How to fit a bathroom lock

Video by Charlie DIYte. This walk-through is based on the video "How to Fit a Bathroom/ Closet Door Lock" from Charlie DIYte, who takes a clear, practical approach to the whole job. His section on marking out the mortice position accurately before you drill is the part most people skip — and the part that causes most of the grief.

1. Choose the right lock for your door

Bathroom privacy locks come as a mortice sashlock (the bolt sits inside the door edge) or a tubular latch with separate bolt. For most internal bathroom doors in UK homes, a 64 mm or 76 mm mortice bathroom lock is the standard choice. Check the backset — the distance from the door edge to the centre of the keyhole or spindle hole — and match it to your door furniture. The most common backsets are 44 mm and 57 mm.

Also check the door thickness. Most interior doors are 35 mm or 40 mm thick. The lock case needs to fit within that thickness without breaking through either face. It sounds obvious, but it is worth holding the lock up against the door edge before you commit to anything. If you are replacing an existing lock, take the old one out first and measure the mortice — nine times out of ten you can find a direct replacement that drops straight in.

2. Mark out the lock position on the door edge

Hold the lock case against the door edge at the right height — typically around 990 mm from the floor, to align with the handle furniture. Trace around the faceplate with a sharp pencil, then use a marking gauge to scribe the centre line of the door edge for the lock body. Getting this centre line accurate is the most important step in the whole job; a lock that is even slightly off-centre will be visible once the faceplate is fitted.

Use the lock itself as a template to mark the spindle hole position on both faces of the door. Punch or drill a small pilot hole from one side, then complete the hole from the other side to avoid breakout on the face. A flat bit or hole saw of the right diameter is what you need here — the lock manufacturer will specify the size, usually 16 mm to 22 mm for the spindle.

3. Chisel the mortice slot in the door edge

Drill a series of overlapping holes with a sharp flat bit or a forstner bit to remove the bulk of the waste, staying just inside your pencil lines. Then clean up the sides and ends with a sharp chisel, working carefully to get the mortice as close to the lock case dimensions as possible. The lock should slide in with light hand pressure — not a tight knock-in, but not loose either.

Check the depth as you go by dropping the lock case in periodically. The faceplate needs to sit flush with the door edge; if the mortice is too shallow, the faceplate will stand proud and interfere with the door closing. To be fair, a millimetre or two can usually be hidden under the faceplate recess once that is chiselled in, but it is better to get it right the first time than to keep adjusting.

4. Chisel the faceplate recess

With the lock sitting in the mortice, score around the faceplate outline with a sharp knife or chisel. Remove the lock, then carefully pare out the recess to the depth of the faceplate thickness — usually about 2 mm. Work from the scored line inwards and take light shavings rather than trying to do it in one go.

Test the fit again. The faceplate should sit perfectly flush with the door edge, with the bolt mechanism operating smoothly. If the recess is a touch too shallow, a few careful strokes with a sharp chisel will sort it. If it is too deep, pack it out with a thin strip of card or ply behind the faceplate — it will not be visible once screwed down.

5. Fit the lock and the door furniture

Screw the lock faceplate to the door edge using the screws provided. Then fit the spindle through the spindle hole in the door and attach the handles or thumbturn rose plates to both faces. Most bathroom lock furniture comes with a thumbturn on one side and a plain rose or a coin-release slot on the other — the coin-release side goes on the outside of the bathroom so the lock can be opened in an emergency.

Tighten the grub screws or through-bolts that fix the handles to the spindle, but do not overtighten them to the point of cracking the rose plate. Check the bolt operates smoothly from both the thumbturn and the handle before you put any weight on it. If the bolt drags or sticks, the spindle may be slightly out of alignment — loosen the fixings, recentre, and retry.

6. Mark and fit the striker plate on the door frame

Close the door and operate the thumbturn to extend the bolt. The bolt will leave a mark on the frame (a dab of chalk on the bolt tip makes this clearer). Open the door and use that mark to position the striker plate, scribing around it and chiselling out the recess and the bolt hole behind it in the same way you did for the faceplate.

The striker plate recess needs to be deep enough for the bolt to engage fully — at least as deep as the bolt throw, typically 10–15 mm. Screw the striker plate flush to the frame, then test the lock several times. The bolt should engage cleanly without lifting or dropping, and the door should feel solid when locked. If the bolt is catching the edge of the plate rather than dropping into the hole, adjust the striker plate position slightly up or down until it lines up. A little bit of trial and error here is normal — frames are not always perfectly aligned with the door edge.

When to call a handyman

Call Richard if the door has already been chopped about by previous attempts and needs the mortice area built back up before a new lock can be fitted, if the door frame is damaged around the old striker plate, or if you simply want the job done in half an hour without any fuss. He also covers general door repairs, handle replacements, and stiff or dropping doors throughout Sandwich and East Kent.