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.

• Push the guard bolt in and then push on the latch.  If the latch won’t depress, the guard bolt is working correctly.
• Slowly push the guard bolt inward and every additional 1/16″ from full extension push on the latch to see if it will depress.
• Measure the distance from the edge of the door to the end of the guard bolt when the latch cannot be depressed and call that measurement A.
• Close the door and measure the gap between the edge of the door and the strike and call that measurement B
• If B is bigger than A the adjustment is incorrect and the gap between the door and jamb is too large.
• Adjust the size of the gap by adding shims under the hinge plate on the jamb.
• Loosen one hinge plate at a time and place a shim under the hinge, then tighten. (Washers work well as shims because you can put the screw through the hole and it will hold it in place).
• Once you have measurement B smaller than measurement A, it is correctly adjusted for depth and one more check to do.
• With the door closed, note if the guard bolt enters the hole in the strike plate.  If it does, that adjustment is not correct.

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.