Why is my UPVC door hard to lock?

Why UPVC doors seize in summer, the lift-and-lock check, and the difference between an alignment problem and a failing gearbox.

Difficulty: beginner
Time: 45 min
Read: 4 min
Safety: warning

Short answer

Almost always alignment rather than the lock. UPVC expands in heat, so the door that locks fine in March binds in July. Lubricate first, then check whether it locks easily when you lift the handle with the door pulled towards you — if it does, the door has dropped and needs adjusting at the hinges.

A UPVC door is a plastic frame around a metal reinforcement, and it moves with temperature more than people expect. Add hinge wear and a heavy door, and small misalignments turn into a lock that needs both hands.

Safety

A multipoint lock that is being forced is close to failing, and it usually fails locked — with you on the wrong side of it. Deal with a stiff door before it seizes rather than after. Never force the handle hard: the gearbox inside is the expensive part and forcing it is what breaks it.

Step by step

Lubricate before anything else.Light machine oil or a proper lock lubricant into the keyway, and silicone spray onto the multipoint strip and the keeps. Not WD-40 in the cylinder — it displaces the lubricant and leaves the mechanism drier.
Do the lift-and-lock test.Pull the door firmly towards you and lift the handle. If it now locks easily, the mechanism is fine and the door has dropped or the frame has moved. If it is still stiff, the problem is in the lock itself.
Look at the keeps for wear marks.The metal plates on the frame that the hooks and rollers engage. Bright scrape marks show where the lock is hitting rather than entering, and their position tells you which way the door has moved.
Adjust the hinges.UPVC door hinges adjust for height, lateral position and compression, usually with a hex key under a cap. A quarter turn at a time, closing and testing between each.
Adjust the keeps if the hinges do not reach.Most keeps have slotted fixings and move a millimetre or two. Loosen, move slightly in the direction the wear marks indicate, retighten and test.
Consider seasonal movement.A door that binds only in hot weather is expanding. Set it slightly looser than perfect so it works across the range rather than adjusting it twice a year.
Check the cylinder separately.If the key is stiff but the handle lifts easily, the cylinder is the problem. Try a different key — a worn key is a common cause. A cylinder is inexpensive and straightforward to replace.
Recognise a failing gearbox.The handle feels spongy, or lifts without engaging, or locks in one direction only. That is the multipoint gearbox and it is replaceable — measure the backset, the centres and the strip length before ordering.
Deal with it before it fails completely.A gearbox that is being forced will eventually break, most often when locked. That means a locksmith and possibly a damaged door rather than a planned part swap.
Check the door has not sagged at the panel.On older doors the panel can settle within the frame. That is a fabrication issue rather than an adjustment one and needs a door company.
Lubricate annually.Once a year on the strip, keeps, hinges and cylinder. Most stiff UPVC doors have simply never been lubricated.

Tips

Always lift the handle fully before turning the key. Not doing so is the most common cause of a 'stiff' door that is working exactly as designed.
Photograph the hinge caps and screw positions before adjusting. It gives you a way back.
If the door locks when open but not when closed, it is alignment. If it is stiff when open too, it is the mechanism.

Common mistakes

Forcing the handle when it feels stiff — The gearbox is the weak point and forcing it is what shears it — usually when the door is locked and you are outside.
Spraying general-purpose penetrating oil into the cylinder — It displaces the proper lubricant and attracts dust, so the lock is stiffer within months. Use a lock-specific lubricant.

If it doesn't work

Locks easily when you pull the door towards you

Cause: Door has dropped or frame has moved — alignment, not the lock — Fix: Adjust the hinges to lift and reposition the door, testing a quarter turn at a time.

Stiff only in hot weather

Cause: Thermal expansion of the UPVC — Fix: Set the adjustment slightly looser so it operates across the whole temperature range.

Key stiff but handle fine

Cause: Worn cylinder or worn key — Fix: Try a different key. If still stiff, replace the cylinder — it is inexpensive and a simple swap once measured.

Handle lifts but nothing engages

Cause: Multipoint gearbox has failed — Fix: Replace the mechanism. Measure backset, centres and strip length, or have a locksmith identify it.

Questions people ask

Why is my UPVC door stiff to lock in summer?

UPVC expands with heat, so a door aligned in cool weather binds when hot. Lubricate, then adjust the hinges slightly looser so it operates across the whole temperature range.

How do I know if it is the lock or the door alignment?

Pull the door firmly towards you and lift the handle. If it locks easily like that, the mechanism is fine and the door needs adjusting. If it is still stiff, the problem is in the lock.