I ran into this when we pulled up the old tile and cabinets in our 1967 era home. The bathroom is on pier and beam, but back in the day, there wasn't cement board around so they poured about two inches of concrete between the floor joists on top of the subfloor then laid the tile.
After the tile was pulled up it looked like a minefield, so I had someone come in and repour the concrete and smooth it out. Now, however, one of the several contractors that has given me bids tells me that I need to tear it all up (since it turned out to be uneven anyway,) because the concrete and tile could be dangerously heavy unless the subfloor consists of boards that are some exact number of inches wide.
Please help. I have been living in dust for months now, and have had the tile laid but it was done incorrectly so I am in the market for a real contractor. So far I have had 4 estimates and only this one guy tells me that this is dangerous. He also tells me that if he removes the sheetrock and sees the (old aluminum) wiring, he has to rewire the entire bathroom because that is the law. I think this is a shyster, but it worries me. He is also the only one to mention a $500 permit required to move the shower drain.
Yes, well, we knew the house had aluminum wiring when be bought it. It has survived since 1968 so I'm not going to worry about it. Right now, I need to get my master bedroom/bath back to some kind of liveable condition. I've moved into the guest room.