Why should new tile not be installed over existing tile that is below grade?
Tile over tile is not recommended on a floor below grade. A basement is below grade and concrete below grade will almost always have moisture in it. A single layer of tile allows any moisture in the concrete to escape through tile joints. When you place new tile over existing tile, you cover the tile joints and cut off the escape route for the moisture. This traps the moisture beneath the tile and can cause loosening of the old tile as well as the new tile.
