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At Trade Door Handles, we've been specialist suppliers of locks and latches for decades, stocking everything from bathroom locks and tubular latches to 2, 3 and 5 lever sash locks and deadlocks for commercial and domestic properties. Alongside selling the hardware, we're asked almost as often how to actually fit it — and a mortice lock is one of the jobs that puts people off most, simply because it means cutting into the door itself.
This guide walks through fitting a mortice lock from start to finish, including the tools you'll need, the steps in order, the most common mistakes, and how the process differs slightly if you're fitting a sash lock rather than a deadlock.
What You'll Need to Fit a Mortice Lock
Supplied by Trade Door Handles, trusted UK supplier of architectural ironmongery since 1976, with quick nationwide delivery.
Before you start, gather the right tools together rather than stopping halfway through fitting your mortice lock:
- A sharp wood chisel (25mm is the standard width for most mortice lock bodies)
- A mallet
- A drill with flat wood bits, slightly wider than the lock's case thickness
- A pencil, tape measure and try square
- A screwdriver to match the fixing screws
- Masking tape, useful as a depth guide on your drill bit
- Safety glasses, since chiselling and drilling both throw off wood debris
You'll also need the correct mortice lock itself before you begin — take a look at our full range of Mortice Locks and Latches and Mortice Key Locks if you haven't already bought one, and always check the case size and backset match your door before cutting anything.
Choosing the Right Lock Before You Start
It's worth confirming exactly which type of mortice lock you're fitting before picking up a chisel, since this affects what you'll be marking out. Our Mortice Key Sash Locks combine a latch and a deadbolt in one case and need handles fitted on both sides, while a deadlock is key-operated only and doesn't need a spindle hole at all. If you're fitting a cylinder-operated version rather than a traditional lever key lock, our Euro Profile Sash Locks collection uses the same fitting method but needs an additional cylinder hole rather than a keyhole. Getting the lock type confirmed first avoids marking out the wrong cut-outs on the door.
How to Fit a Mortice Lock Step by Step
Once you've got the right lock and tools ready, fitting a mortice lock follows the same broad sequence regardless of lever count or brand:
- Mark the mortice lock position. Measure up from the floor to your preferred handle height and draw a centre line along the edge of the door. Hold the lock against the door at this height and transfer the top and bottom of the case onto the line.
- Drill out the mortice pocket. Using a flat wood bit slightly narrower than the lock's faceplate, drill a series of overlapping holes between your marks, to the depth of the lock case.
- Chisel the pocket square. Clean out the waste wood between the drilled holes with a sharp chisel and mallet until the mortice lock body slides in cleanly, sitting flush with the door edge.
- Mark and cut the faceplate recess. With the mortice lock body in place, score around the faceplate with a pencil, then chisel a shallow recess so the plate sits flush once fitted.
- Mark the keyhole and spindle holes. With the lock still in the pocket, mark through the keyhole and spindle positions onto both faces of the door, then drill or cut these through carefully from each side to avoid splintering.
- Screw the mortice lock into place. Drill small pilot holes for the fixing screws first, to stop the door timber splitting, then secure the faceplate.
- Fit the keep to the door frame. Close the door to mark where the latch and bolt meet the frame, then chisel a recess for the keep in the same way as the faceplate, deep enough for the bolt to extend fully.
- Test the mortice lock. Check the latch operates smoothly from both handles and that the key turns the deadbolt freely before finishing off with any final adjustments.
How to Chisel the Mortice Pocket Cleanly
The mortice pocket is the step most people get wrong, usually by removing too much wood too quickly. Work in thin, controlled layers rather than trying to lever out large chunks in one go, and keep checking the mortice lock body against the pocket as you go rather than guessing the final depth. A pocket that's slightly too tight is easy to fix with another light pass of the chisel; one that's cut too wide will leave the lock loose and rattling once fitted.
Fitting the Keep to the Frame
Common Mistakes to Avoid When Fitting a Mortice Lock
- Cutting the mortice pocket too wide. This leaves the mortice lock loose inside the door and can weaken the door edge.
- Skipping pilot holes. Screwing directly into timber without pilot holes is one of the most common causes of split door edges.
- Getting the backset wrong. Always confirm the backset of your new mortice lock matches your door handles and existing hardware before cutting anything.
- Rushing the keep. A keep that's fitted even a few millimetres out of position will stop the mortice lock's bolt engaging properly.
Fitting a Sash Lock vs a Deadlock: Does the Process Differ?
The core fitting method is the same for both, but there are a couple of practical differences worth knowing before you start:
- Sash lock — needs handles fitted on both faces of the door, so you'll also be marking and drilling a spindle hole alongside the keyhole. Our 2 Lever Sash Locks, 3 Lever Sash Locks and 5 Lever Sash Locks all fit in exactly the same way, with the lever count affecting security rather than the installation method.
- Deadlock — no spindle hole is needed, since the mortice lock is key-operated from both sides with no latch or handles involved.
If you're fitting a lock to an external door for insurance purposes, check it carries BS3621 certification before you start — this only applies to certain 5 lever locks, so it's worth confirming at the buying stage rather than after it's already fitted.
Can You Fit a Mortice Lock Yourself, or Should You Call a Locksmith?
Fitting a mortice lock is a realistic DIY job for anyone comfortable using a chisel and drill carefully, and most of the process comes down to patience rather than specialist skill. That said, if you're fitting hardware to a fire door, a listed building, or any door where the certification needs to remain valid, it's worth getting a professional installer or joiner involved, since incorrectly fitting a mortice lock can affect a door's fire rating or invalidate its certification.
Summary
Fitting a mortice lock follows the same broad steps regardless of lever count: mark, drill, chisel, fit the faceplate, then fit the keep.
Cutting the mortice pocket in thin, controlled layers avoids a loose or rattling lock.
Pilot holes for every screw prevent the door timber splitting.
A sash lock needs an additional spindle hole for handles; a deadlock does not.
Check for BS3621 certification before fitting a mortice lock to an insured external door.
Get professional help for fire doors or listed buildings where certification must be preserved.
Frequently Asked Questions
How do you fit a mortice lock?
Mark the mortice lock's position on the door edge, drill and chisel out a mortice pocket to fit the lock body, fit the faceplate flush, mark and cut the keyhole and spindle holes, then fit the keep to the door frame.
What size chisel do you need to fit a mortice lock?
A 25mm wood chisel suits most standard mortice lock cases, though it's worth checking your specific lock's case width before starting.
Do you need to drill pilot holes for a mortice lock?
Yes. Drilling small pilot holes before fitting the screws helps prevent the door timber from splitting.
Does fitting a sash lock differ from fitting a deadlock?
The core method is the same, but a sash lock also needs a spindle hole drilled for the door handles, which a deadlock doesn't require.
Can I fit a mortice lock myself?
Yes, for most standard timber doors this is a manageable DIY job, though fire doors and listed buildings are best left to a professional to avoid affecting certification.
What is a keep on a mortice lock?
The keep, also called a strike plate, is fitted to the door frame to receive the lock's latch and bolt, and needs its own recess cut in the same way as the lock's faceplate.
