I'm about to start on a DIY ensuite installation and would like the floor tiles to be flush with the bedroom floorboards which are 21mm thick. I was thinking I could remove the boards in the new bathroom and something like Hardiebakker could be screwed directly to the joists, but am I right that these sheets won't give me enough strength on their own? Do I need to keep the floorboards down and put the Hardiebakker on top of them? Or a compromise could be to put down ply instead of the floorboards then HB on top of that, but what thickness ply could I get away with? 15mm plus 6mm HB brings me to the same depth as the floorboards but then the tiles would make the finish too high. Or should I be putting the 12mm HB down for floors? Just wondering how I can get the look I am aiming for or do I need to accept that my ensuite floor will be higher than the bedroom boards. I suppose shaving down the joists before I install the ply could work but seems a bit extreme.