windows opened and slammed by a dozen different people a year, handles get swung on, and letting agents replace the cheapest possible part when something breaks. Handles and gearboxes there fail years earlier than in an owner-occupied house.

In Didsbury and Chorlton, most of the housing stock is Victorian and Edwardian, and the uPVC is a retrofit — often on the back elevation and the bathrooms, with sash or timber casements kept at the front. Retrofitted units from the nineties are now well past the point where friction stays and espags wear out.

New-build flats in Salford and the city centre sit at the other end. Since October 2015, Approved Document Q has required easily accessible windows in new dwellings in England to be manufactured to PAS 24, a tested security standard. The hardware is better, but it is not immortal, and the cams and stays still need cleaning and lubricating.

Does a broken window lock affect your insurance?

It can. Most home policies carry a condition that you take reasonable care and secure the property when it is unoccupied, and some go further with an explicit requirement for key-operated locks on accessible windows. A window that physically cannot be locked, left in that state for weeks, is the sort of thing an insurer will ask about after a claim.

Nothing dramatic happens the moment a handle breaks. What matters is that you fix it within a sensible window of time and keep the invoice, which is straightforward proof that you dealt with it. Our guide to window locks and home insurance sets out the difference between a policy condition and a warranty, because the two are treated very differently at claim stage.

When to call a locksmith

Call when the handle spins, when it will not complete its travel, when the sash visibly drops or catches, or when a ground-floor window has been left unlockable overnight. Those are the four points where waiting costs more than acting — a bound mechanism that gets forced for another month turns a gearbox job into a full strip replacement.

Do not call a double-glazing company expecting to be told the hardware is repairable; plenty will quote you for a new window. It rarely needs one. Our uPVC door and window repair service covers handles, espagnolette mechanisms, friction stays, keeps and alignment, and if a window has been damaged in an attempted break-in our burglary repair team can make it secure the same day.

Get a fixed price before anyone starts, and if you have several windows of the same age, have them all looked at in one visit. The second and third window on the same call-out costs a fraction of the first, and hardware that went in at the same time tends to fail at the same time.