I'm remodeling a bathroom and have replaced some rotten subflooring (1 x 12 planks). Now I want to install a tile floor. Should I affix cementboard directly to my subfloor or do I need a layer of plywood? Should I use mortar to attach the cementboard to the subfloor/plywood? Thanks!

Install 1/2 cement board and use the thinset and trowel that you will use to adhere the tile with. Attach with the appropriate cement board screws.