Is hollow sounding floor tile a slab foundation problem?
I frequently get calls from homeowners who have found that some of their floor tiles sound hollow when tapped on with a metal instrument like a screwdriver or a pocket knife. The short answer is: no, hollow sounding tiles is not an indicator of a foundation issue.
The problem is with the thinset used to bond the tile to the slab foundation surface
The hollow sound is due to the thinset breaking down. It is a tile application problem, not a slab foundation problem. When the foundation distorts, the tiles will break or fracture, but the tiles will remain tightly bonded to the foundation surface.
What is thinset?
Thinset is a type of mortar that is virtually always used to bond floor tile to concrete. It is called thinset because the traditional way to bond tile to concrete was to use what is called a mortar bed and it is usually at least 1/2 inch thick. A thinset installation is usually around 3/16-inch thick.
Thinset versus mortar-bed applications
In my experience, you will not find very many mortar bed installations. I am not sure why this is the case. I suspect that the reason comes down to cost. I cannot prove this, but in my experience, a mortar bed job is a better quality, longer lasting job. A mortar bed can be reinforced with steel. It can also be used to level out a floor. A thinset job can also be adjusted to level out a floor, but only minimally. With a mortar bed job, tile cracking due foundation distortion and hollow sounding tiles are much less likely.