Different parts of the country use various installation methods which may or may not be included in the TCNA Handbook. While it is best to follow an established and proven method, some installation methods that are not in the Handbook do function without problems. Spot bonding tile, however, is not one of them.
Spot bonding either floor or wall tile calls for a glob of mortar to be placed on the back of the tile and spread around with a margin trowel. Most times there is no attempt to key the mortar into the back of the tile, which results in a questionable mechanical bond. The tile is then placed onto a dry, unclean, un-mortared substrate which does not encourage any type of mechanical bond to this surface either. Needless to say, almost every industry-recognized recommendation for the proper installation of tile is compromised with this technique.
According to ANSI specifications, there is an approved method for installing floor and wall tile. Section A18.104.22.168.2 states: “Apply mortar with flat side of trowel over an area no greater than can be covered with tile before the mortar skins over. Using a notched trowel of type recommended by mortar manufacturer, comb mortar to obtain even setting bed without scraping backing material. Cover surface uniformly with no bare spots and with sufficient mortar to ensure a minimum mortar thickness of 3/32” (2 mm) between tile and backing after tile has been beaten into place. Tile shall not be applied to skinned-over mortar.”
Using the above ANSI method, the mortar is keyed into the entire substrate, yielding a good mechanical bond with no bare spots. Here is one of the many downfalls to the spot bonding method – the bare spots or voids. As you can see in the accompanying photo, the installer “thought” good coverage was being obtained, but clearly it was not. Mortar voids in floor tile installations are almost always fatal to the installer, creating hollow-sounding tile or even worse: broken tile. On walls in wet areas, this void normally will trap water, which will slowly bleed out through the cementitious grout joint causing a temporary discoloration. Yes, it will go away, but most consumers find it to be unacceptable.
These few words don’t begin to cover all the problems that spot bonding creates. But if installers know and follow industry recommendations (which are there to help provide a better job), the quality level of tile installations will only rise.