Mine has been down since 1965 on double layers of 3/4 treated plywood and is
still solid with no cracked grout, BUT most installations now seem to have a
bottom layer of ply and a top layer of backerboard.
Either can work, although Cement backerboard is the better choice,
especially in an area that will likely get wet. If plywood, you must
use an exterior grade (like exterior BC).
That said, go to this site (http://johnbridge.com/vbulletin/index.php )
and make sure your subfloor and joists pass maximum deflection
requirements for the floor you select. Unlike a lot of pro type sites,
the pros on the John Bridge site go out of their way to make certain
us DIYer types get it done right.
Note, a ceramic tile floor can handle twice the deflection as a
natural stone floor. Also note, backerboard is NOT considered to add
ANY strength to the floor, while plywood will.
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