Door with Narrow Frame

Aluminium and steel doors commonly have a stile too narrow for a standard lock case, and no amount of feature comparison helps if the body will not fit inside the frame member. Slimline locks use a compact escutcheon and a narrow mortice case built for exactly this problem.

Measure the stile first

The stile is the vertical frame member the lock sits in. Measure its face width, and if you can, the internal cavity depth. Those two figures determine what will fit.

Stile width	What it usually means	Format
Under 40 mm	Very narrow commercial aluminium	Specialist narrow case, limited product choice
40 mm to 50 mm	Standard commercial aluminium entry	Slimline mortice case, most of this category
50 mm to 70 mm	Wide aluminium or light timber	Slimline or some standard bodies fit
Over 70 mm	Standard timber or solid core	Full standard range available

Other constraints on narrow doors

Backset - narrow stile locks often use a reduced backset, commonly 25 mm to 35 mm, not the residential 60 mm.
Glazing bead - the glass edge inside the stile limits how deep the case can go.
Escutcheon width - must sit within the stile face or it will overhang onto the glass.
Door movement - lightweight aluminium leaves flex, which affects strike alignment over time.
Weather exposure - many narrow stile doors are also external, so IP rating still applies.

If the door also carries a vertical pull handle, that is a separate and compounding constraint. See Has Vertical Push or Pull Handle and take both sets of measurements before ordering.

Related categories

Vertical Push or Pull Handle · Glass Sliding Doors · External Gates · Mortice Locks

Common questions

How do I measure stile width?
Measure across the face of the vertical frame member beside the glass, from the door edge to where the glazing bead starts. That face width is the figure a slimline lock has to sit within.

Can a standard smart lock be modified to fit a narrow stile?
No. The mortice case dimensions are fixed and cutting a wider pocket compromises the frame. Use a lock designed for the stile width, or move to a surface-mounted or padlock solution.

Do slimline locks have a smaller backset?
Usually. Narrow stile cases commonly run 25 mm to 35 mm rather than the 60 mm typical on residential timber. Confirm the figure on the individual product page against your existing prep.

Guides and reference

Ch 02 - Stile width · Standard backset sizes · Ch 04 - Compatibility matrix