i know that hardi board makes a 1/4 inch material that works great. use only the best portland products.a grinder w/a diamond blade will make good cheap cuts. make sure to follow all ins.-including tape u`re hd board joints. if the ply looks solid, keep it. wiil need to set all fastners and level joints if too rough. may have to use an extension flange or dbl. bees wax on waste. don`t grout around edges (outside)of tile. new caulks can be colored to match and won`t crack out. ck w/tile supplier.