ANSI A108.02-188.8.131.52.1 states in part: “For tiles with at least one edge 15” or longer, the maximum allowable variation is not more than 1/8” in 10’ and no more than 1/16” in 2’ from the required plane, when measured from the high points in the surface.”
Determining the condition of the floor or wall is relatively easy by using a 10’ straightedge. Simply marking the substrate with some method such as circling the low spots and placing an X on the high spots quickly shows where work is needed to meet the ANSI specification. Using a combination of cementitious patching compound (either trowel-applied or self-leveling) to fill the low spots and grinding down the high spots normally will provide a surface that will be suitable for installing tile within the prescribed tolerances. By the way, thin-set or large and heavy tile (formerly medium-bed) mortars should NEVER be used to flatten the surface.
As seen in the attached photo taken during a recent Advanced Certifications for Tile Installers (ACT) test, the installer is using a long straightedge to determine if the trowel-applied patch is flat enough to receive a 12” x 24” porcelain tile with a 1/8” grout joint to be within the lippage requirements. Realize the allowable lippage for this tile installation under ANSI Specification A108.02- 4.3.7 is just 1/32”.
Before starting your next job, look at the surface that is to receive tile and understand what it will take to make it flat enough to install the tile. Requesting a change order before the job starts increases you chances of getting paid for the quality work provided. Attempting to “fix” the floor or wall surface as you go with thin-set mortar will almost always result in an unsatisfactory finished product. Do your customer, and yourself, a favor. Do it right…the first time.