Cbu

Can you or should you thinset cbu directly to 3/4 x 4 t&g floorboards on 2x10 joists 16" oc. Only 3 bays that wide though, as far as deflection goes, there are a couple of beams, it is the foyer by the stairs. Ican sure up 2 of them but the third has ductwork. How do you measure deflection anyway. The largest tile is 20 x 20. Those are the perticulers. My question is, can I thinset 1/4 or 1/2 CBU on F. Bs. Or do I need plywood on top of that, I would like to keep it as tight as possable, it is next to 3/8 hardwood that is 5/16, I don't want the transition to turn into a step. There are 2 rooms & a hallway. Thanks in advance. Any questions kepner@netzero.com

You NEED plywood, 1/2" or better under all CBUs or membranes, if heights an issue, use 1/2" ply, over the dimensional lumber, then a membrane such as Ditra.