1. Flat Substrate PreparationThe preparation of a substrate is important in any installation, but with large format tile a flat surface is crucial. The dips found in all concrete slabs or wood sub-floors are a landscape of mountains and valleys when setting tile sizes 18” x 18” and greater. TCNA installation standards require a substrate to meet a maximum variation of 1/4” in 10’ for both vertical and horizontal thin-bed tile installations. Another recommendation to consider for substrate flatness is no more than 1/16” deviation in the substrate at the longest dimension of the tile or stone; for a 16” x 24” tile the substrate cannot exceed 1/16” of irregularity in flatness in 24”. A substrate that exceeds these standards should be filled with an appropriate self-leveling underlayment or ground to the correct tolerance, not filled with additional mortar. A flat substrate ensures a high quality, fast installation. Note: Floor deflection is also an important consideration – check your floor span and tile weight to local building codes. Expansion joints should not be covered but should be continued through to the finished surface, and cracked substrates should be covered with an appropriate crack isolation membrane.
2. Mortar SelectionOne of the most significant advancements for large format tile and stone is the development of thin-set mortars with medium-bed capabilities. Mortars shrink as they cure - the displacement of water causes the mortar to retract and pull the tile as it hardens. This can leave an uneven surface from tile to tile known as “lippage,” an unwanted finish result. Mortars with medium-bed features are formulated to control the amount of shrinking that occurs during the curing process, allowing the tile to stay in position. With the introduction of innovative lightweight mortars, medium-bed mortars are easy to spread, offer non-slump/non-slip support, and are available in rapid setting formulations that quickly lock tile into place - so that within a matter of hours it’s ready for grouting. Whereas standard mortars are capable of a bed 3/32” to 3/16” thick, a mortar with medium-bed features can span from 3/32” up to 3/4” thickness after beat-in, providing maximum support and coverage.
3. Appropriate Trowel Notch for Proper CoverageWith large format tile, mortar coverage transferred from the substrate to the back of the tile without leaving voids can be a challenge. Using the correct sized notch trowel provides a proper setting bed and reduce the amount of time spent on pulling or resetting tile due to lack of coverage. To increase mortar coverage for tile greater than 12” x 12” use a ½” x ½” square notch. For tile cupping or bowing, and ungauged stone, use a ¾” x ¾” u-notch trowel to make up differences. The correct notch sized trowel and the appropriate medium-bed mortar will help to ensure that these tile meet a 95% or better coverage rate. With the introduction of thin porcelain tiles 100% coverage is mandatory, especially at the corners and edges to ensure proper support. Installers should pull an occasional tile to check coverage rates during the installation process. Back buttering large tile or stone may be an option to consider if proper coverage is not reached.
4. Tile FlatnessTile flatness or stone thickness within each piece and any significant differences in various pieces must be considered. Certain tile patterns, such as a running bond or cut diagonal tile against a manufactured edge, can also accentuate slight cups or bows in tile. The ability to adjust tile in the mortar bed will help tailor the alignment of each tile in these situations. Today manufacturers are producing tile with tremendous accuracy in flatness and dimension with great success – exceeding tile flatness specifications addressed in ANSI A137.1. The use of a medium-bed mortar and the right notch trowel will help compensate for irregularities found in most gauged and ungauged stone.
5. Grout JointsA part of the growing trend of large format tile is the shrinking of grout joints’ widths to 1/16” or 1/8”. With this small grout joint, the challenge is fine adjustments needed to create a flat floor. Some tiles have cushioned edges to compensate for tile-to-tile alignment, yet there are large tiles and stone tiles produced with blunt square rectified edges. This leaves little to no wiggle room for tile alignment and relies on the craftsmanship of the installer, the installation materials, and the correct tools for a successful installation.
These five steps will help in the preparation and awareness of installing large format tile. Always refer to the manufacturer’s recommendations as the first source of information for tile or stone installations and always refer to the TCNA (Tile Council of North America) for the proper tile installation guidelines and standards.