Dante M. Catoni wrote:
My bathroom floor is constructed as follows from the bottom up
2" X 8" joists 16" on center
3/4" X 7" boards running perpendicular to joists
1/2" x 7" boards running parallel to joists (very wavy)
3/16" plywood?
ceramic tile

Within a few weeks of buying this house the tiles started cracking and
the grout popped out. I just took out most of the tiles by hand.

I was planning to remove the 1/2" boards and put down
5/8" exterior grade plywood
thinset
1/4" Hardibacker
then tile again

The floor will end up a little higher than it is now which is about
even with the hall, but it might still be ok. Would it be better to
go down to the joists and start with 1" ply? If I put thicker
plywood on the joists can I go directly to cement board or are there
two layers for a reason? Is there anything else I should consider? I
want to do the job right and make it last.

What was there sounds as if it should have been OK. Obviously, it wasn't so
step #1 is to determine why...what's flexing and why?

a) Is the ply firmly attached?

b) Ditto the "wavy" boards and what do you mean by wavy?

c) How about the bottom sub-floor? Any movement when you bounce on the
floor?

d) How do you know what those layers are? Already ripped up a section?

I'm thinking the fastenings are old/shot/inadequate and that a bunch of 3" x
#10 screws through the whole works into the joists might solve your problem.
Unfortunately, no way to tell for sure.

dadiOH