Latches

More correctly called a spring latch or a deadlocking latch, you definitely need to know the difference.  Latches are lock bolts designed to automatically lock the door when it is pushed closed.

Latches have one side that is beveled and that side impacts the strike and pushes the entire bolt into the door.  Once it is aligned with the hole in the strike plate, the bolt springs from the door to lock it in place.

The only problem with that design is the ability to slip something like a credit card between the door and the jamb and push the bolt back with it.  A very easy defeat that doesn't leave any sign of the unauthorized entry.

The solution to that design problem was a guard bolt addition to the latch.  When operating and installed correctly the guard bolt prevents the latch from moving or being depressed to unlock the door.

Operation is simple but the adjustment is routinely incorrect unless installed by a locksmith.  Here's how you check that yours is installed and working correctly.

Only the latch bolt should enter the hole in the strike plate.  Hopefully the strike plate hasn't been altered to allow the guard bolt to enter, only mis-located.  You can use the same tools that would be used to move the strike up or down, (see Building Settled), to move it to the proper depth that will keep the guard bolt depressed when the door is closed.

Don't forget to reinforce the jamb.  If you still have that air space that will allow someone to deflect the jamb, they are effectively increasing measurement B and your door could be vulnerable to that credit card attack.

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