The subfloor will need to be made as ridged as possible so a minimum of 9mm or idealy 18mm marine ply should be used and screwed between 6 and 8 inches all over. Dont forget to prime it and let id dry before laying your tiles.
Spence
Because cushion floors show every mark under the sun through the material and it being in a bathroom, i would recommend 6mm wpb ply. This will need to be nailed between 6 and 8 inches using ring shanks no longer than the thickness of the existing floor and ply combined. If you have any...