A mortise lock is the primary locking mechanism of an aluminium sliding door, housed inside a pocket machined into the lock stile rather than mounted on the surface. Almost every modern aluminium slider uses one, so the practical question is not whether to have a mortise lock but which type of bolt it throws, what dimensions suit your suite, and how it interacts with the cylinder, the handle and the keeper.
What a mortise lock is
The word describes how the lock is fitted rather than how it works. A mortise is a pocket cut into a door to receive hardware, and a mortise lock is a lock body that sits inside it, as opposed to a rim lock which mounts on the face of the door.
On an aluminium sliding door the mortise is straightforward, because the lock stile is a hollow extrusion. The lock case slides into the cavity, a faceplate closes the edge, and the mechanism is entirely concealed within the section.
This is why mortise locks dominate aluminium sliders. The construction of the door already provides the pocket, so the concealed option costs no more to accommodate than a surface mounted one, and it looks and performs better.
The parts of a mortise lock
Five components work together, and knowing which is which makes ordering replacements far easier.
The lock case is the metal body containing the mechanism. Its dimensions determine what will fit your stile.
The faceplate is the visible strip along the edge of the stile, through which the bolt emerges. Faceplate length and profile vary by manufacturer and suite.
The cylinder is the keyed barrel, a separate component that drops into the lock case and drives it. Cylinders are replaceable independently of the lock body, which we cover in our article on cylinder locks for aluminium sliding doors.
The follower or spindle transfers handle movement into the case, so the door can be operated without a key from inside.
The keeper or strike is fitted to the jamb and receives the bolt. It is part of the lock system even though it is not part of the lock, and misalignment between bolt and keeper is the most common cause of a lock that has become stiff.
Why sliding doors use hook bolts
This is the detail that distinguishes a sliding door lock from a hinged door lock, and it follows directly from how the two doors move.
A hinged door swings, so a straight deadbolt thrown into a jamb resists the door being pushed or pulled. The bolt sits across the direction of travel.
A sliding panel travels along the track, parallel to the jamb. A straight deadbolt would resist that movement only until the panel was levered slightly away from the frame, at which point the bolt would clear a shallow keeper and the door would open.
A hook bolt solves this. Rather than a straight bar, it is a shaped hook that rotates out of the faceplate and engages behind the keeper. Pulling the panel away from the jamb tightens the hook against the keeper rather than releasing it. The harder the panel is levered, the more firmly the hook holds.
Claw bolts apply the same principle with two or more hooks operating together, spreading the load and increasing resistance.
Where a mortise lock on a sliding door is described as a deadlock, it is almost always a hook or claw bolt operating as a deadbolt rather than a straight bolt.
Dimensions that matter when ordering
Mortise locks are not interchangeable across suites, and three measurements decide whether a lock will fit.
Backset is the distance from the edge of the stile to the centre of the cylinder or spindle. It has to match the pre machined holes in the door and the handle set. A lock with the wrong backset cannot be made to fit without machining the panel.
Faceplate dimensions, meaning length, width and whether the edge is square or radiused, must match the existing cut out. A shorter faceplate leaves a visible gap in the stile.
Case depth must fit within the hollow section of the stile. Deeper cases suit larger commercial sections and will not enter a slimmer residential extrusion.
Alongside these, the handing matters. Many sliding door locks are handed left or right, and while some are reversible, many are not. Establishing which way the door slides before ordering avoids the most common ordering error.
If you are replacing a lock rather than specifying a new door, the reliable approach is to remove the existing lock and match it, or to identify the door suite and order the lock specified for it.
Fitting
On a new door, the mortise is machined during fabrication. The stile is routed to receive the case, the faceplate is recessed flush, and the cylinder and handle holes are drilled to the correct backset. Done in the factory, this is precise and repeatable.
Retrofitting into an existing door is harder. The mortise has to be cut accurately in an aluminium extrusion without compromising the section, which is the practical reason like for like replacement is usually preferred over changing to a different lock type.
Two things determine whether the finished installation works well.
Keeper alignment. The bolt must enter the keeper cleanly without being forced. A keeper set a few millimetres out will still allow the door to lock, but the mechanism works under load every time and wears quickly.
Panel height. A door sitting low on worn rollers drops the bolt below the keeper. This is the single most common cause of a mortise lock that has become stiff, and adjusting the rollers usually resolves what appears to be a lock fault.
Hardware manufacturers used in Australia
Mortise lock bodies are suite specific, so the lock generally follows the aluminium system rather than being chosen independently.
Doric is among the most widely specified sliding door hardware manufacturers in Australia, with mortise lock bodies matched to the major commercial aluminium suites.
Lockwood, part of the ASSA ABLOY group, produces mortise locksets used extensively across Australian residential and commercial doors.
Allegion supplies a range of Australian door and window hardware brands including Whitco, long established in sliding door locking.
dormakaba supplies architectural and access hardware, more commonly specified on commercial projects.
Hardware is generally distributed through Lincoln Sentry and Access Hardware, which stock across these brands.
Coastal considerations
A mortise lock is better protected than most sliding door hardware, since the mechanism sits inside the stile rather than exposed on the face. Salt still reaches it through the faceplate opening and through the cylinder.
Near the coast, in the eastern suburbs, Northern Beaches and Sutherland Shire, the failure sequence is usually predictable. The cylinder stiffens first, then the hook bolt mechanism becomes reluctant, then the springs weaken and the bolt no longer throws fully.
Because the cylinder is a separate part, the first stage is cheap to address. Leaving it produces the later stages, where the whole lock body needs replacing and the panel usually has to come out.
Specify stainless components where the manufacturer offers them, and treat a stiffening key as a signal rather than an inconvenience.
Maintenance
Mortise locks need very little, and the correct maintenance is largely about what not to do.
Operate the lock through its full travel a few times a year, with the door both open and closed, to confirm the bolt throws fully and enters the keeper cleanly.
Keep the keeper clear of debris. Grit in a keeper stops a hook seating properly, and a hook that only partly engages provides a fraction of its designed resistance.
Use a dry lubricant such as a PTFE or graphite based product. Do not use oil based sprays, which attract dust and eventually seize the mechanism.
Never force a key or handle that resists. Resistance almost always means misalignment, and forcing it damages a mechanism that a roller adjustment would have fixed.
Where the mortise lock sits in the system
It is the primary lock, and the other hardware on the door either serves it or supplements it.
The cylinder drives it and is replaceable independently, so key control and cylinder security are separate decisions from the lock body itself.
Multi point systems are mortise locks that engage the jamb at several heights from one handle movement rather than at a single point. On tall panels this removes the leverage that a single hook is vulnerable to, and it compresses the jamb seal along the full stile. We cover them in our article on multi point locks for aluminium sliding doors.
Auxiliary bolts address a different attack entirely. No mortise lock prevents the panel being lifted clear of its sill track, which is the most common way an aluminium slider is defeated. That requires a bolt engaging the head track, as covered in our article on patio bolts for aluminium sliding doors.
The glass remains the most vulnerable element on any sliding door, and no locking specification compensates for a pane that can be broken quietly.
Australian standards
AS 4145 covers locksets and hardware for doors and windows, with AS 4145.2 addressing mechanical locksets and setting performance and security levels.
It applies to the lockset as a product rather than requiring a particular security grade on a domestic sliding door, so a homeowner has no obligation to fit a specific level. Commercial and strata projects may have specification requirements, and these are worth confirming rather than assuming.
Note also that AS 5039, which covers security screen doors and window grilles, does not apply to the lock on the sliding door itself. A mortise lock cannot be AS 5039 compliant, because the standard governs security screens rather than door hardware.
Confirm current requirements for your projects, since standards are revised periodically.
Summary
A mortise lock is the primary locking mechanism of an aluminium sliding door, concealed inside the lock stile. On sliding doors it throws a hook or claw bolt rather than a straight deadbolt, because a hook tightens against its keeper when the panel is levered rather than releasing.
Backset, faceplate dimensions, case depth and handing determine what will fit, and these are suite specific rather than universal. Keeper alignment and panel height determine whether the lock works smoothly for decades or wears out early.
Treat it as one part of a system. The cylinder handles key control, a multi point version handles leverage on tall panels, an auxiliary bolt handles anti lift, and laminated glass handles the pane. Our aluminium sliding doors are supplied with matched mortise locking hardware and multi point options.
Frequently asked questions
What is the difference between a mortise lock and a rim lock?
A mortise lock sits inside a pocket in the door, and a rim lock mounts on the surface. Aluminium sliding doors almost always use mortise locks, because the hollow stile already provides the cavity.
Why do sliding doors use hook bolts?
Because a sliding panel moves parallel to the jamb. A straight bolt would clear a shallow keeper as soon as the panel was levered slightly away. A hook engages behind the keeper and tightens under that same force.
Can I replace a mortise lock myself?
Like for like replacement is usually straightforward if you can match the backset, faceplate and handing. Changing to a different lock type generally requires machining the stile and is better done by someone who works with the suite.
How do I know which mortise lock fits my door?
Remove the existing lock and match the backset, faceplate dimensions and case depth, or identify the door suite and order the lock specified for it. Handing matters too, since many are not reversible.
Why has my sliding door lock become stiff?
Most often the panel has settled on worn rollers and the bolt no longer aligns with the keeper. Check roller height and clear the keeper of debris before replacing the lock.
Does a mortise lock stop the door being lifted out?
No. That requires a separate bolt engaging the head track. It is a different attack from the one a mortise lock is designed to resist.