Choosing the right deadbolt for your front door

Start with the door, not the lock

Most of us only think about the front door lock when something goes wrong — a key snaps, the barrel stiffens, or someone tries the handle at three in the morning. The rest of the time it sits there doing quiet, unglamorous work. A deadbolt is the part that actually holds the door shut against force. Everything else on the door is convenience, weather sealing or decoration.

So before you buy anything, spend five minutes with a torch looking at the edge of the door. What you find there decides which locks are even an option.

• Solid timber or composite: a mortice deadlock can usually be fitted or upgraded, and this is where a five-lever lock earns its keep.
• UPVC or aluminium: the door already relies on a multipoint locking system, and cutting a fresh mortice into it is rarely sensible. Look instead at upgrading the euro cylinder and the door furniture.
• Flats and shared entrances: check what your lease or managing agent permits before changing anything that affects a fire door.

What a five-lever deadlock actually does

A five-lever mortice deadlock is the traditional British answer to a particular problem: someone trying to get in quietly, without breaking glass. Rather than pins, it uses five levers inside a steel case, and the key has to lift each one to precisely the right height before the bolt will move. That makes it slow and awkward to pick, and there is no easy bump-key shortcut.

Look for a lock tested to BS 3621, the standard for thief-resistant locks, or BS EN 12209. Many insurers still ask for a five-lever mortice deadlock on the final exit door, so check your policy before you spend — there is little point buying a lock your insurer will not credit you for.

Inside the case, look for hardened steel plates and anti-drill pins. The bolt itself should deadlock: once thrown, it cannot be pushed back with a card or a length of wire through the gap.

Deadlock, nightlatch or cylinder?

These three do different jobs, and the strongest front doors usually combine two of them.

• Five-lever mortice deadlock: the best all-round resistance on a timber door. Fitted into the door's edge, it is hard to attack without making noise and difficult to defeat quickly.
• Mortice nightlatch: spring-loaded, so it latches behind you automatically. Convenient, but a latch alone offers very little resistance. Treat it as a companion to a deadlock, never a replacement.
• Euro cylinder deadlock: easy to fit and easy to change when keys go missing. The cylinder is the weak point, so choose a three-star model that is anti-snap, anti-pick, anti-drill and anti-bump. It should not protrude more than about 3mm beyond the handle, or it gives a burglar something to grip.

Measuring up: thickness, backset and case size

Most UK external doors are around 44mm thick, and a mortice lock needs a case that sits in the door without hollowing it out. Two measurements decide whether a lock will fit:

• Case size: the overall length of the lock body, commonly 64mm or 76mm.
• Backset: the distance from the faceplate to the centre of the keyhole, typically 44mm or 57mm.

Measure your existing lock rather than guessing from a photograph online. If you are replacing like for like, you can often reuse the existing mortice, though the old keyhole may need filling and making good. If the door is noticeably thinner than 44mm, or hollow-cored, a mortice deadlock is not the right answer — talk to a locksmith about alternatives instead.

The keep: small detail, big difference

The keep, sometimes called the striker plate, is the metal plate set into the door frame that receives the bolt. It is the most commonly overlooked part of the job, and a beautifully made lock is wasted against a poorly aligned keep.

• The bolt should pass cleanly into the keep and sit at least 20mm deep. Shallower than that and a crowbar has an easy purchase.
• The keep must line up vertically and horizontally with the lock. If the bolt catches, or the door needs lifting to lock, the keep is out of position.
• Fix it with long screws — at least 75mm — that reach the structural timber of the frame, not just the softwood architrave.
• If the frame is soft, or the gap between door and frame is wide, consider a steel reinforcing plate or a deeper keep box.

Do this check with the door closed and latched normally, not swung open. A lock that works perfectly on an open door can be hopeless once the door is sitting on its seal.

Fitting, maintenance and staying safe inside

Fit the lock so the bolt throws smoothly with the key, and take your time chiselling the faceplate flush — a proud faceplate catches on the frame and wears the mechanism. Once it is in, test it a dozen times from both sides before you call the job done.

Think about personal safety as well as intrusion. Never leave a key in the lock within reach of the letterbox, and fit a letterbox guard or cage if the lock sits close to it — keys have been fished out with a bent coat hanger for decades. At the same time, agree a fire plan with everyone in the house. A thumbturn inside is quicker to operate in smoke and darkness, but a key-only deadlock is harder for an intruder to exploit if they are already inside. Whichever you choose, make sure everyone can use it without thinking.

Finally, oil the lock lightly once a year using graphite powder rather than an oily spray, which attracts grit and eventually gums up the levers. If the key starts to feel stiff, or the door needs a shoulder to close, sort it out straight away — that stiffness is usually the first sign the keep has shifted or the door has dropped.