French doors look wonderful on a house but you need to make sure they have adequate locks. If you don't, they serve as an invitation to a burglar. You can hire a locksmith to put a lock on the French doors or you can save the money and do it yourself. It's not a difficult job and replacing a deadbolt lock on French doors doesn't require much by the way of tools. You should be able to accomplish the job in an hour or less.
Step 1 - Removing the Old Lock
Have the French doors open so you can access everything easily. Start by unscrewing the strike plate, which is on the edge of the door that doesn't hold the lock mechanism. This will be held in place by two screws. Unscrew the latch plate which is on the edge of the door with the lock. Again, this will be held on by two screws.
Remove the body of the lock. To do this, you'll have to unscrew the lock cover. The screws will be on the inside of the door on the lock. Remove the lock cover and you'll find two more long screws which are used to hold the lock body in place against the plate which stays on the inside of the door. Remove these carefully (it could take a while) and remove the lock body. You will need to twist it to take out the deadbolt.
Step 2 - Installing the Strike Plate
Remove the strike plate from your new lock kit. Match it against the opening of the old strike plate on the French door. If the screw holes match up, fill them carefully with plastic wood before proceeding. This ensures that the screws will grip firmly and can't be easily loosened. If the screws don't line up, but the chiseled opening for the deadbolt fits well, you can proceed. The screws need to be at least 1-½ inches long to give a secure fit.
Step 3 - The Lock Body
Install the lock body in the round hole already in the French door. You'll need to slide it in at an angle so the deadbolt (which should be in the retracted position) will fit.
To secure in place, thread the two long screws through the small plate that will fit inside the door and screw into the correct holes in the body of the lock. Before tightening, use the key on the outside and inside of the door to open and close the lock. Try it first with the door open to ensure the mechanism operates properly, then repeat with the door closed to check whether the deadbolt slides smoothly in and out of the strike plate.
If necessary, make adjustments by loosening the long screws which will let you move the deadbolt up and down a little. If the new deadbolt is longer than the old one, you might have to use a chisel to deepen the hole behind the strike plate (remember to remove the strike plate first if you need to do this).
Once you're satisfied with the easy operation of the key, the mechanism and the deadbolt, you can tighten the long screw to make sure there is no movement or play. Add the inside lock cover. With the door open and the deadbolt extended, screw the latch plate in place.