We purchased a new house 7 years ago, unfortunately the shower pan is plastic and has discolored and stained. Was thinking of replacing the plastic pan with straight tile instead. The shower sits on the concrete slab foundation with the drain in the center. Do I have to use a rubber liner over the cement or not? If I do have to can I lay the tile directly onto the rubber or do I need to put down a thin layer of cement before laying it?

Any help thank you.