Rim locks · explained · Manchester
Why your front door has two locks doing different jobs.
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- Repair £30-55
- Rim cylinder £30-55
- BS3621 nightlatch £75-130
- 20-40 min
Look at a typical Manchester front door. There is a lock screwed onto the inside face, up near the top, that clicks shut behind you. And there is a keyhole lower down with nothing visible attached to it. Two locks. Almost nobody knows why, and the answer decides whether your insurance pays out.
The one that clicks: the rim nightlatch
Screwed to the surface of the door, which is what "rim" means. It has a sprung latch with an angled face, so the door shuts and it clicks home on its own.
Its job is convenience, not security. A sprung, angled latch is a latch: it can be pushed back in. That is the credit-card trick, and it works because that is what a spring does.
The one that doesn't: the mortice deadlock
Buried in the edge of the door. Square-ended bolt, no spring, moves only when the key turns. Nothing slips it and nothing pushes it back.
This is the one your insurer means. When a policy says BS3621 on the final exit door, it is almost always talking about this lock, not the one that clicks. See mortice locks.
Which means the mistake is obvious
You pull the door shut behind you and walk off. The nightlatch clicked. The deadlock did not, because a deadlock never locks itself.
You have left the house on a spring. And most policies do not just require a BS3621 deadlock, they require it to be engaged. A claim can be declined on that alone, and the lock was fitted, and it was compliant, and it was open.
The fix costs nothing: turn the key.
The feature you own and never use
Better nightlatches deadlock. Turn the key from outside and the latch itself locks, so it cannot be slipped and nobody can reach through the letterbox and operate the handle. On the inside, the snib does the same.
If you have a deadlocking nightlatch and you have never given the key that extra turn, you have paid for a security feature and never switched it on.
What goes wrong
| Symptom | Cause | Price |
|---|---|---|
| Latch sticks, won't pull back | Dried grease and dirt in the case | £30-55 |
| Door shuts, latch misses | Door dropped. Not the lock. | £35-65 |
| Key needs jiggling | Rim cylinder worn | £30-55 |
| Snib won't hold | Worn mechanism | £30-55 |
| Whole thing loose on the door | Screws pulled out of old timber | £25-50 |
Nightlatches take more abuse than anything else in the house, because the door slams onto that latch several times a day for decades. Almost all of it services rather than replaces.
Lost your keys? You want the cylinder, not the lock
A new rim cylinder is £30-55 and every old key dies the moment it goes in. The latch case, the screws and the woodwork all stay where they are. You only need a whole new lock if the case is damaged or you are upgrading to BS3621.
Yale locks and what people mean by them · Lock change · Deadbolts
Questions people actually ask
Why does my front door have two locks?
Because they do different jobs. The one screwed to the inside face that clicks shut behind you is a rim nightlatch, and its job is convenience: it locks itself so you cannot walk out without the door being shut. The keyhole lower down is a mortice deadlock buried in the edge of the door, and its job is security: a square bolt with no spring that only moves when you turn the key. The nightlatch can be slipped because a sprung angled latch can be pushed back. The deadlock cannot.
If I just pull the door shut, is my house locked?
Only on a spring, and this is the most common and most expensive mistake in British houses. Pulling the door shut engages the nightlatch, which is a sprung latch that can be slipped. It does not engage the mortice deadlock, because a deadlock never locks itself. Most insurance policies do not just require a BS3621 deadlock to exist, they require it to be engaged when the property is unoccupied. A claim can be declined on that alone: the lock was fitted, it was compliant, and it was open. Turning the key costs nothing.
What does the snib on a nightlatch actually do?
Two things. Slide it one way and it holds the latch back so the door will not lock, which is what you do carrying shopping in. Slide it the other and it deadlocks the latch, so it cannot be slipped and nobody can reach through the letterbox and work it. On a deadlocking nightlatch, an extra turn of the key from outside does the same. If you have that feature and have never used it, you have paid for a security measure and never switched it on.
My nightlatch sticks. Repair or replace?
Repair, almost always. A nightlatch takes more punishment than any other lock in the house because the door slams onto that latch several times a day for decades, and the usual cause of sticking is dried grease and dirt inside the case rather than a broken mechanism. A strip and service is £30 to £55. The other common cause is that the door has dropped and the latch is hitting the edge of the keep instead of entering it, which is a realignment, and a new lock fixes neither.
Is my nightlatch BS3621?
Almost certainly not. BS3621 nightlatches exist, at £75 to £130 fitted, but the standard one that came with the house is very unlikely to be one. Look at the faceplate on the edge of the door for a Kitemark and the number stamped on it. If it is not there, it is not. The usual and cheaper way to comply is a BS3621 mortice deadlock as the second lock, which is what most Manchester front doors already have in the lower keyhole, and then actually turning the key.