I recently moved into a second story condo and have encountered what
might be a serious problem. When I was looking at the condo, I
noticed a gradual dip in the middle of the carpeted living room
(approximately 10' X 10' and perhaps 1 inch deep at the center). We
hired a building inspector to check it out and were told that the
plywood floorboards were slipping due to settling, though the condo
was structurally sound. He also said that this could easily be fixed
at a minimal cost by pulling back the plywood and reinforcing the
frame with some 4X4's. Well, today I peeled back the carpet to take
a look and discovered that the subfloor isn't plywood, it's concrete!
What the ?!?

Anyways, the condo is about 20 years old and the previous owners
assured me that the dip hasn't increased in the past 6 years or so.
Assuming that the frame is in decent shape, what would be the best
(and most economical) way of leveling the floor? I only plan on
living here for another 3-5 years and don't want to spend too much in
improvements. At the same time, I'd hate for my floor to collapse
onto my neighbors downstairs. Any advice? Thank you in advance.