A recent comment on Facebook said that 95% mortar coverage is not always required on shower walls. While the person making the comment has the right to do so, it goes against the long-established ANSI Standards and Installation Requirements found in the Tile Council of North America (TCNA) Handbook.
The ANSI Standards:
According to ANSI A108.5 section 2.2.2 states, “Apply mortar with flat side of trowel, keying in with pressure, over an area no greater than can be covered with tile before the mortar skins over. To obtain a setting bed, use a notched trowel of type recommended by mortar manufacturer, comb mortar with notched side of the trowel in one direction. Cover surface uniformly with no bare spots and with sufficient mortar to ensure a minimum mortar thickness of 3/32” in (2 mm) between tile and backing after tile has been forcibly embedded. Tile shall not be applied to skinned-over mortar.”
A108.5 section 2.2.3, in part, states, “Contact area shall not be less than 80%. Contact area on all exterior installations or interior wet installations shall not be less than 95%. The 80% or 95% coverage shall be sufficiently distributed to give full support of the tile with particular attention to all corners and edges of the tile. Embed the tile in the mortar by pushing in a direction perpendicular to the combed ridges to fully collapse them and achieve specified coverage.”
When ceramic and porcelain tiles are installed following these standards and industry best practices, the job is pleasing to the eye and will deliver many years of service. If these requirements are not followed, an unhappy consumer may be on the phone to the installer.
In a stall shower or tub (considered a wet area) when the combed mortar ridges, (it doesn’t matter whether they are vertical or horizontal) are not collapsed, as described above and demonstrated in the NTCA Trowel and Error video, open channels will exist in the mortar. The problem develops when a cementitious grout is used which, by nature, allows moisture to penetrate the grout joint.
When the trowel ridges are collapsed, the moisture stays in the grout joint until it uniformly dries out after showering. However, when the uncollapsed notches exist, the moisture will collect and increase in volume until they are full of water, forming a small reservoir. After showering, the grout joints dry out, but these reservoirs continue to feed water to the nearest grout joint, making it darker than the adjacent joints for several days as seen in the above image. This trapped water seeks equilibrium, meaning it will seep out through the grout until the humidity level is equal both behind the tile and in the open shower.
The consumer will not be happy with these continuously light and dark grout joints resulting in a callback which is not easily or inexpensively corrected. About the only solution is to carefully rake out the existing grout and apply an A118.3 epoxy grout which will not absorb water.
Think about the bigger and more expensive picture before deciding that 95% mortar coverage is not necessary in all showers.