Locks are your home’s main defence against all intruders. Latches simply keep doors closed. It’s vital to choose and fit them correctly.

T he type of latch normally found on all hinged internal doors is a simple spring-loaded affair with a ‘springbolt’ (2). This is a protruding tongue which is rounded on one side so it will slip into the hole in a metal ‘striking plate’ fitted to the door jamb, but is easily freed again by turning the handle. The mechanism is called a ‘mortise latch’ because it sits in a hole or ‘mortise’ cut into the edge of the door, while the spindle for the handles passes through the door.

Locking is more complicated. For an internal door, a ‘two-bolt mortise lock’ (also called a ‘sashlock’ – 1) is standard. One tongue (usually the upper one) is a springbolt, which you can use for ordinary coming and going. When you need the door properly locked you use the other, which is a ‘deadbolt’. That is, it can be moved in or out only by turning a key. When out, it fits into a second hole in the striking plate, and is ‘deadlocked’, which means it can’t be pushed back by an intruder sliding something between the lock and plate.

The key works by pushing various levers in the lock mechanism, which in turn slide the tongue. The more levers there are, the harder the lock is to pick. Most internal lever locks have only one or two, and are thus not very secure – though they will probably keep out children, for example. Anyway, it’s not generally considered a good idea to lock internal doors when you’re out- intruders may simply smash them down.

External doors obviously need more protection. Those which you can lock while still inside the house, such as the back door, can have appropriate additional security devices – hand-operated sliding bolts and so on. But the door which you finally use to leave the house presents problems.

There are two main solutions: another mortise lock, or a ‘rim lock’. The tongue mechanism of a rim lock, instead of being inserted right into the thickness of the door, is screwed to the inside face. Cheap models (12) have a lever action, and are quite unsuitable for external doors. Much more useful are rim locks which incorporate a cylinder passing right through the door, containing pin or disc tumblers

Mortise locks and latches

The main advantage of a mortise lock is that, once it’s fitted, its mechanism is inaccessible. On the other hand, you need a lot of care to cut a mortise into which the lock will fit snugly; and cutting the mortise itself may sometimes weaken the door.

The door must be at least 45mm (13/iin) thick, and you should check if the stile (the upright part of the frame) is wide enough – though some locks are available in narrow-stile patterns.

Instead of levers. Cylinders are also found in some mortise locks -such as (6).

Standard rim cylinder locks, and again, some mortise locks, such as (6), have a ‘nightlatch’ -that is, a springbolt which can’t be opened from outside except with a key. They can be deadlocked from inside via a small extra knob called a ‘snib’.

However, even they afford no real protection unless the springbolt can also be deadlocked when leaving the house. On some models this is done by ‘double-locking’ from outside with an extra turn of the key; with others (9) and (10) it happens automatically as you close the door. Some also enable you to deadlock the internal handle with the key -usually from the outside, but sometimes from either side.

A good lock is reinforced in several different parts of its construction, so that it can’t be forced, drilled or sawed through.

Rim locks and latches

These are generally easier to fit because they don’t need a hole cut into the thickness of the door. With a thin door a rim lock may be your only option: most makes will fit doors as thin as 38mm (1 But in the case of rim cylinder locks, there is sometimes a maximum door thickness of about 60mm (23/oin). Again, some models come in narrow-stile patterns.

Rim locks have a ‘staple’ (a matching box that is fixed to the door jamb) instead of a simple striking plate.

A high number of ‘differs’ (combinations of levers or tumblers) means less likelihood that any given key will fit the lock, and thus less chance of unauthorised entry. So the more levers or tumblers the better. Five are the minimum for security.

Any deadbolt will be reinforced (for example with a hardened steel roller) prevent its being sawn through, and should ideally go into a box-type striking plate mortised into the door frame, where it will still be protected. It should project at least 15mm (5/8in). The lock mechanism should be protected by a hardened steel plate, at least on the outside, to prevent anyone drilling into the mechanism to free the lock.

Many manufacturers offer special high-security models, with more differs and greater physical protection. Some also offer key registration: when you buy the lock, you register the number of the key with them, and extra keys can only be obtained from them against your signature.

Remember that even the finest lock is only as good as the door and frame. Solid flush doors are better than panelled ones, but wood panels are stronger than glass. The door should fit well – with no more than 3mm (Vain) gap at top and sides – into a sound door frame which is itself securely fixed into the wall.

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