Hi I am about to start work on a ground floor bathroom that currently has an old concrete deck(no DPM or insulation etc). I'd like to raise the floor a bit anyway to make it level with the adjoining room so I was thinking of: laying down a DPM, then some 60mm kingspan boards, then clagging 10mm tile backer boards on top, tapping them etc as it will be a "wetroom" set up and then tiles on top. This is the only method I have managed to find on the old Google search so far which applies to a set up like mine but I would be a bit worried about the stability of this method. Would there still be any kind of possible movement, even if I staggered all joins in the boards? Would I not need to lay down some 60mm timber strips to screw the backer board joining edges to or would I need to mess about laying ply sheets as well??? I can’t seem to find any info online so any help would be much appreciated thanks!