Illustration 1
Porcelain tiles have always been prized because they are stain-proof, impervious to moisture, freeze/thaw stable and highly resistant to wear. One drawback, however, is that most porcelain tiles are traditionally rather small. One-inch square and hex tiles, commonly found on shower floors, are good examples.

Large porcelain tiles made with wet clay tend to curl, cup and twist during firing, but thanks to the dust-pressed method, where tile bodies are made from rather dry particles and formed in a die under tremendous pressure, porcelain tiles are now widely available in much larger sizes. On a recent tour of a tile manufacturing facility in Spain I saw flat, 2-by-3-foot porcelain tiles about ½-inch thick exiting a modern production tunnel kiln.

Illustration 2
Not all mass-produced large format tiles are porcelain, but most are vitreous or impervious and have gained rapid acceptance by designers and consumers because of their beauty, functionality and—because they require fewer grout joints than do small tiles—ease of maintenance. Large format tiles are described as those measuring 10 inches or larger. Installers are more efficient with these than with smaller tiles, but to ensure that the adhesive bond between the tile and setting bed meets the industry minimum of 85 percent uniform coverage in dry areas (95 percent in wet applications), special tools and a newly developed installation method are required. I refer to this technique as the straight-and-slide method, and it is the most effective and efficient way to ensure that larger tiles are fully bedded in adhesive with the least amount of strength-robbing air trapped beneath them.

Special tools and methods are required because the methods and tools used to install smaller tiles don’t work well on larger formats. Large tiles tend to trap air that can significantly reduce the adhesive bond. Switching to a larger size square-notch trowel is an effective strategy for small tiles, but large format tiles cover so much area that sometimes, switching to a larger square notch actually allows more air to become trapped beneath the tile! Even using a beating block and hammer, usually a dependable method for increasing the adhesive contact area, may be ineffective, especially if the adhesive is combed in a traditional swirling pattern. Illustration 1 shows how the ridges of adhesive act like a dam to keep air under the tile.

Illustration 3
The two main factors in eliminating air trapped under large format tiles are the shape of the notch and the alignment of the combed-out ridges of adhesive. Ridges of thin-set mortar combed by square notches larger than 3/8 inch tend to mushroom as they are squeezed between the tile and the setting bed (Illustration 2). By careful study and a cooperative industry effort, it was determined that rounded, U-shaped ridges of adhesive significantly reduced trapped air. More study showed that using a U-notch trowel to spread straight, parallel lines all but eliminated excess and unwanted air (Illustration 3).

The traditional technique of making swirling patterns with the notched trowel may be acceptable for spreading thin layers of contact-type adhesives, but when done with thin-set mortar, the practice results in a relatively thick and very uneven layer that may negatively affect the surface finish, contribute to adhesive voids below the tiles and create a significant amount of labor to keep grout joints clean and unclogged. Maintaining a consistent angle with the trowel and applying a precise and uniform layer of thin-set are key elements for ensuring that large format tiles are securely bonded.

Illustration 4
Once the adhesive has been combed, the second part of the straight-and-slide technique begins with “hinging” (Illustration 4) the tile down into the adhesive bed (in-line with the adhesive ridges so air will be swept out as the tile is collapsed), moving the tile about ½ inch perpendicular to the direction of the adhesive ridges, then moving it back into its final position (Illustration 5). The technique requires that each tile be initially placed very close to its final position prior to the ½-inch slide. As the size of the tile increases, the amount of pressure exerted by the installer during each slide has to increase: for maximum performance, a minimum 3/32-inch layer of thin-set mortar should remain between the tile and the setting bed.

The straight-and-slide method turns the leading edge of the sliding tile into a plow, coating it with thin-set mortar that must be scraped and sponged clean. This requires extra time, but far less work than what back buttering or spotting (two ineffective and inefficient methods for improving adhesion) require. In the hands of an expert installer, back buttering can achieve 95 percent to 100 percent adhesive coverage; however, for a do-it-yourselfer or the occasional installer, the method can result in too much ooze or unwanted voids.

Illustration 5
Spotting (also known as 5-spot) is the practice of applying five globs of thin-set on the corners and center of the back of a tile, pressing the tile into position against its neighbors, then removing it and filling the voids with additional thin-set using the flattened spots as a guide. Originally practiced to help speed up the back-buttering process for beginning installers, the technique has degenerated into a doomed procedure where five spots of thin-set are applied to the back of the tile, but the tile is not removed for refilling after it is first installed. The resulting voids beneath each tile contribute to a hollow sounding floor and create an installation whose compressive strength is incapable of supporting even the lightest of loads.

Even worse, in an exterior or a wet interior application, 5-spotting promotes the growth of germs, harmful bacteria and other organisms, as well as the seemingly ever-present and discoloring black mold. Voids readily fill with moisture that can cause freeze/thaw damage in winter, and provide an incubation chamber for unwanted visitors at any time.

The straight-and-slide method depends on flat tiles and a flat setting bed. Although the minimum industry tolerance for variances of flat, level, plumb or slope is ¼ inch in 10 feet, the tolerance needs to be tightened for large tiles. A 1/8-inch variance in 10 feet would be the suggested minimum for 12-inch tiles, with tiles larger than 18 inches needing a surface that is flat to within 1/16 inch in 10 feet. Seemingly small variances in the substrate can cause unacceptable lippage when dealing with tiles larger than 16 inches. Small tiles conform easier to setting bed irregularities than large tiles, and so require a simpler surface preparation. As tiles get larger, an installer may need to apply mortar and featheredge or self-leveling compounds over wood and concrete substrates to ensure a smooth finish for large format tiles.