Before you get started

All doors and windows are potentially vulnerable and should be adequately secured to prevent unwanted intrusion.

The two most common locks to be fitted on entrance doors are a mortice lock and a cylinder rim lock.

A mortice lock is opened with a key from both inside and outside the door. This is referred to as a deadlock.

Mortice locks have 2 to 7 levers. The more there are the harder the lock is to pick. An entrance door must have at least a five-lever lock approved to BS3621.

Most mortice locks are of standard size. An old one can be changed for a new one if the distance from the door edge to the handle and/or keyhole is the same (check backset).

Cylinder locks are easily changed or replaced when you move house, or if you lose your keys, as they are simply slotted into place and are widely available in standard sizes (check backset).

A cylinder rim lock with a deadlock has a key for the outside and a knob operated by the key on the inside. The internal key provides the cylinder lock with a deadlock option. In other words, it cannot be opened if glass in the door is smashed and the knob turned.

A cylinder lock and five-lever mortice deadlock should be fitted 45-60cm apart to be as effective as possible.

Mortice door bolts can be fitted in the edge of doors. One close to the top and one close to the bottom on the opening edge of a door will give extra security. They may also be used on the hinging edge.

Useful supplements to security include door limiters, security chains and surfacemounted bolts, which can all be easily fitted.

UPVC doors may be damaged by retro-fitted security. If in doubt consult the supplier or the manufacturer about modifications that might invalidate the guaranty. Modern designs normally incorporate multi-locking systems that don’t need extra security fittings.

Doors with glazed panels should be fitted with a deadlock as they are less secure than solid doors. Consider replacing glass panels with laminated or toughened glass.

Make outward opening wooden French doors and windows more secure by fitting them with a mortice sash lock and hinge bolts.

Patio doors are often accessed by thieves so it is sensible to fit extra locks. Push-to-lock, key-to-open locks are easy to use and fit if the frame can accommodate them.

Fit a mortice security bolt


Mark position of bolt on door edge, then drill a hole to the depth of door bolt as directed by manufacturer.




Insert bolt and mark a line around the outside edge of the end plate as a guide for chiselling.




With a chisel, carefully remove enough wood from within the pencil outline for the door bolt end plate to be neatly recessed and fit snugly.




Adjacent to the chiselling, hold the bolt against the inside of the door and accurately mark the position of the keyhole with a carpenter’s awl.




Using a drill bit size recommended by the manufacturer, drill a hole through to the hole for the bolt. Do not drill right through the door.




To secure the bolt, first drill pilot holes for fixings. A 2-3mm wood bit is an ideal size for the pilot holes. Slide in the bolt and fix in place.




Make pilot holes before fixing the key plate in place. The plate must be correctly aligned with the keyhole.




Check that key works. As directed by manufacturer, drill a hole for bolt in door frame and fix mortice plate.



Replace a five-lever sashlock


Remove the handle on the inside by unscrewing the retaining screws that hold the handle in place. Place the handle to one side to reattach later.




Remove the spindle bar that fits through the lock and both handles and place safely to one side as this will be reinserted.




Remove the screws that secure the lock. You may need to thread a screwdriver through the spindle hole and pull towards you to release lock.




Fit and screw the replacement lock in position. If necessary, chisel out a little more wood from the door edge to ensure the lock fits snugly.




Reposition the spindle and inside handle. Check that the new lock works smoothly and engages correctly with the strike plate on the door frame.



Replace a cylinder lock


Unscrew the lock body. This needs to be removed but will not be replaced. Just refit the part once the new lockis in place.




Unscrew the two retaining screws that go through the door and hold the cylinder part of the lock in place. Keep the mounting plate in position.




Remove the old cylinder from the outside. Slide the new cylinder through the drilled hole in the door and refix with the long screws.




You can now refit the lock body, but if the connecting bar is too long, remove the lock body and cut the bar down using a junior hacksaw.




When the lock body is firmly screwed in position, check that the door opens and closes and the new key works smoothly in the newly fitted lock.


What you’ll need

Fit a mortice security bolt

Mortice security bolt

Drill/driver and bits

Flat bit (size as directed by manufacturer)





Carpenter’s awl (bradawl)

Pilot drill bit


Safety glasses


Replace a five-lever sashlock

Five-lever sashlock


Chisel (if required)

Hammer (if required)

Safety glasses


Replace a cylinder lock

Cylinder lock


Junior hacksaw