Door Installation

After the proper door assembly has been selected for a particular application, it is critically important that the door be hung so that the security features in its hardware (latchset, locks, security hinges, and so forth) are not defeated by sloppy installation. The antishim feature of the latchbolt only works with proper installation. The antishim feature prevents an attacker from opening your door with a credit card some other semiflexible shim, or a pocketknife. Most good-quality door latchsets, even residential latchsets, have an antishim feature.

To see how the antishim feature works, find a door with a key-in-the-knob lock. Turn the knob or handle. The latchbolt is the part that retracts from the strike plate when the knob or handle is turned. Open the door and look at the edge of the door. The latchbolt protrudes and has the end-on appearance of the letter “D.” The flat side of the “D” has what appears to be a smaller “D” (or a pin) with its flat side to the latchbolt’s flat side.

This smaller “D” is the antishim. It can move either with the larger latchbolt or independently. First, push in on the larger latchbolt with your finger. You can push it entirely into the door. Release it and let it come back out.

Now, with a finger, push only the antishim (the small “D”) into the door and hold it there. Try to push the latchbolt into the door. The antishim should prevent you from doing so. Let the latchbolt come back out. Let the antishim out slightly, perhaps 1/8 inch. Try to push the latchbolt in again. Allow the antishim to come out a little at a time until the latchbolt can push fully into the door edge. A properly installed strike plate will depress the antishim this far while allowing the latchbolt to extend into the strike plate, holding the door closed. In a properly installed door assembly, the latchbolt can be retracted only by operating the knob or lever.

With a little bit of practice and a sharp-pointed knife, an attacker can open an improperly installed door almost as quickly as with a key. This is an example of how a very routine, often overlooked aspect of hardware installation can create a serious security vulnerability.

Key-in-the-knob locksets also can be defeated using a pipe wrench or a large water-pump plier. Independent deadbolts must be installed on doors that have to resist forcible entry.

Doors that swing toward the insecure area must have hinges that do not have removable hinge pins and that do have security studs on the hinge leafs.

Critical doors often are equipped with sensors connected to the alarm system. In most instances, these sensors function after the door has been opened slightly. In some applications, it may be desirable or essential to know when the latchbolt has been retracted, even if the door is not open. For example, someone inside the secure area wishing to open the door later from the outside may slip a barrier such as a thin piece of wood or metal between the latchbolt and the strike plate (the Watergate burglars taped the latchbolt).

At first glance, the door appears closed, but it will open with is a push or pull from the outside. Sensors can be installed behind the latchbolt strike plate to sense the presence of the latchbolt. If the latchbolt is not present or whenever the latchbolt is retracted, the sensor generates an alarm.