A door locking mechanism is the part of a lock that moves a bolt, latch or hook into the frame so the door can’t be opened. Most UK homes use one of three: a euro cylinder driving a multi-point lock (uPVC and composite doors), a mortice lock (timber doors), or a rim nightlatch (often paired with a mortice lock). Knowing which you have tells you which standard to look for and where the weak spots are.
This guide explains 15 common types of door locking mechanism found in UK homes, how each works, where it’s used and what to check.
The standards that matter
Before the individual locks, here are the UK standards you’ll see stamped on faceplates and packaging. They’re the quickest way to tell a good lock from a basic one.
| Standard | What it covers |
|---|---|
| BS 3621 | Thief-resistant locks operated by key from both inside and outside. The current edition is BS 3621:2017. Often asked for by insurers on timber doors. |
| BS 8621 / BS 10621 | Related standards for locks with keyless exit from inside (BS 8621) and locks that switch between keyless exit and a secure mode (BS 10621). |
| TS 007 | Star rating for euro cylinders and handles. A 3-star cylinder on its own, or a 1-star cylinder with 2-star handles, reaches the 3-star level. |
| Sold Secure SS312 Diamond | A high-security test standard for cylinders, recognised alongside TS 007 3-star. |
| PAS 24 | Security standard for complete doorsets and windows, not individual locks. |
Locks for uPVC and composite doors
1. Euro cylinder
The euro profile cylinder is the keyhole part on most uPVC and composite doors. Turning the key rotates a cam in the middle of the cylinder, which works the lock mechanism inside the door. It’s held in place by a single fixing screw through the edge of the door.
Older or basic cylinders can be vulnerable to lock snapping, so the rating matters. West Yorkshire Police say a 3-star solution is the minimum to aim for when upgrading, and that cylinders must be measured correctly so they don’t stick out from the door or handle.
2. Thumb-turn cylinder
A thumb-turn cylinder has a key on the outside and a small knob on the inside, so you can lock and unlock from inside without hunting for a key. That’s convenient and helpful for a quick exit. The trade-off is that a thumb-turn near a letterbox or glass panel could be reached from outside, so pair it with a good letterbox guard and choose one with a proper 3-star or Diamond rating.
3. Multi-point lock
Behind the handle of most uPVC and composite doors is a multi-point lock: a long strip running down the edge of the door with a central gearbox. Lifting the handle throws hooks, rollers or bolts into keeps along the frame; turning the key then locks them in place. Because the door is held at several points, it’s much harder to lever open than a single-latch door.
The catch is habit. If you close the door without lifting the handle and turning the key, often only the latch is holding it. Problems such as a stiff handle usually come from the door dropping out of line, not a broken lock; our guide to the weak points of uPVC doors explains what to check.
Locks for timber doors
4. Mortice deadlock
A mortice deadlock sits inside a pocket cut into the edge of a timber door, so only the keyhole and faceplate are visible. Turning the key throws a solid bolt into the frame. There’s no latch and no handle operation; it simply locks and unlocks.
For external timber doors, look for one marked BS 3621. West Yorkshire Police recommend a 5-lever mortice lock tested to BS 3621 on wooden doors, and note that insurers often require it. Check your own policy wording.
5. Mortice sashlock
A sashlock combines a deadbolt and a sprung latch in one case. The latch is worked by the handles, so the door can be pushed shut and opened without a key during the day, and the key throws the deadbolt at night. It’s common on back doors with lever handles. The same BS 3621 marking applies if you want insurer-friendly security.
6. Rim nightlatch
Often called a “Yale-type” lock, the nightlatch is the box mounted on the inside face of the door with a cylinder on the outside. It latches automatically when the door closes. Basic versions can be slipped or opened by reaching through a letterbox, so for a front door choose a deadlocking nightlatch. Some nightlatches are made to BS 3621. On most timber front doors, a nightlatch works best paired with a mortice deadlock.
Electronic locks
7. Smart locks
Smart locks let you lock and unlock with a phone app, fob, code or fingerprint. On UK uPVC doors, many work with a compatible euro cylinder and the existing multi-point lock rather than replacing it. Points to check:
- The security rating of the cylinder or lock, ideally TS 007 3-star or Sold Secure Diamond where it uses a euro cylinder.
- What happens when the battery runs low, and whether there’s a key override.
- Whether it still needs you to lift the handle to engage the multi-point lock.
- App security and account settings, such as two-factor login.
8. Keypad and push-button locks
Code locks are common on side gates, utility rooms and communal doors. Mechanical versions need no batteries; electronic versions often add features like temporary codes. They’re handy where several people need access, but change the code regularly and check what security standard, if any, the lock has been tested to.
Locks for patio, French and bi-fold doors
9. Patio door hook lock
Sliding patio doors usually lock with hook-shaped bolts that swing up into keeps on the frame, so the sliding leaf can’t be pulled open. The hooks don’t stop a door being lifted out of an older frame, which is why the Met advise checking for an anti-lift device.
10. Shootbolts and espagnolette locks
French doors and bi-folds often use shootbolts: rods that shoot up into the head of the frame and down into the threshold when the handle is turned. On French doors the second leaf usually has its own shootbolts at top and bottom, and the main leaf locks into it. Make sure both leaves are fully engaged every time.
Bolts and secondary locks
11. Mortice rack bolt
A rack bolt is fitted inside a hole drilled into the edge of a timber door and operated with a splined key from the inside only. It adds a hidden extra locking point at the top or bottom of the door, useful on back doors and French doors.
12. Barrel and tower bolts
Surface-mounted bolts are simple sliding bolts screwed to the inside face of a door. They’re only as strong as their screws and the material behind them, so use long screws into solid timber. They’re useful extra security for doors you rarely use, not a main lock.
13. Hinge bolts
Hinge bolts are fixed steel pins in the hinge edge of the door that sit in holes in the frame when it’s closed. They help stop the hinge side being forced, which matters most on outward-opening doors where the hinge pins are outside. West Yorkshire Police suggest hinge bolts or security hinges for outward-opening doors and on each hinge of French doors.
14. Sash jammers
A sash jammer is a small lever-type lock screwed to the inside of a uPVC door or window frame that swings across the opening edge. It’s an inexpensive extra, but it needs screwing into the profile, so check your door warranty first, as West Yorkshire Police also advise.
15. Padlocks and hasps
For sheds, garages and gates, a hasp and padlock is often the main lock. The weak point is usually the hasp, not the padlock, so choose a hasp whose screws are hidden when closed, and look for a Sold Secure rating on the padlock. Sold Secure grades products Bronze, Silver, Gold or Diamond, from opportunist to dedicated attacks.
Beyond the lock: holding the door itself
Every mechanism above relies on the lock staying intact. A floor-braced door bar works differently: it holds the door shut even if the lock is attacked.
The Door Jammer – HOLDOOR hooks under the inside handle of a uPVC door and braces it against the floor, so the door stays shut even if the lock is snapped, bumped or picked. It is Sold Secure Bronze rated and Secured by Design accredited (Police Preferred Specification), adjusts from 70cm to 120cm to fit most UK single-handle uPVC doors with the handle on the left or right, and fits in about a minute with no drilling or tools. It’s made in Britain from British steel and is used from inside, when someone is at home. It isn’t for sliding, French, double or patio doors.
In a fire: find the key, lift the Door Jammer off (or unlock and remove the Door Handle Guard), unlock the door and leave the building.
Frequently asked questions
How do I know what lock I have?
Look at the door edge. A long faceplate with several bolts or hooks means a multi-point lock, usually worked by a euro cylinder. A shorter faceplate with a keyhole on a timber door is a mortice lock; check it for a BS 3621 mark. A box on the inside face of the door is a rim nightlatch.
Which lock do insurers want?
It depends on the policy. Many ask for BS 3621 mortice locks on timber doors and specific locks on uPVC doors. Read your policy wording rather than assuming.
What’s the most secure lock for a uPVC door?
A multi-point lock used properly, with a TS 007 3-star or Sold Secure Diamond cylinder fitted at the right length. Add a door bar for extra protection when you’re home.
Can I change a euro cylinder myself?
Often yes, if you measure carefully and buy the right size. If you’re unsure, police advice is to use a Master Locksmiths Association approved locksmith.
Next steps
- Identify the locking mechanism on each external door.
- Check the markings: BS 3621 on mortice locks, TS 007 3-star or SS312 Diamond on cylinders.
- Upgrade anything unmarked, starting with your front and back doors.
- For a single uPVC door, add the Door Jammer – HOLDOOR so the door stays shut even if the lock is attacked.
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