The most secure French door lock is a deadbolt. There are two types of deadbolts. A two-key deadbolt lock is unlocked using an outside and inside key. The single key deadbolt is safer for a homeowner in case of an emergency because it is unlocked on the inside with a turn knob. A deadbolt is used on French doors that have one working door and one stationary door.
Step 1 - Purchase a Dead Bolt Lock Kit
A kit comes with a deadbolt, instructions and a template. The paper template makes it easy to align the placement of the lock hole and the latch hole.
Step 2 - Use the Template
Tape the template that comes with the door lock to use as a guide for placement of the lock and latch holes.
Step 3 - Make a Lock Hole
To drill a hole for the French door lock use a hole saw. Stop drilling before going completely through the door. Then continue drilling the hole from the opposite side this will prevent splintering.
Step 4 - Drill Latch Hole
The latch hole is created by drilling with a spade bit on the edge of the door. While drilling you will cross the lock hole that you have already made.
Step 5 - Install the Latch Plate
Place the deadbolt and the latch plate into the latch hole. Trace an outline of the latch plate on the edge of the door with a pencil. Now carve a mortise for the latch plate using the chisel. Affix the latch plate with screws into the mortise.
Step 6 - Install the Lock Body
The lock body installation can vary depending on the manufacturer of the French door lock. You should follow the instructions that come with the deadbolt. Generally, the next thing you do is install the assembly by attaching the inner and outer cylinders with screws. Test the lock with the key to ensure that it is working properly.
Step 7 - Add Deadbolt and Strike Plate
Determine where the deadbolt hits the adjoining door edge of the stationary door. To do this partially close the working door and mark the position. Take the strike plate and using the utility knife score around it then chisel a mortise. Set the strike plate in its mortise. Mark where the hole will go for the deadbolt and drill the hole in the edge of the stationary door. The hole should be the same length as the deadbolt. Finish by screwing the screw plate into the mortise.