I was going to install floating plank flooring in our tiny powder room, but discovered that the floor near the toilet was buckled. Apparently plywood was laid down over the old sheet vinyl and butted up to the toilet base. I am in the process of pulling out the old 1/2" plywood and there is mold under it. How do I deal with this? And what kind of underlayment should I put on it to prevent this happening again, cement board? I might tile the room in the future. The plywood under all this seems sound.