Hollow (debonded) tiles

A tile is held in place by the tile adhesiveTile adhesiveA ready-mixed cement-based mortar, sold as a powder and mixed with water, used to stick tiles to walls and floors. It is spread with a notched trowel. or mortar underneath it. When that bond fails, the tile is left sitting on air. Tapped lightly, for example with the handle of a screwdriver, it gives a hollow sound instead of the solid sound of a tile that is well bedded.

It happens often on balconies and flat roofsFlat roof (roof terrace)The flat concrete roof on top of a building. It is the most common kind of roof in Greece and the part that takes the most sun and rain., because water gets in through open joints and cracksCrackA split or gap in render, concrete, a wall, a coating or wood. Fine cracks are usually only on the surface; ones that grow, keep reopening or run diagonally need attention., and sun and cold keep the floor moving. It also happens when the adhesive was spread in a few dabs rather than over the whole back of the tile.

A hollow tile can stay like that for a while, but at some point it cracks or lifts. A few hollow tiles are usually replaced one by one. When there are many, what is going on underneath needs checking, because water is often getting into the floor. No coating should go over hollow tiles before they are fixed, because it would have nothing firm to hold on to.