basic door latch question

OK.. this has to be basic.. but I don't know the answer, so I'll ask.
On door latches there are 2 parts that extend out from the door (which go into the strike plate). Both can be presssed towards the door. One is larger in diameter and is the part that latches inside the strike plate when the door is closed. But there is another, smaller diameter part that rides right along side the larger one. What is that for? If you push hard enough, that one can also latch inside the strike plate. Is it suppose to?
I have an exterior door that does not fit well and leaks air. I think it needs to be rehung, but if I press hard enough to get that second bar to latch, it's better.
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The second bar is "behind the main striker plate. I believe its purpose is to keep you from being able to use a credit card from opening the entire latch. If you try to push the card slowly into the tapered side of the door latch, from the outside, you can get the latch to retract into the door. But the separate bar does not push back gradually, so you can't get the door open.
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So with this explanation, that second, smaller bar would need to "latch". Hmm.. or maybe if it's slightly depressed (against the strike plate , but not extended into the opening in the stike plate) maybe it prevents the larger bar from being push into the door.
wrote:

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With the info posted here, I did an experiment.
The large bar is what locks the door in place into the strike plate. The smaller bar should not fall inside the strike plate, but rather be depressed about half-way. When it is, it prohibits the large bar from being depressed into the door. So to answer my original question, I think the door should fit snug just as the large bar extends into the stike plate, and at that time, the small bar should be just behind the large bar, but resting on the strike plate and not inside the opening of the strike plate.
So back to my real issue. My door is not fitting properly. I think it's hung slightly out of square as the lock side bottom is lower than the hinge side bottom, and the crack along the lock-side is not equally spaced. When closed, I can see daylight towards the bottom of the lock-side edge, and when it rains I get some water coming inside. The sweep at the bottom also rubs too much on the lock-side (as compared to the hinge-side), and is worn down and cracked and split.
So what I really need is an education on how a properly hung door should fit, and then how to hang the door in that way.
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It shoudl be square in the opening if the floor is level and the top is level and the sides are purely vertical.
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What kind of door is it?
If you are capable of installing a new door and the old one has just too many issues, a new pre-hung steel door can be got for like $130. If the frame is out of square, to properly fix it the old one may have to come out anyway.
For 130 and a couple of bucks for shims you get new door, new frame, new weatherstrip, new hinges, everything aligned and square (if installed properly).
You need to make the call which is best bang for the buck.
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The door is only 2-3 years old. It's a Jeld-Wen steel. I think the installer was a bit inexperienced, rushed, or figured "oh well, it's just a shop".
Could someone point me to some tips for hanging a door?
So initially I will get out the 4' level and check the hinge and lock- side jambs for plumb (or plumb bob). Then I will check the top for level. The door should then fit squarely inside the jamp with an even gap all around.
I guess I'd have to pull off the inside and outside trim if I need to adjust. So this may get ppd until spring or a warm winter day!
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wrote:

That is incorrect.
The smaller bar is a "double lock" that is NOT supposed to latch behind the strike plate.
It is there for the reason you stated, to prevent someone from entering with a credit card. BUT, it doesn't hold the door closed. What it does is lock the main latch in position so it cannot be defeated with a credit card.
Lock the door and depress just the little pin, as if it were resting against the strike plate. Now the main latch will not retract when you push on it.
If you think about it, what good would that little pin do to hold the door closed all by itself. Any moderate amount of brute force would bend it after the main latch is slipped out of the way with a credit card.
Loose-fitting door should be fixed with replacement weatherstripping. What was there is probably gone or worn out.
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coloradotrout wrote:

It detects whether the door is closed. If so, the latch cannot be manipulated by anything other than the door knob.
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