How much air under tile is acceptable?

Question

We hired a contractor to do redo our tile. One had to be replaced because it caved inward when a tool was dropped on it. We peeked under the adjacent tiles and there are giant areas without mortar. How much air under the tile is acceptable?

Answer 1

How much air under the tile is acceptable? Virtually none.

Tile installers use notched trowels for a very specific reason: To allow the mortar to make consistent, supportive and bonding contact with the entire tile. This only works if the mortar was mixed properly and the trowel is used properly. It looks to me like the former is the problem here.

See how the mortar in your photo looks like it slumped, and how it's somewhat smooth with protruding bumps? This tells me that it was overwatered. It was too loose when applied. Therefore it sagged and failed to make good contact with the tile. This happens when an installer is in a rush and wants the mortar to be easy to spread, or when they were simply careless with mix ratio, or when the mortar was retempered--rewatered to extend its working time.

It could also be that too small a notch was used. Very large tiles--those over about 12"--require a larger notch to give the mortar more fluidity under the tile. ¼" x ⅜" or even up to ½" notches are common. Here we see what looks like a ¼" x ¼" notch, or a ¼" by ⅜" notch where the trowel was laid over too far, shortening the mortar ridge height.

If you have more than a very occasional failure like we see here, you have contractual negligence, in my opinion. You should expect repair at no cost to you.

Answer 2

The Tile Council of North America (TCNA) publishes a handbook for designers. It's purpose is to provide bathroom designs and quality tolerances so that a designer can create construction documents without knowing much about tile work. If you don't know much about tile, then it would be wise to specify this document as the quality standard you expect from somebody doing your tile work.

If those brown tile edges from your photo are from the same tile as the cratered tile, then you have ceramic tile. The TCNA Handbook specifies 80% coverage in dry areas and 95% coverage in wet areas, where "coverage" denotes the area of tile bonded with the underlying thinset. Those ridges in the thinset allow the air beneath the tile to escape as pressure and movement from above "collapse the ridges" during installation. Uncollapsed ridges is a strong signal of improper installation. In addition to @isherwood's criticisms, there's a possibility that the installer spread large areas of thinset all at once and that this thinset stiffened too much before he finally placed the tile. The backer board material can cause this stiffening by sucking the water out of the thinset.

The relevant part from the TCNA Handbook:

Mortar Coverage for Ceramic Tile

Average contact area for dry areas is 80% and for wet areas is 95%. Mortar coverage is to be evenly distributed to support edges and corners.

It is not possible or practical to achieve 100% coverage consistently and such should not be specified.

Mortar Coverage for Natural Stone Tile

Mortar coverage must be sufficient to prevent cracks in the stone resulting from voids in the setting bed. In dry and wet areas, the minimum coverage is 95% with no voids exceeding 2 square inches and no voids within 2" of tile corners. All corners and edges of the stone tiles must be fully supported, and back-parging, or back-buttering, is recommended in all areas. Coating the back of the tile, however, does not constitute coverage, which is the area where the mortar makes contact with the tile and the substrate.