I've always over boarded chip board but starting to wonder Y ?? iam tiling a hallway floor at the min which i overlaid with ply, the customers questioned why i was over boarding and priming the undersides of the ply and the step he would have going from the hallway to the kitchen, so i gave him the info regarding moisture etc and stated you can't tile onto chipboard etc.. he pointed out and showed me his kitchen that was tiled 12 years ago directly over chipboard. and hasn't failed .. ? also reading some old posts on hear and most say its fine to tile directly over chipboard if it has been treated ? ( green colour ) . also was thinking would it be ok to prime chipboard with SBR then use a 2 part flexi to avoid the step and cost ?