Ceramic tile does not bend or stretch. If a floor sags under furniture or foot traffic, tiles won’t flex—they’ll crack. And if the floor expands from moisture or the tile shrinks because of cold emperatures, the tile has to be free to move. If it’s bonded to the floor, you guessed it: it will crack.
Code-compliant joist spacing and subflooring are fine for wood floors, carpeting, and resilient flooring—but not for ceramic tile. Follow industry specs, or better yet, exceed them. Install a deeper floor joist or a closer joist spacing, and double up on the plywood subflooring.