When faced with the prospect of adding granite to your home, there are a variety of factors that can influence your decision, not the least of which is what type of previously-installed materials are already in place. Knowing whether you can install granite over an old tile surface can mean the difference between a life-long installation or one that will fail in just a few short months.
Complexity of Underlayments
One of the easiest ways to picture how an underlayment works is to think about how a pearl is formed within the tissue of mollusks. A tiny piece of sand, or some other particle, is introduced into the tissue. Irritation follows, with the hard casing of the pearl surrounding the irritant forming as a result of that constant irritation. In the case of underlayment and substrates, they can act as a sort of irritant which, if installed incorrectly, can lead to damage to the surrounding "tissue," which in this case is the installation itself.
The difference between a mollusk and your tile or granite installation is that your tile is not a natural organism with built-in defensive capabilities to help it deal with an irritation. This is why ceramic and natural stone installations around the world require a 100 percent stable, controlled and structurally sound substrate--also known as an underlayment--in order to guarantee the longevity of the installation in question. If there is the slightest issue with movement the entire installation can fail, causing tiles to separate from the floor, grout lines to crack and other types of failure.
Judging the Previous Installation
Regardless if you are looking at a ceramic or natural stone tile installation, the one thing that holds true across the board is the importance of the substrate beneath, the foundation of the entire installation. If there is the slightest flaw in that foundation, the entire process can fail. Given the nature of homes and buildings as they move over time, it is virtually impossible to know with any certainty whether the previous installation is solid enough to provide a structurally sound foundation for the new installation of granite tile.
With that being said, there are occasions when an old tile floor can be a suitable substrate, but a thorough examination must be performed. If you are looking at a home that is 10 years old or even more, you can check the durability of the old tile installation by looking for cracks in the grout joints, hairline fractures in the tiles themselves, as well as hollow-sounding spots beneath the tile, all of which are indicative of an installation that is failing with time. However, if an installation is done properly, a tile installation will very literally last several generations, and these tile installations can provide an adequate foundation for the new granite install.
The Long and Short of It
Whether you can install granite over tile is really an open question that will vary depending on the nature of the previous installation. Almost every registered professional across the nation will tell you that there is no way to know for sure whether a previous installation is structurally sound enough to provide a strong enough foundation, and while there are exceptions to the rule, they are few and far between. Keep in mind that most contractors will refuse to warranty a granite installation over a previous tile install specifically because they have no way of guaranteeing that the previous installation is perfect.
At the end of the day, it really is up to personal preference as to whether you decide to go ahead with a granite install over an old tile surface. If you can check the structural integrity of the old installation to your satisfaction then you can do as you see fit, but keep in mind the aforementioned warranty issues should you choose to use a professional contractor.