The need for adequate mortar coverage on the back of installed tile has most likely been an issue since thin-bed mortars were developed over fifty years ago. During this time many articles and technical bulletins have been written to inform installers how to achieve “proper coverage.”
The 2011 ANSI Specifications, section A108.5 states, “Average uniform contact area shall be not less than 80% except on exterior or shower installations where contact area shall be 95% when no less than three tiles or tile assemblies are removed for inspection. The 80% or 95% coverage shall be sufficiently distributed to give full support to the tile with particular attention to this support under all corners of the tile.”
To obtain the desired coverage, there are a host of trowels and methods that may be used to meet this requirement. Using a trowel with notches large enough to provide this coverage may work, however the use of straight line troweling of the mortar will dramatically increase the probability of full coverage. Some installers “back butter” the tile prior to installing it in the mortar which was previously spread. These methods or the combination of them should yield adequate coverage.
So why is this “coverage” question such an issue? Without adequate mortar, the tile may come loose (debond), sound hollow, crack, deteriorate in freezing climates or temporarily darken grout joints in shower walls and floors.
There is another situation that is a significant problem which is difficult to diagnose. When removed, the tile in question appears to have no mortar coverage at all. The tile looks as though it has just come out of the box, clean and untouched. The reason for this is that this tile was cut on a wet saw and not thoroughly dried. The water on the back of the tile, especially on porcelain tile (.5% porosity), acts as a bond breaker and will not allow the fresh mortar to bond to the back of the tile. When this water evaporates out through the grout joints, the cause of the bond failure is gone leaving an unbounded tile with no apparent cause for the failure. The attached photo shows this along with another problem of not enough mortar to lock in the metal “L” angle.
Remember two things. Get enough mortar coverage and dry the back of your wet cuts before installing.