Here are some tips a good tile contractor should never do: never lay tile onto plywood or floorboards, gypcrete gypsum concrete, on top of tiles unless there is room for a proper prep work, on top of vinyl or glue or green board (green drywall) for a shower. It is a must to have a proper subfloor installed prior to tiling. Waterproofing the shower walls are a must do. Although, green board is moisture proof, it is not waterproofed, not good enough for a shower wall. Never use mastik/glue (ready mixed adhesive) on the floors/showers.