On One Hand: It's Best to Remove ThemThe new floor will not seal as well if it is glued directly over the existing flooring, especially if the old tile is not in pristine condition. Small cracks or dents in the old tile will affect the integrity of the new floor. Also, air may become trapped between the floor layers, which will cause noise when the floor is walked on. In addition, installation over ceramic tiles will raise the height of your floor.
On the Other: It Depends on the ConditionIf the previous floor is in great condition, it will not matter if you remove it. Make sure to clean and prep the previous tile for adhesive. Take care that enough adhesive is placed between the layers to prevent air bubbles from becoming trapped.
Bottom LineAlthough removing ceramic tiles is both costly and timely, it is the preferable solution for longevity in the new floor's life. If you do not have the time or money, inspect the old tile for breaks and cracks before installing the new layer of flooring.
Source:DIY Focus: Laying Ceramic Floor Tiles
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