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uPVC Window Will Not Lock
The window shuts but the handle will not turn to throw the lock, or it turns and nothing engages.
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What's happening?
A modern uPVC casement locks with an espagnolette: a steel gearbox behind the handle driving a long strip up and down the sash edge, with mushroom-shaped cams or hooks along it that pull into keeps on the frame. When the handle will not turn, either the cams are not lining up with their keeps, or the gearbox itself has failed. When the handle turns freely but nothing grips, the drive strip has usually sheared or come off the gearbox pinion. The distinction matters because the first is nearly always alignment — a sash that has dropped a couple of millimetres, or a frame that has moved in hot weather — and the second is a mechanism replacement.
What usually causes it?
- Sash dropped on worn hinges, so the locking cams sit below their keeps and jam on the frame
- Espagnolette gearbox worn or seized, commonly after years of being forced against a misaligned keep
- Sheared drive strip or a stripped pinion, giving a handle that spins with no resistance
- Thermal expansion — a south-facing sash grows enough on a hot afternoon to take the cams out of register
- Keeps loose, worn oval, or shifted when the frame was last adjusted
- Debris, old paint or hardened grease in the keeps and along the drive strip
- A handle spindle of the wrong length or a snapped spindle, so the handle no longer engages the gearbox at all
Is it dangerous?
It is a security fault rather than a physical hazard. A casement resting closed on its gasket with no cams thrown is held only by the handle detent — it can be pushed open from outside. Treat a ground-floor or accessible window that will not lock as something to resolve the same day. There is a second consideration if this is a bedroom window that was fitted as your escape window: a jammed sash that will not open is as much of a problem as one that will not lock, because escape windows are expected to open.
Can I fix it myself?
Two things are reasonable. First, check for the obvious obstruction — grit or hardened paint in the keeps, or a screw that has backed out. Second, lift the sash slightly by the handle edge while turning the handle; if it locks when supported, the sash has dropped and the diagnosis is alignment. Do not force a handle that will not turn: the gearbox castings are the weakest part of the mechanism, and forcing turns an adjustment job into a replacement. Never lubricate with a spray oil that gums up — the drive strip wants a light PTFE or silicone lubricant.
When should you call a window fitter?
Call a window fitter the same day for any accessible window that will not lock, and promptly for an upper-floor one. Call before forcing it if the handle has become stiff over weeks, because that is the classic run-up to a sheared gearbox. If the handle has already snapped off or spins freely, the mechanism is out and needs matching by measurement.
What will the window fitter actually do?
- Identify the lock system by measuring the backset, centres and cam pattern of the espagnolette
- Adjust hinges to lift the sash back into register, and reposition or renew the frame keeps
- Replace the gearbox or the full multipoint strip where the drive is worn or sheared
- Fit the correct spindle length and a handle to match, and set the detent so the handle throws cleanly
- Lubricate the drive strip and cams with a suitable lubricant and cycle the window to confirm even engagement
- Check the remaining windows on the same elevation for the same drop, as they usually age together
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