Flooring to tile onto - TilersForums.com

[SteveM]

Hi, new member here! I'm about to tile the walls and floor of our shower room. The floor at the moment is chipboard that isn't very well fitted and there aren't any joists at the edges of the room. My plan had been to remove it, strengthen the joist layout below then fit 25mm plywood but I cant source any for local pickup or delivery (remote-ish Scotland). My plan now is 18mm marine ply with 6mm hardibacker on top. Does this sound reasonable or would there be an easier alternative? I'm also going to re-plasterboard the walls, is using square edge ok with no tape or should I use tapered and tape and fill the joints? Fo I need to bury and fill the screw holes?

Thanks for any advice!