Why does my door lock perfectly when it is open but not when it is shut?

This is the most valuable five seconds in door diagnosis. Standing the door open and working the handle removes the frame from the equation entirely: the bolts throw into thin air. If everything works in that state, no part of the lock is broken, and every pound you might have spent on a new lock is saved.

Safety

Step by step

1. Do the test properly.Swing the door fully open, clear of the frame. Lift the handle to its stop. Turn the key. Turn it back. Drop the handle. If all of that is smooth and light, the lock, the cylinder and the handle are all sound.
2. Find where the bolts are landing.Throw the bolts with the door open, then walk the door slowly up to the frame and look through the gap. You are checking whether each hook lines up with the mouth of its keep. Look at the top hook and the bottom hook particularly — they show the error most.
3. Read the wear marks instead if the gap is too tight to see.Wipe the keeps clean, rub a little chalk or a smear of grease on the leading face of each hook, close and lift the handle as far as it will go, then open and look at where the mark transferred. It shows exactly how far out and in which direction.
4. Decide which error you have.Marks below the keep slots mean the door has dropped and needs raising. Marks above mean it has been raised too far. Marks in the right place vertically but the handle still lifting far too easily, with the door rattling, mean the door is standing out and needs compression rather than height.
5. Correct height at the hinges first.The vertical adjustment on the hinges moves the whole door and therefore all the bolts together. Work in small increments across all hinges rather than winding one to its limit. Test after each adjustment with the door open and then closed.
6. Then correct the lateral position.The in-and-out adjustment at the hinges moves the hinge side. Use it if the door sits proud at the hinge edge or if the gap down the closing edge has become tapered rather than parallel.
7. Adjust the keeps last, and only for compression.Once the bolts enter their keeps cleanly, the keeps are moved in or out a millimetre at a time to set how hard the door is pulled onto the seal. This is a fine adjustment and it must come after the hinges, never instead of them.
8. Set compression by feel, not by maximum.The handle should lift with firm but comfortable pressure. A door wound up so tight that it needs both hands seals beautifully and destroys its own gearbox within a year or two.
9. Recheck at the other end of the year.Composite and dark uPVC doors move with temperature. A door set perfectly in February may be tight in July. Aim for a setting that works at both ends rather than perfection in one season.

Common mistakes

• Buying a new lock because the door will not lock — This test proves the lock is fine before anything is spent. A dropped door will destroy the new lock exactly as it destroyed the old one.
• Winding the keeps out to meet a dropped door — The keeps adjust in and out, not up and down. Moving them cannot cure a height error, and moving them a long way leaves the hook engaging on the very edge of the keep where it will eventually pull through.
• Adjusting one hinge to its limit — It racks the door and puts all the load on one hinge. Height changes are shared across every hinge, a quarter turn at a time.

Questions people ask