I tiled right over the old tile in our guest bathroom. The tile in there was perfectly flat and in very good shape with no signs of mildew so I had no reason to suspect leaks or water damage, and that is why I went this route. To make sure it holds up for years to come I added another waterproofing paint on layer just in case. This saved both money and time and I have no regrets!! If you have more questions on this, I wrote a blog post about it here with all the products I used: https://erinzubo