22 year old home. Shower stripped bare (almost) Original floor tiled. The walls were green board.... The only thing that 'waterproofed" was a 6 inch piece of cement wonderboard surrounding the tile area. It has a 3 inch deep 'groove" around were the wonderboard is embedded in. Was thinking of poring cement in surrounding 'trough', make sloping cement floor and put pan liner over original tile floor, followed by thin layer of thinset over liner to begin tiling. Is this a good idea?