Large format porcelain tiles have been around for some time but have exploded in popularity over the past few years. Traditionally, tiles came in a relatively standard size such as 8" x 8" floor tile and 4" x 4" wall tile. Today, tiles are available from tiny glass mosaics that are 3/8" x 3/8" to large format porcelain tile slab that can be as large as 5' x 10'. Compared with standard tile installations, Large format porcelain tiles present new challenges for tile installers. When installing porcelain slab tiles, certain procedures and methods must be adhered so that the tile does not crack and leave behind a failed installation. Handling, lippage, subfloor preparation, mortar coverage, and curing / protection all require extra consideration during the installation process of large format tile.
A tile is considered large format when it is 16" x 16" or larger. However, a tile may also be considered large format if at least one edge is greater than 15" such as the popular wood-look porcelain planks. Large format tiles can be made of natural stone, ceramic, or porcelain with even larger and large format tile, requires specific installation methods.
For consumers, Large format porcelain tiles is aesthetically pleasing with in a wide variety of colors and designs available, and as a design element,porcelain slabscan be used to make a smaller room appear bigger. Many people also enjoy the look of thin grout lines commonly used with Large format porcelain tiles. Additionally, because each tile offers more coverage per tile than traditionally sized tile, there are fewer grout lines. With fewer grout lines, maintenance becomes easier because a tile face is typically easier to clean than a grout line, and as many of us know, even sealed or epoxy-based grout can be difficult to keep clean and looking its best.
Porcelain slabs, it is imperative that the subfloor or substrate is properly prepared before laying any tile and the larger the tile size, the more essential this becomes. It must be completely level and flat. Any dips or humps can lead to lippage that large format porcelain tilesinstallations are prone to. Lippage is when one edge of a tile is not level with the adjacent tile's edge. This leads to the finished surface having an uneven appearance, degrades the quality of the installation, and can be a tripping hazard.
Additionally, uneven surfaces will make it difficult to achieve the proper mortar coverage and bond, another important aspect of installing large format tile. For some large format porcelain tiles, only specific substrates are approved by the manufacturer.
Variance in Substrate
With traditional tile that is less than 15" x 15", there can be up to a 1/4" variance in the substrate over a 10' span. For tile with at least one edge greater than 15", a maximum variance of 1/8" over a 10' span and 1/16" over a 24" span. A 6-foot level or 10-foot straight edge can be used to measure variance in the substrate. Any imperfections in the subfloor will be highlighted by large format tiles, so If the subfloor exceeds these specifications, the subfloor must be corrected with a self-levelling underlayment, patch, or grinding it down.
A self-levelling underlayment, such as Laticrete NXT Level and Laticrete NXT Level-Plus, is a cement-based product that can be poured on the subfloor to fix small imperfections in the subfloor and even out the surface. As it cures, the self-levelling underlayment produces a smooth and flat surface ready for tiling. For minor variances, a trowel applied self-leveling patch, such as Laticrete NXT Patch, can be used to bring the substrate to tolerance. Patching compounds can also be used to fix small variances on walls as well. The industry recommends using substrate preparation products and installation products from the same manufacturer to ensure compatibility.
Using the appropriate thinset/mortar is very important for a successful installation. With traditionally sized tile installations, thinset is used to bond the tile to the substrate. Large format porcelain tiles that often have a 1/4" (6mm) thickness, regular thinset can be problematic. For many large format tile installations, it is often recommended to used a medium-bed mortar to account for irregularities in the tile and substrate.
Using a thick layer of regular thinset instead of a medium-bed mortar can cause lippage or cracks because as the thinset cures, the displacement of water causes the thinset to shrink. As it shrinks and hardens, the thinset pulls on the tile and can result in damaged tiles. On the other hand, using an inadequate amount of mortar can lead to hollow spots. These hollow spots indicate that the tile is not fully supported, and the tile becomes susceptible to cracking at these points.
Medium-bed mortars are formulated to limit the amount of shrinkage during the curing process and are recommended for large format tile installations. Mortars designed for medium-beds will be able to support the tile without the tile slumping into the mortar. These mortars, however, can be difficult to trowel and work with in general. Newer types of high performance medium-bed mortars are formulated to not only be lighter, but are also much easier to trowel. When selecting a mortar, it is always advisable to consult the material manufacturer for mortar recommendations, as not all mortars are suitable for every application.
Full and complete mortar coverage is an essential part large format tile installations. Without the proper coverage, the tile and gout become susceptible to cracking along with a loss of bond to the tiles. To achieve a full coverage, start by using the appropriate trowel and trowelling technique. For many large format tile installations, a larger trowel such as 1/2" x 1/2" square notch or a 3/4" x 3/4" rounded notch will help attain adequate coverage. For larger tiles, a U-notch trowel may be necessary for correct coverage.
With a correct trowel, a proper setting bed will be produced that to aid in eliminating voids and reducing time spent resetting tiles that do not have full coverage. There is no exact standard for trowel size selection, so trowel size may vary for different jobs and substrates. However, the larger the tile size, the larger the trowel that is generally needed. For instance, a 1/2" x 1/2" square or round notch trowel is commonly used for tiles sized from 13" to 20" while a 3/4" x 3/4” round notch trowel is typically used for tiles larger than 20". For thin porcelain tile panels, a specialty euro / zipper trowel is often used to achieve full coverage.
When applying the thinset to the substrate, the thinset should be combed in a linear, uniform direction. With the trowel held at a 45-degree angle, drag the trowel across the thinset in one direction leaving behind straight and full "ribbons" of thinset. Using straight trowel lines will also help prevent voids or air from becoming trapped between the tile and substrate for proper coverage and adhesion
While a medium bed mortar will generally require for proper coverage, back buttering also helps to achieve full coverage. Back buttering is the process of using the flat side of the trowel to apply a thin layer thinset or mortar to the back of the tile before setting it. This helps to fill any voids on the back of the tile. Back buttering is especially important with natural stone products such as travertine, granite, marble, slate, etc. because of the naturally occurring spaces and imperfections on the back of the tile. By filling the spaces in with thinset before setting, weaker areas are strengthened and better coverage is obtained.
For thin porcelain tile panels, a different method is used for back buttering. Instead of using the flat side of the trowel to produce a flat even coat on back of the tile, the notched side of the trowel is used to create the same ribbons of thinset as used on the substrate. These thinset ridges should be in a level line across the back of the panel and will line up parallel with the ridges of thinset on the substrate. By troweling the both the backside the panel the same as the substrate, 100% coverage can be achieved which is essential for the success of thin porcelain panel installations.
porcelain slabs are more susceptible to cracking than smaller sized tiles. Movement along with tiny cracks in the subfloor can transfer to the tile and result in cracks. Traditionally, reinforced mortar beds had been used to allow the substrate and tile covering to move independently of each other and prevent the transfer of cracks from substrate to covering. However, reinforced mortar beds add a significant additional height to the installation and isn't practical by today's standards for new construction and remodels.
Uncoupling membranes are designed to replace the traditional reinforced mortar beds without the addition of significant height. While each manufacturer will have their own design, in general uncoupling membranes are composed of a waffle-like grid structure with an anchoring fleece on one side, are light weight, come in a roll, and are easy to install. Incorporating an uncoupling membrane or other crack isolation underlayment will help to prevent cracks from occurring.
Even with a perfectly flat subfloor, lippage is still another concern for large format tile installations. Lippage occurs when the adjacent edges of two tiles are not even. The bigger the tile, the more problematic lippage can become. Not only does lippage appear more prominent with larger size tiles, it also is presents a greater tripping hazard. Incorrect mortar bed, wrong setting techniques, and warpage can all lead to lippage.
Using an appropriate sized mortar bed, such as a medium-thick mortar bed, with correct setting practices will help to prevent lippage. Another extremely valuable tool available today are tile leveling systems. Both Tuscan Leveling Systems and MLT Leveling Systems are easy-to-use tile leveling systems that will greatly help in creating lippage-free installations. These systems generally consist of a few components including base/bottom plates, caps, straps, and installation gun and can be used for both floor and wall applications.