Why Aluminium Doors Drop, and What the Lock Case Has to Do With It

The complaint arrives in the same words every time: the lock has failed, the door will not latch, the key turns but the bolt misses the strike. In a narrow-stile aluminium door the lock is almost never the fault. The leaf has dropped — often by two or three millimetres at the lock edge — and the latch is now presenting itself below the hole it is supposed to enter. Replacing the lock case fixes nothing, and it is the most common wasted purchase in this category.

Understand what a narrow stile is and the reason becomes obvious. The stile is the vertical member at the edge of the leaf, and on a commercial aluminium door it is often only 30 to 50mm wide, because the client wanted glass and not frame. That leaves a very small section of extruded aluminium to carry the entire weight of a large glazed leaf, resist the leverage of someone pulling on a handle a metre from the hinge, and house a lock cavity cut into the middle of it. The lock pocket removes material from the one part already doing the most work.

Three things move, in this order. The hinge or pivot side wears first, because every opening is a lever with the leaf as its arm — a 2.2 metre glazed door concentrates a great deal of moment onto a top pivot. Second, the corner joints of the frame open slightly; a mitred aluminium corner held by cleats and screws is stiff but not infinitely so, and once it racks by a fraction of a degree the diagonal shortens and the free corner falls. Third, and only third, the door furniture shows it — because the latch is a precision fit into a strike with perhaps 2mm of tolerance, and it is the first component in the assembly with no slack left to absorb the movement.

This is why the hardware choice that matters most on a narrow-stile door is not the lock at all: it is what carries the leaf. A floor spring set into the slab takes the weight into the structure rather than into the frame corner, which is why a heavy glazed door on a floor spring stays square for years while the same leaf on butt hinges racks its frame. If you are specifying a door that is tall, wide, glazed, or used hundreds of times a day, the pivot decision is the anti-sag decision, and it has to be made before the floor is poured.

Now the part that genuinely is a lock-case decision. In a 30mm stile there is very little metal either side of the lock pocket, so the case has to be narrow, and its backset — the distance from the door edge to the centre of the follower — is fixed by the profile rather than chosen. Our narrow-stile cases run at 30, 40 and 45mm backsets for exactly this reason; a 60mm backset case simply will not fit a narrow stile, and forcing a wider pocket into the extrusion removes the material the stile needs to stay stiff. Ordering by backset first, and by everything else second, is the correct sequence for aluminium.

Two more details decide whether the installation survives. The first is latch throw. A door that has moved a millimetre still latches if the bolt is long enough to find the strike, and fails if it is not — which is why a hook bolt or a longer deadbolt throw is worth specifying on a wide leaf, and why our sliding hook-lock cases exist for aluminium sliders where a straight latch has nothing to bite. The second is the strike itself: an adjustable or slotted strike gives the installer a millimetre or two of vertical adjustment, and that is often the entire repair on a door that has dropped slightly. It costs almost nothing at order time and saves a site visit.

So the checklist, in the order that prevents the problem rather than chases it. Decide how the leaf is carried — floor spring for anything heavy, tall or glazed. Measure the stile and pick the backset from it, not from a catalogue habit. Choose a bolt with enough throw for the leaf width, and a hook rather than a latch on sliders. Specify an adjustable strike. And when a door does drop, adjust the pivot and the strike before ordering anything: the lock case is the symptom, and replacing a symptom is how the same fault comes back next season.

One honest note on what we publish. Our lock cases carry backset, centre distance, case size and bolt throw on the product pages, because those are the numbers this decision turns on and several of them came off our own production drawings rather than out of marketing copy. What we do not yet publish is a load rating for the doors these cases suit — that depends on the leaf, the glazing and the pivot far more than on the lock, and a number invented for the lock alone would be worse than no number. Send the leaf size, the stile width and the profile system, and the export team will tell you which backset fits and what to carry it on.