As installers approach a floor tile job, there are numerous questions that need to be answered before the first tile is set. Is the floor substrate suitable for receiving tile and is it flat?

The American National Standards Institute (ANSI) A108.02-4.1.1 states; “All surfaces shall be structurally sound, clean, dry, and free of oily or waxy films and all foreign matter. Concrete surfaces shall be free of form oil, curing compound, laitance, and cracks.”

When these requirements are met, there is one more important question. Is the floor flat enough to successfully install large tiles without lippage? ANSI A108.02-4.1.4.3.1 states; “For tiles with at least one edge 15 in. (0.38 m) or longer, the maximum allowable variation is no more than 1/8 in. in 10 ft. (3 mm in 3 m) and no more than1/16 in. in 2 ft. (1.6 mm in 0.6 m) from the required plane, when measured from the high points in the surface.”

To determine if the floor is flat enough, the best tool to use is a 10-foot straightedge as described in the above standard. Moving the straightedge across the floor will quickly provide the needed answer. An uncomplicated way to measure the low spots under the straightedge is to use horseshoe spacers of assorted colors. For instance, the standard calls for no more than a 1/8” gap which can be quickly measured with a red 1/8” spacer. If the gap allows a red spacer to snuggly slide under the straightedge, you are good to go. If the gap allows a yellow (3/16”) or black (1/4”) spacer to pass through or an even wider gap is seen, the floor will need to be made flat.

The out of flat situation can be corrected by grinding down the high spots, filling the low spots with a troweled-on patch, or applying a self-leveling underlayment (SLU). Exercise caution if grinding is selected and be certain to contain the dust with the proper tools attached to a HEPA filtered vacuum. The troweled-on patch will work if the area is not extensive. However, if the entire floor needs to be flattened, a SLU, now known as Flowable Hydraulic Cement Underlayment (FHCU) may be the preferred method.

To prepare for the FHCU, all gaps and holes along the walls or within the floor area must be sealed completely so that the flowable material does not leak out during the installation. This can be done with caulking or spray foam. Additionally, the perimeter walls should receive a layer of ¼” foam sill sealer which acts as shock absorber for any movement.

The key to success with a FHCU is the primer which must be produced by the same manufacturer. The primer is applied per the manufacturer’s recommendations and allowed to dry. The FHCU is mixed with water per the instructions on the bag, poured onto the floor and allowed to dry.

The result, as seen in the attached image, is a beautifully flat floor which is confirmed by the 10-foot straightedge. This floor is now ready to receive the new 24” x 24” porcelain tile. The unseen detail of this newly flat floor is that the floor, before the pour, had multiple low spots over ½” which is over four times the allowable variation.

Proper substrate preparation is critical to a successful tile installation while also making the tile much easier to install.