I recently pulled up my bathroom floor. The guy who pu it down was aparrently an idiot. (for many reasons) Anyway, there were vinyl prestick tiles over a plywood subfloor. Shortly after we moved into the house we noticed that the tiles started to buckle and the sub floor as well. A few days ago my wife called me into the room and showed me where the subfloor was buckled you could hear water squishing around under it. So I pulled the toilet and the vanity and pulled up the floor. I am thinking that since he didn't use a spacer between the closet flange and the toilet waste water was allowed to seep out and buckle the plywood subfloor and in turn buckle the tiles.
So, I am thinking about installing ceramic tiles(with a flange spacer). Can I put thinset over the black mastic adhesive that is on the slab. I really don't want to do this job nore than once. I haven't done much with tile except repair work so can anyone tell me what my options are.