Do you have to tape and fill seams on cement board when applying ceramic tile over it like when doing drywall?
Yes, you'll use fiberglass mesh tape to tape the seams and a coating of thin-set mortar
No you don't have to but if there is a chance of movement of the panels, say on an outside installation where temp changes will cause expansion and contraction it may prevent cracking of the tiles at the seams.
With no offense meant to answer 1, Tape NEED NOT be applied to concrete backer board; for the purpose of tiling over. Thin set can be skimmed into the seams and screw holes. Certainly the board should be flush at it's face with any adjacent. If the house is wood frame; I suppose one might think that at some point it might shift; but we hope, not in our lifetime. Steven Wolf
With all due respect to those who say it's not necessary, I think you're looking for trouble if you don't fill the seams with fiberglas mesh tape and flexible thinset mortar. No harm can come from it, and it can only help prevent problems. Also, if you're installing in a damp environment (bathroom, especially), cover the screw heads with some oil-based paint and let it dry for a day or more before putting the mortar on. I've seen cases where people didn't do that, and after a few years rust from the screw heads started to penetrate the top surface of the tile.
