On May 16, 11:04*am, "Bob F" wrote:
Bob F wrote:

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.

Who told you you needed a minimum of 1 1/4" of plywood under the
backer board? I'm not aware of any product that requires more than
3/4".

R