I'm putting down 12x12 ceramic tiles in my kitchen remodel. The exposed floor, having torn up the old linoleum, is 3" wide pine planking which is over the original subfloor of wider planking on the diagonal. I'm using thinset mortar and the floor is level. My question regards expansion/contraction of this pine planking. I'd like to go right over it with the thinset, but if I don't put down a 1/4" luan will I get cracking in my thinset and grout?