I have been very paranoid since an overflow we had a few months ago. It was the bathtub in the upstairs bathroom. It flowed all thorugh the bathroom but didn't make it onto the hall carpet. There was no caulking around the toilet. It leaked through the downstairs ceiling. It didn't get into any fixtures or anything. The overflow was caught quickly and I soaked everything up with towels and mops and turned the bathroom fan on. I've never seen any water stains or felt anything soft on any of the ceilings where it leaked through.

However, I do notice the upstairs shower, which did have caulking around it, seems to creak when I move around in it (and I'm really light). Could this be a sign that there was damage I just can't see and that it's rotting underneath the shower? This might sound silly but, if it is, could the whole shower fall through?

I would really appreciate any help, insights, advice. Thank you.

No, the whole shower will not fall through the ceiling...

What happened is that the sub-floor, which is interior plywood or particle board, warped a bit pulling the nails that hold it attached to the floor joists up with it. Now, when you step on it, it moves a tiny bit. What you hear is the plywood rubbing along the nails as it is pushed down - and then when it returns back up to its upper most position.

What kind of flooring is in that bathroom ? Tile ? Carpet ? Vinyl ? Hardwood ?

Vinyl. I think. Oh, and the toilet does have caulking around it now. This just happened to happen right after it had come loose and peeled off. :o

I agree with Milo and it may take months for that subfloor to totally dry. This will likely lead to mold but it is a very big project to tear everything up.

If that cracking really bugs you and you want to deal with it than you can re-nail the ply with ring-shank nails, over and through the vinyl, into the joists. You can also use screws. Than, you will have to install new vinyl over it and over entire floor.

Ballengerb has good point with mold. If you don't want to take any chances on mold than cut up and lift the old subfloor, let dry, spray entire area with mold desinfectant and install new subfloor and new vinyl. Obviously, this option is a lot bigger project to do.

Caulk around toilet base and shower base, too...

How will I know if it leads to mold? Is there anything else I can do to help that doesn't require pulling up the floor? Should I avoid using that shower/bath for now?

There is really no way of finding out if there is mold growth w/o opening up the floor.

You can keep using shower but you want to make sure you don't allow any more water onto your floor.