QUESTION
We installed 24”x24” porcelain tile in a hotel lobby using a thin-set mortar over a concrete slab. After completion, the owner noticed a few areas that sound hollow when tapped. The tiles look fine—no cracks or movement—but they’re concerned this indicates a problem. What causes this?
ANSWER
Hollow-sounding tiles are a common concern and don’t always indicate a failure. A hollow sound simply means there’s a void beneath part of the tile—usually due to variations in slab flatness or incomplete mortar coverage—not necessarily that the tile is loose
or debonded.
The ANSI A108.02.4 standard requires concrete substrates to be flat within 1/8” in 10’ for tile installations using the thin-set method. If those tolerances aren’t met before setting begins, it can be difficult to achieve full contact between the tile and mortar. Even with a strong bond, small air pockets can create a hollow sound when tapped.
Here are some important steps to help prevent this on future projects:
- Check and correct slab flatness before starting. Low areas can be filled or leveled using materials specifically designed for this purpose: patch, flowable hydraulic cementitious underlayment (aka, self-leveling underlayment, or SLU).
- Use proper troweling techniques—directional troweling and back-buttering large-format tile helps ensure full coverage, transfer and bond.
- Verify coverage during installation by periodically lifting a tile. For all floors in a dry area, ANSI A108.5.2.2.3 requires 80% coverage or as “specified in the contract documents, whichever is greater.”
If there’s uncertainty about the cause or extent of a hollow-sounding condition, NTCA recommends contacting a qualified independent tile consultant to perform an on-site evaluation. Find NTCA Consultant Members on the NTCA website here:
While hollow sounds may be unsettling, they do not automatically mean the installation is failing. Proper substrate preparation and adherence to ANSI and TCNA standards remain the best prevention and protection for successful installations.
