If a floor is not flat enough, per tile industry standards, to receive tile, the problems connected to this situation are numerous.  

The American National Standards Institute (ANSI) specifications require the substrate (the surface to which the tile is installed) to be flat for the proper installation of ceramic and porcelain tile. ANSI A108.02 states the following; section 4.2.2 Substrate flatness for tiles 15 in. (0.38m) or longer: “For tiles with at least one edge 15 in. (0.38 m) or longer, the substrate shall have a maximum permissible variation of 1/8 in. in 10 ft. (3 mm in 3 m) from the required plane, and no more than 1/16 in. variation in 24 in. (2 mmm in 610 mm) when measured from the high points in the surface.” 

If the floor substrate of an upcoming job does not meet this requirement, it can be corrected in several ways. The most popular and fastest methods are by using a trowel-applied flash patch or a flowable hydraulic cement underlayment (FHCU) over the manufacturer’s recommended primer. Failure to correct the out of flat situation makes the tile installation more difficult to achieve plus it involves increased labor and materials to complete the job. Building up the thin set mortar thicker than recommended by the manufacturer may work temporarily, although it will fail eventually, but the thicker mortar will result in excessive shrinkage. During the curing process, the thicker than recommended thinset will pull the tile down below the adjacent tile causing excessive lippage (where one tile is higher than the adjacent tile). 

This tile installation’s failures are numerous, but the blatant ones include excessive lippage, interesting grout application techniques, and potential legal issues.

Photo: Scott Carothers.

As you can see in the photo, the installer tried to hide the mistake by ramping up the contrasting color grout (which made the situation worse) to the next tile causing a very unsightly appearance and increased labor to effectively clean the floor in the future. This alone is bad, but even worse is the potential tripping hazard that was created. In this instance, the tile installer could be found at fault in a court of law and required to pay damages.

However, if the floor flatness is corrected to the industry standard and the appropriate trowel is held at the designed 45-degree angle, the tile should be flat. Right? Here is where the skill and experience of a tile professional comes into play. Knowing how to maneuver the tile to achieve the required minimum mortar coverage and transfer to the back of the tile without lippage is critical for a pleasing result. Sometimes the consumer gets what they paid for and sometimes they do not.

The choice on what to do and how to handle the existing conditions is often left up to the discretion of the installer. Take this opportunity seriously; make the best educated decision; and do it right the first time.