Today I am going to present two techniques, one offensive and one defensive. The first is how to shim a door using the ol’ “credit card trick.”  The second is how to protect your doors from being shimmed.

This is the first post in a series tagged Defense Against Methods of Entry (DAME). The DAME suite of posts will cover ways to improve your locks, harden your home, and protect yourself from common lock defeat methodologies.

Moral Justification for Lock Defeat Techniques

Since this is the first article here where I am presenting some lock-defeat technique, I’m going to lead off with something of a disclaimer. Some of you may be wondering “why would you teach people to defeat locks?” Simple: if you do not understand how they can be defeated, you cannot mitigate these defeats. It is literally as simple as that.

But also, just so it’s said: never perform any lock bypass/defeat/picking techniques on any lock that you do not own or have explicit permission to access! Breaking into someone else’ home or business is a serious crime, and I do not condone such activity.

Shimming or the “Credit Card Trick”

The “credit card trick” – more appropriately known as shimming the latch – is extremely prolific. It’s so prolific it even has a nickname. You’ve probably seen examples of it in movies and on television. You may have seen it done, and you may have done it yourself. It is so prolific because so much door hardware is susceptible to it. Few people actually know why and how it works. There are two facts you should know in order to understand how this technique works, and when it won’t. There are a couple of contraindications for this technique that yous should be aware of.

  • This technique will not get you through a door with a locked deadbolt. A piece of plastic applying force in a perpendicular direction will not disengage the deadbolt. This technique is useful only for doors secured only with a locking knob set or lever set.
  • This technique does not work on doors that open outward. Generally this technique is reserved for inward-opening doors, like most of the exterior doors on your home. In a future post I will talk about a very similar technique for outward-opening doors.

With that out of the way, let’s talk about why this works. The short “why” answer is that the door was either installed incorrectly, or is incorrectly adjusted. The long answer is…well, longer.

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The Vulnerability Behind the Technique

Locking knobs and levers rely on a spring-loaded latch to keep the door locked. You probably don’t think much about this latch until you can’t get into your door, but it’s what’s doing the work. The latch is rounded on one side and flat on the other. The rounded surface is what allows you to slam the door. The rounded portion strikes the strike plate (photo further down) which pushes the latch back into the door, allowing it to close.

Note the two, separate moving parts: the latch (the large, rounded portion) and the “dead latch,” the independent moving piece along the back of the latch.

The flat, back side of the latch is a little more complicated. Along this flat surface is another, separate part that moves independently of the latch. It is called the “dead latch.” If you mess around with it you’ll learn a couple of things. First, if the dead-latch is depressed the latch can’t be pushed back into the door; it becomes “dead.” If the dead-latch is extended the latch becomes “live” meaning it can be pushed back into the door.

A strike plate. The latch and dead latch are designed to interact with a strike plate.

Your door’s latch is designed so that when your door is closed, the strike plate (the metal plate on your door jamb that interacts with the latch) keeps the dead-latch depressed. If all of this works correctly, your door is safe from the “credit card trick” because the latch can’t be depressed.

If the dead latch is depressed independently of the latch, the latch is now “dead,” meaning it cannot be fully depressed into the door.

So what makes the credit card trick work? The thing is, most strike plates don’t prevent that little latch from extending. Let’s take another look at what I mean. Below are three photographs. The first is a latch and dead latch, both fully extended. I have placed some yellow paint on the end of the dead latch so we can tell if it seats correctly, or falls into the strike plate.

Note the small amount of yellow paint on the end of the dead latch.

The next photo (below) shows the dead latch after I have closed the door normally. The dead latch is depressed (not extended) and is currently functioning correctly. We would not be able to shim this door at this time because the latch is “dead” and cannot be pushed back into the door.

A firm tug on the door from the outside is enough to allow the dead latch to fall into the strike plate. At this point the dead latch has been deactivated and the latch is once again “live.” This latch is ready to be shimmed.

Our yellow paint on the end of the dead latch has disappeared, indicating that the dead latch is not functioning correctly and the door can be shimmed.

A shim is just a thin piece of material that is introduced between the latch and strike plate. Because the latch is live, it can be pushed back by the shim, retracting the latch without turning the knob.

IMPORTANT: In the photos above I am demonstrating from the interior of the door. If you are attempting to shim this door, you will not be able to see this process happen because you will be on the other side of the door. You will not be able to see the gap between the door and the jamb. You will have to rely on other clues to know that the latch is not dead. Namely, you may hear it “click” into place when you tug on the door. Some homeowners will take this step for you.

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Here is a good example. Many years ago, before I knew anything about locks,  I lived in a condo on the beach. Every morning when I would leave for work I would lock my door knob from the inside, then pull the door shut (the condo did not have a deadbolt). I would hear an audible “click” as the latch fell into the strike plate. Not knowing any better, I pulled the door snugly until I heard the second “click” of the dead latch falling into the strike. I thought I was doing a good thing; little did I know I was defeating much of the (admittedly minimal) security of my door’s lock.

But…why?

Why would lock manufacturers and builders make your securing dependent on such an easily bypassed mechanism? They don’t do it on purpose but there are a lot of potential causes for this vulnerability. First, your door and lock might have been installed incorrectly. Carpenters aren’t locksmiths. When a house is constructed locks are normally installed by the general contractor who, in all likelihood, doesn’t know a whole heck of a lot about locks. Same deal if locks have been replaced or upgraded by previous owners of your home – who knows what that homeowner might’ve done.

Even if the lock was installed perfectly, most doors are out of adjustment within just a few months. This happens because of settling, shoddy installation, cheap components, swelling and shrinking of the door jamb with changing weather, or some combination thereof. Worse, there is usually soft, pliable weather stripping around the door. If you can pull the door knob from the exterior and compress the weather stripping, there is a very good chance you can get that dead-latch to fall into place.

Soft, pliable weather stripping provides some built-in play in the door.

Whatever the reason, this is a dangerous vulnerability. What I am revealing here is nothing that hasn’t been revealed in books, YouTube videos, podcasts, and blog posts long before this one. The point: a lot of people know about this. The information is widely available, and the necessary tools can often be quickly fabricated out of garbage. What is less talked about is how to correct such a vulnerability.  Let’s look at the technique, then we’ll look at the fix.

How To Shim a Door

You only need one piece of equipment to pull of this technique (assuming it will work in your situation). That piece of equipment is a somewhat tough, yet flexible piece of plastic. The best thing is shim material like that sold by Sparrows Lock Picks. If you don’t have that I have found that gift cards – like the Starbucks card in some of the pictures – are usually an acceptable substitute. Hereafter I will refer to whatever you use as the “shim.”

Regardless of what you use, I recommend rounding off the corners. Material that is rigid enough to shim the latch can also damage weather stripping. You don’t want to damage your own weather stripping. Nor do you want your material getting caught up on weather stripping, potentially fouling your attempt.

Next, before your begin shimming the latch, you need to ascertain that either there is no deadbolt present or the deadbolt is not locked. If there is no deadbolt, you’re good to go. If there is, you can see if the deadbolt is engaged or not by using your shim. People often take security shortcuts for the sake of convenience, and deadbolts sometimes don’t get locked. To check, simply insert your shim between the door and jamb at the height of the deadbolt, and see if it runs into the deadbolt.

Once you are sure the deadbolt is not locked, you can begin. First, pull the door toward you as hard as you can. This gives the best possible chance that the dead-latch will extend fully. Listen carefully for the dead latch to fall into the strike plate. If you don’t hear it this could simply mean that the dead latch is already set. Insert your shim between the door and the jamb, just above the knob’s latch. Insert it at an angle as shown in the photo below.

Coming in at an angle like this helps the card to get between the strike plate and the latch, by introducing it a little at a time. Keep applying downward and inward pressure on the card.

At the same time, grasp the door knob and “wiggle” the rapidly in an inward/outward motion. It is possible for the latch to bind against the strike plate; wiggling the door  can take some pressure of the latch and allow the shim to slip in.

Once the shim is inserted deeply enough it will separate the bolt from the strike face and the door will open. Voila! If the door doesn’t open after a few second, check your technique and try it again. If it still doesn’t open…it’s probably not going to.

Defense

Defending against this technique is fairly easy: have a correctly installed deadbolt on all your exterior doors that is used at all times. Some of you may be unable to install a deadbolt. While deadbolts can cost as little as $12, you do need some tools to install one. That may be cost prohibitive for some people. You may also live in an apartment where the installation of a deadbolt is not allowed. I  have lived in such places, (though I never will again).  So, bottom line: deadbolt if at all possible, but it may not be possible for you some of you.

If installing a deadbolt is not possible, your best bet is to adjust the door so that it cannot close “too far” allowing the dead latch to fall into the strike plate. The simplest way I have found to accomplish this is by adding something to the door jamb to limit how far it can close.

You are limited only by your imagination. If you have the appropriate tools you could rip down narrow strips of wood and glue it to the jamb once the sufficient width has been found. A wooden, #2 pencil placed against the jam might prevent the door from over-closing. Believe it or not, on at least three separate doors I have been able to correct the issue by placing a couple of pieces of 9mm brass into the weather stripping. You may have to play with this; 9mm brass may be too large or small, preventing the door from closing sufficiently, or still allowing it to close too far.

Make no mistake; this is a serious vulnerability. Test your door. If your dead latch falls into the strike and you don’t have a deadbolt, fix this issue ASAP.

Editor’s note: This article was originally published by Justin “Graveyard” Fields for Swift | Silent | Deadly. This article contains affiliate links.