“Full coverage” can take on a couple of meanings. If we’re talking about insurance, it means that you have added protection against accidents. Another meaning, which can also tie into insurance, is adhesive coverage. Are you achieving the correct amount of bond to “insure” a successful installation? Lately, I’m seeing quite a few installations where proper coverage has not been maintained, which has led to tile and hardwood issues.
When it comes to tile, what is coverage? The 2018 TCNA Handbook states:
Mortar Coverage for Ceramic Tile: “Average contact area for dry areas is 80% and for wet areas is 95%. Mortar coverage is to be evenly distributed to support edges and corners. It is not possible or practical to achieve 200% coverage consistently and such should not be specified.”
Mortar Coverage for Natural Stone Tile: “Mortar coverage must be sufficient to prevent cracks in the stone resulting from voids in the setting bed. In dry and wet areas, the minimum coverage is 95% with no voids exceeding 2 square inches and no voids within 2 in. of tile corners. ALL corners and edges of the stone tiles must be fully supported, and back-parging or back-buttering is recommended in all areas. Coating the back of the tile, however, does not constitute coverage, which is the area where the mortar makes contact with the tile and the substrate. It is not possible or practical to achieve 100% coverage consistently and such should not be specified.”
Directional Troweling: “To ensure proper coverage of the bonding surface of 8x8 in. and larger tiles, and to provide full support of edges and corners, select a notched trowel sized to facilitate the proper coverage. Key the mortar into the substrate with the flat side of the trowel. Comb with the notched side of the trowel in one direction. Firmly press tiles into the mortar and move them perpendicularly across the ridges forward and back to flatten the ridges and fill the valleys. This method can produce maximum coverage, with the corners and edges fully supported, without back-buttering or beat-in. Periodically remove and check a tile to ensure proper coverage is being attained.”
The failures I’ve seen on both tile and hardwood could have been avoided had the installers used the correct trowel notch and proper substrate preparation.
For hardwood, the National Wood Flooring Association states to follow the manufacturer’s recommendations as there are numerous types of adhesives and trowel notch sizes to accommodate the requirements for each manufacturer.
The installation method chosen may also change the coverage amount requirements. Different sized trowel notched may be needed, depending on the construction of the wood product and whether you’re using the adhesive as a bonding agent only or as an adhesive and moisture control membrane.
Bostik’s GreenForce, for instance, lists the following on their trowel selection: “Trowel size is suggested to maximize coverage of adhesive. Periodically check coverage of adhesive during installation. For adhesive only, >80% coverage and transfer to the back of the flooring is required for all engineered wood flooring; >95% coverage and transfer is required for all solid wood flooring or bamboo flooring products.
“For use as an adhesive and moisture control membrane, in order to form a membrane that functions properly for moisture vapor protection and/or sound reduction, the right trowel needs to be selected to achieve both 100% coverage of the substrate and 100% transfer to the back of the flooring. Jobsite conditions, profile of the substrate, depth of back channeling in the flooring, and other factors affect the amount of adhesive that must be applied to achieve proper coverage and transfer. Always pull a board at the beginning of and during the installation process to confirm adequate coverage and transfer. Trowel size may need to be changed to achieve the required coverage and transfer.”
Remember, it’s up to us as professionals to make sure we achieve proper coverage. If we don’t, the liability falls on us—and thin-set and adhesives are a lot less expensive than a failure.