The ANSI Specifications contain mortar coverage requirements for each type of tile. Specifically for glazed wall tile (semi-vitreous and non-vitreous), A188.8.131.52.4 states: “Average contact area shall be not less than 80% except on exterior or shower installations where contact area shall be 95% when not less than three tiles or tile assemblies are removed for inspection. The 80% or 95% coverage shall be sufficiently distributed to give full support of the tile.”
Knowing that shower or wet areas require 95% coverage, installers need to meet the challenge. The best way this can be achieved is to experiment with various trowel notches and configurations until the coverage requirement is met. If not achieved, back buttering the tile may be necessary.
Additionally, the condition of the substrate and pattern on the back of the tile are factors that affect the ultimate coverage. Once installed, a tile should be removed randomly to check coverage. Assuming coverage is there is not good enough. You need to check it out.
Unfortunately, many coverage failures occur when the choice is made to use a smaller notch to eliminate the mortar squeeze up between the tiles. This is a fatal mistake because coverage is sacrificed and there isn’t enough mortar to properly bond the tile to the substrate, let alone meeting the ANSI requirement of 95%.
The above photo shows that the mortar coverage is almost non-existent; the mortar barely dots the raised grid pattern on the back and nothing touched the body of the tile. This installation didn’t have any squeeze up, but it didn’t have any coverage either and it would be lucky to stay on the wall. The ironic part of this installation is that on the surface, the tile job “looked” fantastic. The grout joints were consistently sized, the cuts were very neat and there was virtually no lippage. But that is not good enough. Just because an installation looks good doesn’t necessary make it a good installation.