What causes tile to tent?
Tile heaved off the floor, or tented, and sometimes cracked is often a sign that movement joints were not used sufficiently. For tile over concrete, the curing of the concrete places the tile under compression.
Why do installations tent after a number of years?
How long it takes tile to tent is directly a function of at least three variables – the rate of concrete shrinkage, the shear strength of the thinset, and any expansive forces applied to the tile layer (for example, heat). When the tile is poorly bonded, the tile can tent very quickly. If there is a strong bond, often the grout will compress significantly before the tile will lose its bond. Of course, the type of tile is important as well. Thinset has a harder time bonding to porcelain than most other tile. At the other extreme, I have seen a saltillo installation where the tile did not tent but rather spalled as the thinset and grout were stronger than the tile.
When tile fails with a loud report, this certainly indicates that a good bond was present. Only when the shear force exceeds the strength of the bond, will the tile let go. Frequently, either the tile or the concrete will be without thinset residue – as if the thinset was not applied correctly originally. Usually, if the tile is tenting years after the installation, this was not the case. Had the thinset not been applied correctly originally, the tile would have tented long before. Rather, it is important to consider that the cleavage plane will usually occur at the thinset transition – either the bond to the concrete or the bond to the tile, depending on the relative permeability and exact composition of each.
Hence, it is common to see one surface or the other sheared clean of thinset. Even in “explosive” failures where the tile cracks and “jumps” off the floor, usually one surface is free of thinset. Clearly a good bond had been established.
With organic bonding agents and some of the polymer-modified thinsets, continued shear forces degrade the bond over time. So even when tile tents without an explosive report, the original installation may have had sufficient adhesive.
In summary, every installation should allow for movement. Properly designed installations, where expansion and contraction do not create shear forces, should have no problem for the tile to stay well adhered.