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.
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.
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.
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.
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.