I intend laying granite tiles in my bathroom. Its an upstairs room in an extension built about 30 years ago. The floor is tounge and groove replaced about 10 years ago ( after a flood ). What thickness ply should I use ? ( i presume I will need to overboard to get a nice solid base ). Also is it best to do the whole floor ,before fitting the new bathroom? The tiles are £18.00 a shot so dont want to waste any...