Hey there, if it's an exterior door and you're worried about a standard door handle lock being manipulated because of the gap, just use a deadbolt :) If it's an interior door than a slide-latch lock would work. I'd try and fix the door if possible though.

It is an interior door. Slide-latch... With chain? If so, would like something more secure... Suggestions?

What kind of security do you need? Nobody walking in on you in the bathroom or safe room? Is swapping out the handle/knob for one with a lock an option?

Good questions... Here's the complete scoop: This door leads from a basement to a Bilco-door. Up to now the Bilco-door has been secured by its own slide-locks. So the door pictured was just an access door to the Bilco. Note: I do not use or plan on using the Bilco for access into the basement. I have converted the Bilco stairwell to storage accessible from the outside. As a result, I no longer have the Bilco slide-locks to provide security into the basement. Therefore, this door now becomes the security between me/basement and the outside-world. Make sense?

It appears you have a hollow core door in a cinder block wall. If it's security you want, a flimsy door won't be fixed with a new lock. I'd recommend installing a solid core or fireproof door with a keyed deadbolt.

I "think" it's a metal door... I'll check, then be back!

I second what randy427 said above.

Metal door. Back to what locks can I use for the two un-even surfaces?

Yes... Thank you!

I'd go with a deadbolt lock like it was suggested in the first response. This is now essentially an exterior door. Deadbolts are the most secure.

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