Locate a Deadbolt Strike Exactly by Taking an Impression in Foil Tape

The Problem

A deadbolt strike has very little margin. The mortise has to land where the bolt actually throws, and on a door that's already hung, the bolt's position is the product of the lock's height, how the door sits in the opening, and how square the jamb is. Transferring that position by measuring means measuring from the door, walking the number to the jamb, and hoping the door closes where you think it does.

When the mortise lands a couple of millimetres out, the bolt drags on the edge of the plate or won't throw its full length, so the lock no longer engages the depth it's rated for. When it lands further out, the mortise is simply in the wrong place in a finished jamb, and the fix is a filled and re-cut jamb rather than an adjustment.

The Technique

Let the bolt mark its own position.

1. Cut a piece of foil tape big enough to cover where the plate will sit, and stick it flat on the jamb at roughly the right height.
2. Close the door.
3. Throw the bolt against the foil and retract it. Do it a few times.
4. Open the door. The bolt has stamped its outline into the foil — actual size, actual position, actual height.
5. Mark the mortise and the bolt hole off that print, then peel the tape and cut.

The print records where the bolt lands with the door on its current hinges, at its current sag, against the jamb as built — so it already contains every error a measurement would have to account for separately.