I'm installing 450 sq ft of 18"sq porcelain tile on a subfloor that is flat and solid. Can I substitute wonderboard (securely fastened) for a mud base and then set the tile using thinset. Also the existing floor is 12" vinyl tile which is in good shape, can I fasten the wonderboard over the tile or should I remove it and start with a plywood sub-floor. Thanks, Al Z
If there's no problems with the height difference resulting from putting down backerboard, I would always recommend going with it under tile. You can leave the vinyl tile, as long as it's securely bonded. Make sure to spread a layer of thinset first, then fasten your backerboard into the thinset. The thinset will fill any minor imperfections between the two floors and provide additional support. you should be good to go!