I'm about to put floor tiles (ceramic or quarry), down in my bathroom. Just wanted to know if Tongue and Groove (Floor graded moisture resistant) Chipboard was better than Plywood, for covering the floorboards, so as to avoid movement and grout cracking? I'll probably go for 22mm thick on either option. Read on another thread that it's a good idea to leave a little gap between plywood sheets to allow for expansion. Is this necessary? Cheers.