I'm replacing my kitchen floor and have got it down to the plywood. 2 questions please. 1, can /should I use redgard between plywood and backer board? 2, getting conflicting answers, should I apply thinset under and over backer board? Thanks
On #2, you want to use thinset between the backer board and the plywood subfloor. The purpose of this thinset is NOT to attach the backer to the plywood (that is done with the proper screw or nail type fasteners), but rather to even out the differences between the plywood and the backer board and to fill in any minor voids. The backer board needs to be absolutely stable with no flex, otherwise the tiles or their joints will crack after they are installed. Note that the larger the tile, the more likely for the tile to crack under these conditions.