Fit a mortice deadlock into a timber door and its frame, marking, drilling, chiselling and testing the lock.
MDL-01Check the door and choose the right lockMeasure the door thickness — most mortice deadlocks need at least 44mm to take a 64mm or 76mm case. Buy a lock sized to fit, with a backset (distance from door edge to keyhole centre) that suits your door stile. If it's a front door, a 5-lever lock to BS3621 is worth buying — insurers often ask for it, though it isn't a legal requirement.
RULES1 each · Mortice deadlock kit (lock, keys, escutcheons) KNOW THE RULES
If this is a fire door — common in flats, HMOs, or between a house and integral garage — Building Regulations require certified fire-rated ironmongery. Use only a lock listed as suitable for that door's fire rating, and check with your landlord or building control if you're unsure.
MDL-02Protect the work areaLay a dust sheet under the door to catch shavings and sawdust. Wedge or prop the door steady so it can't swing while you work — some people find it easier to lift the door off its hinges and rest it flat on trestles, but working it upright is fine too. Put on eye protection and a dust mask before drilling or chiselling.
1 each · Disposable dust mask MDL-03Mark the lock heightMortice deadlocks usually sit 150-200mm above the existing latch or handle, which normally works out around 990-1050mm from the floor. Use a try square to mark a clear, level line across the door edge at this height, and carry the mark onto both faces of the door.
MDL-04Mark out the mortice using the lock as a templateHold the lock body against the door edge on your marked line and trace around the case with a pencil. Mark the forend outline separately, and mark the keyhole centre point on both faces of the door.
MDL-05Drill out the mortice pocketFit a flat wood bit matched to the width of the lock case. Drill a row of overlapping holes into the door edge, keeping the drill square and level, down to the depth of the lock body — typically 70-80mm for a 76mm case. Go slowly and check depth often rather than drilling all the way in one go.
1 each · Flat wood bit (sized to lock case) MDL-06Chisel the mortice squareClean up the drilled pocket with a sharp chisel until you have a neat, square-sided mortice. Test-fit the lock body repeatedly, trimming a little at a time, until it slides in flush and sits square in the door edge.
MDL-07Recess the forend plateHold the lock in place and mark round the forend plate onto the door edge. Score the outline with a knife, then pare out a shallow recess, usually 2-3mm deep, so the forend sits completely flush once fitted.
MDL-08Drill the keyholeMark the keyhole centre on both door faces using the lock as a guide. Drill through with the bit size stated in the lock's fitting instructions — commonly 10-12mm — working from both sides to avoid splintering the far face. File or pare the hole slightly if needed for a snug fit under the escutcheon.
MDL-09Fit the lock bodySlide the lock into the finished mortice and check the key turns smoothly with no binding. Screw the forend flush into its recess using the screws supplied with the lock.
MDL-10Fit the escutcheonsFit the keyhole escutcheons to both faces of the door, lining them up with the keyhole. Most are either screwed or push-fitted — follow the kit's fixing method.
MDL-11Mark and cut the keep in the frameClose the door and mark the bolt's position onto the frame in pencil, then open the door and square the line across using a try square. Mark the keep plate outline, then drill and chisel a mortice into the frame deep enough to take the bolt's full throw, usually 20-25mm.
MDL-12Fit the keep and test the lockScrew the keep plate flush into its recess in the frame. Close the door and turn the key several times to check the bolt engages smoothly. If it binds, ease the keep mortice slightly with a chisel or rasp rather than forcing the lock.
MDL-13Tidy up and finishVacuum up sawdust and shavings from the door edge, frame and floor. Lightly sand any rough edges around the mortice and keep. A dab of graphite lubricant in the keyhole and on the bolt keeps the mechanism working smoothly.
1 each · Graphite lock lubricant Some links may earn us a small commission at no extra cost to you.
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