Hello Jaz; Thanks for your comments and help with my project. This is my fist attempt at laying a tile floor, so I figured my small 1/2 (4' x 5') bath is good start. I ask many questions so I do the job correctly. I did notice that the 3/8" plywood floor is not perfectly even. If I place a 4' level on it, there is slight rocking in at least 1 place so far ---- at the edge of the floor along the bottom wall plate. I suppose parts of the plywood are either higher than others or other areas simply have sagged/weaken after 45+ years. So, I guess my question is when I lay the 3/4" plywood over top, will it even itself out even if there is no joist in this area along the wall? Will it matter much when the cement board is placed down? The 3/8" floor seems solid and doesn't "give", but just want to make sure. The joist is actually located up under the wall plate, so I can't tell if the joist is not even and the plywood is just following its contoour.