Strike Plate Screws: The Length at Which a Door Strike Stops Being the Weakest Part of the Lock

Many deadbolts are defeated not at the bolt but at the strike plate, whose short screws bite only into the thin frame facing rather than the structural timber behind it. This study computes strike resistance from stated assumptions: a 4 mm screw embedded in softwood resists pull-out at 35.2 N per millimetre of embedment, four screws share load with a factor of 0.7, and the frame itself splits at 5,400 N, capping the assembly. The short 19 mm screws supplied with many locks give a system resistance of only 1873 N. Resistance climbs linearly with length until, at 54.8 mm, the screws would hold as much as the frame itself can bear; the 76 mm screws tabulated reach the 5400 N cap, an improvement by a factor of 2.88 over the short screws, for a total screw cost of USD 0.48. Beyond the crossover, longer screws buy nothing because the frame becomes the failing part. The resulting rule is simple: fit strike screws of at least 65 mm that reach the structural timber, and spend any further money on the frame rather than on still longer screws. SIMULATED, NOT MEASURED.