QUESTION

In South Florida walls are skimmed with latex modified thin set and the tiles are back buttered with the same material. To make up for any wall variation 4 to 5 spots of additional thinset are applied and then installed. This recently resulted in tiles selectively falling off the wall. I need to go back to my subs with the exact correct method so we can set the proper standard for our installers.

ANSWER

ANSWER - What they did is called spot bonding, which is against industry standards unless the adhesive is an epoxy on an interior wall application over cementitious backer board application.

It does no good skim coating the substrate and back buttering the tile if the only contact between the two are the spots.  If you only make 50% contact between the substrate and tile, then you have reduced the potential bond strength by 50%.  Plus where there are voids making the tile will be more susceptible to damage if it is subjected to some impact, and voids that can collect moisture leading to other problems.

The ANSI A108.5 standards state that wall applications should have at least 80% contact between the tile and its substrate with sufficient distribution of the thin-set to give full support of the tile.  Per ANSI A108.19, if the tile is a Gauged Porcelain Tile Panel/Slab (11 sf or larger), then the required thin-set contact between the back of the tile and its substrate is 85% with no voids greater than 2 square inches (size of a golf ball).  A partially collapsed trowel ridge where the width of the valley of the trowel ridge is less than 1/4 inch is not considered a void. Thin-set coverage is to be sufficiently distributed to give full support of the tile, especially at corners and edges.

In addition, ANSI A118.4 and A118.15 for modified thin-set mortars state that the thin-set mortars are not intended to be used in truing or leveling underlying substrates or the owrk of others.  Standard thin-set mortars should not be applied thicker than 1/4" after embedment.  Large and Heavy Thin-set Mortars (previously called medium bed mortars), should not be applied thicker than 1/2" after embedment.  If the wall or floor is out of flatness then adjust it with an appropriate mortar leveling or patching material.

Tile installers, and their employed tile installers (both setters and helpers), shouldo take the UofCTS online Tile Installer Thin-set Standards (ITS) Verification course to teach them and verify that they know and understand the current industry standards.  It takes about 5 hours to complete and they can be taken in on night or over 14 days.  It is available in English and Spanish.  Plus the tile installer can put the acronym ITS next to their name and you can then advertise that all of your tile installers are ITS Verified.  This will save the consumer and the installer money and time by not having problems.  It is cheap insurance...  You can get more information on the course at https://uofcts.org/online-courses/titc-certified/.