If your floor tiles are drummy or hollow sounding, you really do need to take some action. This situation will generally become obvious as soon as the tiles are laid.
It means that the adhesive does not have a good bond between the tiles and the substrate, or that there are hollow voids of missing adhesive under the tiles. If tiles are laid correctly and there is 100% adhesive coverage, and the bond is sound, then the tiles effectively become one with the substrate.
As the floor is used and subjected to foot traffic, the tiles that are not correctly laid will most likely break, crack, chip, or fail in some other way. In extreme circumstances, the tiles will actually lift off the floor
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