Fitting a Mortise Lock

A simple and relatively cheap way to improve the security of you home is to fit mortise locks to vulnerable doors (back or side doors). Mortise locks make it much more difficult for an intruder to force the door.

DIY Extra Tip!

There is an old saying in DIY, "Measure twice and cut once". With a job like this, measurement and marking out needs to be accurate. The better the lock mechanism fits into the mortise, the more secure the door will be. Taking the time to be accurate will also save time overall.

Marking

Once you have bought the lock mechanism, the first thing you need to do is mark where the lock will go. If you are installing the lock into a new door, the best place for it to go is directly in line with the centre rail (the solid strip of wood running across the middle of the door). If you are adding the mortise lock to a door that already has a standard lock fitted, fit the Mortise just above the existing lock, as close to the rail as possible. Hold the body of the lock against the edge of the door and use it as a template to mark the top and bottom of the mortise you need to cut. Now measure the width of the door edge and draw a line down the middle or the mortise mark you just made.

Drilling Out

Take a Drill Bit the same width as the body of the lock and using a strip of electrical tape, mark the depth of the lock on the drill bit. Now drill a series of overlapping holes along the middle line you marked (the tape will let you drill them all to the correct depth). With a sharp Chisel, take out the small ridges of wood not removed by the drill until you have a nice straight mortise.

Fitting the Lock

Insert the lock into the mortise and mark around the faceplate. Remove the lock and carefully Chisel out a recess for the faceplate to sit in. This will need to be no more than 3-4mm deep. Re-insert the lock to check everything fit nicely. Now hold the lock against the side of the door, lining it up carefully with the mortise and with the edge of the door. Mark the position of the key and spindle holes by pushing a sharp point through the keyhole in the lock mechanism and, using a drill bit the same width as the shaft of the key, drill out the keyhole. If you have a Pad Saw, you can use this to shape the hole, if not, three carefully drilled holes will do the job.

Slide the lock mechanism back into the mortise and check that the hole for the lock lines up with the mechanism. If it does, you can mark the screw holes in the faceplate and drill guide holes for the screws using a 3mm drill bit. Screw the lock mechanism firmly into place using 50mm or longer screws.

Fitting the Striking Plate

With the bolt (and latch if there is one) open, close the door against the frame and mark where the top and bottom of the latch and bolt touches the frame. Extend those marks around onto the face of the frame. Now measure from the edge of the door to the centre of the bolt, and use that measurement to mark where the centre of the mortise needs to be on the doorframe. Cut out the mortise in the same way you did on the door, only using a drill bit the width of the bolt. Chisel out any excess wood once the drilling is done. Close the door and make sure the bolt lines up with the hole.

Line up the Striking Plate with the mortise on the frame and mark around it. Chisel out a rebate so the striking plate will sit flush in the frame. Finally, mark the screw holes in the striking plate and drill guide holes. Screw the plate into place and the job is done.