What Causes Voids Under Tiles?

Voids under a tile usually come from an uneven substrate, gaps in the adhesive bed, adhesive that skinned before the tile was placed, or contamination that prevented bonding. A hollow sound can be a clue, but it does not show the size of the void or prove that the tile is about to fail.

Why incomplete contact develops

• The substrate had high and low spots that the adhesive could not bridge evenly.
• Adhesive was spread too thinly, with missed areas or without the required contact pattern.
• The surface of the spread adhesive dried before the tile was bedded into it.
• Dust, paint, weak plaster or moisture interfered with adhesion.
• The substrate moved and the installation had no suitable movement accommodation.

How to inspect a suspicious area

Compare nearby tiles, check whether the tile moves, look for open or crumbling joints and note any dampness. Mark the boundaries of a repeated area. Avoid hard impacts while testing: they can crack the tile and make a small defect larger.

When a local repair may be enough

If one tile is affected, remains stable and sits over a sound, dry substrate, a careful local repair may be possible. Remove the tile and weak adhesive, check the plane and rebuild the contact with compatible materials. Do not simply add another layer over an unknown surface.

When one-tile repair is not enough

• The hollow sound repeats across a large area or in a continuous strip.
• The tile moves, has cracked or is losing grout around its edges.
• There is a leak, damp substrate, screed crack or wall crack.
• The installation is over a heated floor or another substrate that visibly changes shape.

Preventing voids in a new installation

Level and clean the substrate, select adhesive for the tile and conditions, respect its working time and check the first rows for even contact. Estimate adhesive from the product coverage data rather than one universal consumption figure.