Window Sash Lock Spacing: How Many Locks a Wide Window Needs Before Prying Flexes the Rail Open

A wide sliding or casement window fitted with a single central lock can be opened by prying at a corner, flexing the meeting rail until the latch clears its keeper, without breaking anything. This study models the rail as a beam from stated values: a 700 N pry, an aluminium rail of elastic modulus 69 GPa and second moment of area 7.5e-9 m^4, and a 6 mm flex at which the latch disengages. Deflection grows with the cube of the unsupported span, reaching 6.09 mm at 0.6 m and 14.43 mm at 0.8 m, and the widest span that stays below the escape gap is 0.59 m. A window 1.8 m wide therefore needs 3 locks rather than the customary one, at a stated hardware cost of USD 27, while a 1.2 m window needs 2. The rule is to space window locks no more than about 0.55 m apart along the meeting rail, and to count a window's locks by its width rather than by habit. SIMULATED, NOT MEASURED.