Marble on marble
I plan to replace the 12x12 marble tile in my bathroom with a marble tile of another color. Can I tile over the existing marble, or does it need to be removed first and a new backer board installed?
if the existing marble was properly installed correctly and there are no problems; you can install marble on marble. However you have to use a good polymer modified thinset, you need to rough up the surface of the existing marble someway without destroying it(like a very low grit sandpaper) and you also need to think about your transitions at doorwaysw