UPVC door won't lock? Here's what is actually wrong

Few things are more irritating than a door that has locked perfectly well for years and then suddenly will not. The good news is that the cause is almost always one of three things, and none of them mean you need a new door, despite what you may be told.

Symptom one: the handle has gone slack

If the handle moves freely without any resistance, the multi-point gearbox has almost certainly failed. The gearbox is the mechanism inside the door edge that the handle drives, and it pushes the locking points into the frame.

When it fails, the handle no longer engages anything, so the door cannot be locked at all. It is the single most common fault on UPVC and composite doors between fifteen and twenty-five years old, and the gearbox can be replaced on its own.

Symptom two: the door needs lifting to lock

If you have to lift the handle or the whole door to get the key to turn, the door has dropped. This is a hinge or frame problem, not a lock problem.

Doors drop because the hinges wear or the frame moves as the building settles. The locking points no longer line up with the keeps in the frame, so the mechanism cannot fully engage. The fix is to re-hang and re-align the door properly.

Symptom three: it only fails in cold weather

A door that locks all summer and refuses to lock in winter is suffering from thermal movement. The door and frame contract at different rates as the temperature drops, so locking points that met their keeps in August no longer reach them in January.

This is an adjustment job, not a part failure. Once the door is set correctly it should lock reliably all year, though it is worth having it checked if the problem keeps returning.

What a repair should cost

• Door adjustment and re-alignment: £60 – £90
• Multi-point gearbox replacement: £160 – £300
• Handle replacement: £90 – £160
• Cylinder replacement: £90 – £160

Compare that with a replacement door, which typically runs from £600 to well over £1,500 fitted. In the large majority of cases the door itself is fine and only one component has failed.

When a door genuinely does need replacing

Occasionally it does. If the door has warped beyond adjustment, if the frame itself has moved significantly, or if the door has been forced and the internal structure is bent, a repair may not be viable.

A good locksmith will tell you honestly when that is the case rather than spending money on a repair that will not hold. But it is the exception, not the rule, and anyone quoting for a new door without examining the mechanism first should be treated with caution.