R
Rich
Min of 15mm BWP ply and then 6mm cement board, really not worth messing about with. Also might need to add some extra noggins before the ply goes down. Seal the underside and edges of the ply, leave a 2-3mm joint between the boards (that should be staggered) and screw down about every 200mm centers. The cement boards will need sticking down with either tile adhesive or glue, depending on the brand of boards, and then screwed down. Then you should be good to rock and roll