Selecting and Applying Adhesives
Without a clear understanding of the various types of thin- and medium-bed mortars available and their specific uses, an installation will fail to meet established specifications
Many times the process of choosing the proper adhesive for a tile, stone or glass installation can be as overwhelming and intimidating as selecting a big screen television or a new computer. In order to overcome this anxiety, one must first research the vast group of materials available. In doing so, we very quickly find that some manufacturers provide over 30 thin-set mortars alone. No wonder some installers and specifiers are frustrated. But with good information, this process becomes manageable.
To determine the correct adhesive or setting material, we first need to know what each product is and what it does or doesn’t do when in place. In order to do that, we will use information from the 2011 TCNA Handbook that references the applicable American National Standards Institute (ANSI) Specifications and International Standards Organization (ISO) material specifications. Within the scope of this article, we will focus on thin-bed and medium-bed mortars only.
One very important note that needs to be made at the outset is that none of these products are intended to be used in truing or leveling the work of others. In the past and still today, many good installers have routinely used a thin-bed product to “fix” the floor or wall surface before setting tile. These products were not designed nor intended to correct imperfections in the substrate. Use the appropriate trowel applied patch or self-leveling underlayment to correct these flaws before installing the tile.
Dry-Set MortarDry-set mortar is a mixture of Portland cement with sand and additives imparting water retentivity that is used as a bond coat for setting tile.
Its use is appropriate for the installation of ceramic and natural stone tiles over a range of properly prepared substrates including masonry, concrete, gypsum board, cement backer board, fiber-cement board, cementitious-coated foam backer board, cured Portland cement mortar beds, brick, ceramic tile and dimension stone. Consult membrane and setting material manufacturers for use over membranes.
Latex/Polymer Modified Portland Cement MortarLatex/Portland Modified Portland cement mortar is a mixture of Portland cement, sand and special latex/polymer additives that is used as a bond coat for setting tile. The installation specification for this product is found in ANSI A108.5 while the material specification is found in ANSI A118.4 and the ISO “C” category.
This product may be used over suitable surfaces similar to those of dry-set mortar and is installed using the same mortar thickness. The difference between these two is the addition of liquid latex (added when mixing in lieu of water) or a spray-dried polymer (added during the manufacturing process), which is activated when mixed with water at the jobsite. The purpose of the latex/polymer additive is to improve adhesion, reduce water absorption and provide greater bond strength to tiles which are difficult to bond.
While these additives provide increased strength and flexibility, they also create other circumstances that need to be addressed; in that they require air to coalesce and dry. Without air, these products can remain uncured for an extended period of time. Be especially careful when installing porcelain or glass tile over an impervious substrate such as a membrane. This situation may require an extended drying time of 14 to over 60 days before grouting the tile or allowing traffic, water exposure or submersion. Since not all latex/polymer-modified Portland cement mortars are suitable for wet areas, consult the manufacturer for guidance and follow their directions explicitly.
Exterior Glue Plywood (EGP) Latex Portland Cement MortarEGP mortar is a latex/polymer modified Portland cement mortar used specifically for bonding ceramic and natural stone tiles to exterior glue plywood.
The installation of these products with an EGP mortar on a wood surface necessitates that two requirements be met. The floor structure must have two layers of structural wood with the top layer being exterior glue plywood. The use of particle board, lauan plywood, oriented strand board or other similar boards is not recommended. EGP is installed in one layer usually between 3/32 and ¼ inches after the tiles are embedded.
The installation specification for this product is found in ANSI A108.12 while the material specification is found in ANSI A118.11 and in the ISO “C and P” categories.
Medium Bed MortarMedium bed mortars are designed to minimize slump and facilitate thicker bond coats, as compared with non-medium-bed mortars. These characteristics make them useful for setting heavy tiles and/or tiles with ungauged thickness and for setting tile with at least one side greater than 15 inches where the final embedded thickness of the mortar will exceed 3/16 inches. They are intended to be used as direct bond adhesives with a bond coat 3/16- to ¾-inch thick after the tile is embedded, but are not designed nor intended to be used to correct substrate irregularities.
At present, medium bed mortars do not have an ANSI or ISO standard, but committee work is underway to create an approved definition and installation standard. Since this is the case, please refer to manufacturer’s recommendations.
Epoxy MortarMedium bed mortars are designed to minimize slump and facilitate thicker bond coats, as compared with non-medium-bed mortars. These characteristics make them useful for setting heavy tiles and/or tiles with ungauged thickness and for setting tile with at least one side greater than 15 inches where the final embedded thickness of the mortar will exceed 3/16 inches. They are intended to be used as direct bond adhesives with a bond coat 3/16- to ¾-inch thick after the tile is embedded, but are not designed nor intended to be used to correct substrate irregularities. At present, medium bed mortars do not have an ANSI or ISO standard, but committee work is underway to create an approved definition and installation standard. Since this is the case, please refer to manufacturer’s recommendations.
Organic AdhesiveOrganic adhesive or mastic is ready to use with no further addition of liquid or powder that cures by evaporation and is for interior use only. It’s suitable for setting ceramic tile on floors, walls and countertops, where the surfaces are appropriate and properly prepared. Adhesives are applied in one thin layer with a trowel per manufacturer’s directions and must never be used to level or true the substrate. Since drying, temperature and support issues may be a factor, consult the manufacturer.
The installation specification is found in ANSI A108.4 while the material specification is found in ANSI A136.1 and in the ISO “D” category.
Applying Bonding MaterialsThe application of thin-bed bonding materials is a relatively simple operation, but it does require a consistent technique. The straight line troweling method developed by the National Tile Contractors Association (NTCA) in the “Trowel and Error” video provides excellent results of full or nearly full coverage on the back of the tile.
The substrate must be properly prepared, being clean and free of surface contaminants and flat within the appropriate ANSI Specification. Apply the bonding material using the flat side of the trowel effectively keying or burning it into the substrate. Using the proper notch, trowel the bonding material in one direction. Press the tile into the bonding material, moving the tile perpendicularly to the trowel ridges in an up, down and back up motion. This movement collapses the ridges of the trowel notch into the valleys, successfully producing a uniform thickness that yields adequate coverage and transfer to the back of the tile. To ensure adequate coverage, routinely remove a tile to check for both coverage and transfer.
When the appropriate bonding material is used in concert with straight line troweling, the result is a well-bonded tile installation that will provide years of beauty and enjoyment.