Fitting a mortice latch

Door latch types, sizes and fitting tips

Tips on this page cover:

I'll explain everything I know on this page about fitting a mortice latch perfectly. It's important to take your time when setting out and drilling, as if you get either of these wrong you risk damaging the nice new door you've just hung and/or having a badly working latch.

Which size door latch should I use?

The difference in sizes available refers to the length from the face plate to the centre of the square spindle hole, and governs how far into/across the door the handle or knob will be fitted from the leading edge. When choosing which size latch to fit in panelled doors, go for the one that puts the handle closest to the middle of the door jamb otherwise it can look odd when fitted off centre.

Another thing to consider when choosing the door latch size is the dimensions of the doorstops. When I'm fitting a mortice latch in 4 panel fire doors generally in victorian houses that have had loft conversions, I use long 102mm latches because otherwise the door knobs would be so close to the doorstops that you'd wrap your knuckles on them when opening the door.

With ply flush/hollow core doors 102mm will be too long though, because the lock block inside is not always deep enough and you'll have nothing to screw the handle or door knobs to. I normally choose 68-73mm latches for ply flush doors, which work perfectly.

Fitting a mortice latch - tools required

When drilling the fairly large holes required use a slow speed on the drill and adjust the clutch if it has one in order to prevent the drill bit 'snatching', and potentially breaking your wrist!

Fitting a mortice latch - step by step guide

Step 1: Establish the handle height

The normal height for fitting a mortice latch is a metre up from the floor (around 3 feet), but you can either match existing handle heights, ask the client how high they want them or check the drawing/schedule.

Four panel doors and period doors often have a knob and mortise latch fitted to the centre of the lock rail. This can weaken some doors though so check with the manufacturer if you are looking to install the latch there.

Mark the height for the centre in the narrow edge of the door jamb, and square a faint line round with a sharp pencil to where the handle will be fitted, on both sides of the door. Measure the distance from the front of the face plate to the centre of the spindle hole and mark that too (I set a combination square to this measurement to save time when fitting more than one latch).

Use the marking gauge to work out the centre of the door edge. You can guess this at first and push the pin of the gauge into the door working from both sides. You know the centre of the door is inbetween the two pin holes.

Step 2: Drill the door

For standard handles/knobs I drill a 16mm hole in from each side of the door - being careful not to go all the way through. Next, I use a 25mm flat wood bit to drill into the centre of the door edge. I prefer flat wood bits for fitting a mortice latch but you can use augers. If it is a panelled door I put tape on the drill bit to mark the length of the mortise latch. This will help stop you drilling too deep into the door and coming out through the moulded panel.

I put a clamp on the door aswell, especially if it is a thin or hollow core door. This will ensure the door doesn’t split. Use a slow speed and the side handle of the drill if it has one. To prevent drilling out the side of the door stop every now and then to check you are straight and level.

Clear any loose bits from the hole and push the latch inside.

Step 3: Mark round the face plate.

Make sure the centre of the latch is in line with the height previously marked on the door. Some people screw the latch to the door and mark carefully around it with a sharp Stanley knife, but its not accurate enough for me. When fitting a mortice latch to make sure it is parallel I mark a small pencil line at the top and bottom edge of the latch, then use the combination square to square the lines across. I also set up a mortise or marking gauge to scribe the vertical edges.

Step 4: Chop it in.

Bang the 25mm chisel into the door just inside the top pencil line and with the flat edge of the chisel facing up. Turn the chisel upside down and repeat at the bottom - again, keeping just inside the lines. Now using the same technique for chopping out the hinge, remove the timber inside being careful to cut only as deep as the faceplate is thick. Push it in every now and then to check, if you need to make it wider push the latch in, hold it tight to one side and use a sharp stanley knife to cut down the other side. When it fits perfectly drill pilot holes and screw it to the door. If you do chop too much out cut some cardboard to put behind it and keep it flush with face.

Step 5: How to fit the door handle/knob

After you've screwed the latch in push the square spindle through one of the holes in the side of the door, slide the handles/knobs on and screw them to the door. You can pilot drill again if necessary and use the combination square to make sure the handles are plumb/level.

Sometimes door knobs are difficult to screw on without the drill chuck scratching the door knob. In this instance mark the positions of the holes on the door, remove the knob and pilot drill before using a long pozi bit or screwdriver to screw the knobonto the door.

Step 6: Fitting a mortice latch keep / strike plate on the lining / frame.

Push the door to the frame and where the latch strikes it mark the top and bottom of it with a sharp pencil. Square these across the widest part of the lining. Place the strike plate on so the two lines are central in the hole, like in the picture above. If you just draw round it now you'll end up with a loose fit when its chopped out because you are adding the thickness of the pencil lead all the way round. Instead, mark a small sharp pencil line at the top of the keep and the top of the strike plate. Move the keep up slightly, just enough to cover the pencil lines, then mark the bottom of the latch and striking plate. Again square these faintly across the lining.

Step 7: Mark the keep the right distance in from the face of the lining.

To make sure when the door closes it stops flush with the lining and wall, you need to put the keep against the latch, in the position it will be when the door is closed. Transfer a line onto the door from the edge of the keep closest to the door face, as in the picture. Set your marking gauge to this line and scribe it into the lining in between the lines marked in step 6. Place the keep back on the lining so it is in line with all the marks and the new scribe. Mark the final edge and use either a sharp knife or the gauge to cut along it.

Step 8: Chop the keep in

Carefully remove the timber inside the lines, using the same technique as for the door latch. Push the keep on and mark a pencil line all the way round the hole inside for the latch. Remove the keep and drill a 16mm hole about 10mm deep. Use the 10mm chisel to square the hole up, cutting at the top and bottom first. Fit the keep back on and screw to the lining.

Close the door till it clicks shut, flush with the lining.

Fitting the Doorstops

Measure inside the head of the lining for the doorstop.When nailing the doorstop, close the door and nail the head on first using 40mm oval nails or a finish nailgun if you have one. Leave a 2mm gap if the door and lining are to be painted so that it won’t ‘bind’. Do the same for the legs, I check for a bow in the doorstop and fit these so the middle bows away from the door. It’s easiest to nail the top first, then the bottom and push the middle up to the door last.

Still not right?

Click here for help adjusting a door

Whats next?

After fitting a door latch and the doorstops are on you are ready to cut and fit the architraves around.

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