What is best for tiling a bathroom: 25mm plywood floor or hardibacker overboarding on 18mm floorboards? Ceramic or porcelain tiles, with joists at 40mm centres. My builder seems dead against hardibacker as he says it has a tendency to crack and therefore isn't suitable for floors. I wonder if it's the cost, but I thought plywood was about the same price...?? Also, on the subject of plywood floors, my builder has fitted 25mm plywood sheets in the kitchen at my request (fitting quartz tiles on floor). With two wet appliances on the plywood floor (sink and dishwasher), was wondering if anyone has any suggestions about protecting the kitchen floor?