Bob F wrote:
Dante M. Catoni wrote:
*It sounds as though you have a decent subfloor. What I would look
at is the joists. How long of a span do they have without support?

Something else to consider is that the previous owner may have put
down a fast and cheap tile job to sell the house. The tiles may be
popping up because they were not installed properly.

I think you are right about the tile job. No screws were added to the
existing subfloor to prepare it for tile. The ply wood is too thin
and doesn't have nearly enough screws. Now, the Hardiebacker website
says that 5/8" ply is the minimum to put the cement board on top of.
Does that mean if there is nothing else underneath it? So if I
remove the 1/2" layer then screw the 3/4" layer to the joists every
few inches so that it is very solid, could I use 1/2" ply under the
cement board? I would still have over 1" of wood underneath the
cement board.

I have been told to use minimum 1 1/4" of wood under the backerboard.
I added a layer of 3/4" ply under the 3/4" subfloor plywood on a
small bathroom to keep the final level even with the other floors. I
used 2x4s under the added layer attached to the joists to brace the
addition, and lots of screws between the two layers, especially near
the joists.

The additional layer was between the joists, below the normal subfloor level. I
didn't make that clear.