I'm due to lay plank effect tiles, 600x150mm in my first floor bathroom next week but can't decide on the method and materials. The existing floor is 22mm chipboard and I'm worried laying a Plyboard that's too thin will crack the long thin tiles, I'm torn between 9mm and 12mm Plyboard but is this over doing it? What benefits would marine plyboard give? Also, what are the best primers out there to treat the ply before laying tiles? I presume flexible adhesives and grout would be required? Questions; 1. Thickness of ply 2. Benefits of marine ply 3. What primer to use 4. Best flexible materials? 5. Any other tips?