Jen makes some good points. In addition I would recommend you put down the steel mesh that goes under the tile. Its purpose is to take up the tensile load applied to the floor when walking on it. Ceramics, brick, concrete are terrible in tension, but great in compression. The tensile load will be at a maximum at the bottom of your tile, that's why the mesh is there. It will keep your tile from cracking later. If you look at old home with ceramic tile floor, the laid down a thick layer of mortar on the floor to help stabalize it, this is in addition to the mortar use to adhere your tiles. Just a thought. Bryan