I have had the house for 5 years, and just noticed it last year. It seems to be getting worse.
Any idea what could be causing this? Is it serious, or just annoying? How do I fix it? I have some fiberglass and epoxy left over from a boat project; would it make any sense to reinforce the top surface of the floor? (presumably the real fix is from underneath, and that would have to be much more work than a simple reinforcement)