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In a typical bathroom floor tile installation you have the following
sequence:

Plywood subfloor, Thin-Set, Backerboard, Thin-Set, Tile/Grout.

If the grout is not perfectly sealed and the bathroom is heavily used
(with a lot of water on the floor), where does the water stop? I know
Backerboard is water-damage-resistant, but I hear it can wick and
absorb water. What are we relying on to keep the water from soaking
the plywood? Is it just the thin-set?


*I painted my underlayment plywood with exterior primer and paint to prevent
water from being absorbed into it. Ideally you want to stop the water at
the tile surface. Use caulk around the backerboard edges. Use a grout
sealer or an epoxy grout such as Laticrete Spectra-Lok (My choice).