Hi All , spoke to tech at hardie boards , 6mm holds up to 100kg m2, providing fixed with adhesive and fixings. Also, does 10mm of gypsum base coat dries at 1mm a day. Brother had his bathroom done 8yrs ago , tiler from the tile shop, used Hardwall and primer and then tiled over with 600x300 porcelian few days later and has power shower still looking good. But must do to the book, dont want any comebacks