By Greg Mowat, Forensic Tile Consultants
ANSI A108.5, which sets forth the standards for installing ceramic tile with modified and non-modified mortars, states when “95% coverage is specified in the project specifications, back butter each tile with bond coat; or select a notched trowel sized to facilitate the proper coverage.”
The tile should be embedded in the mortar by beating-in, pushing in a direction perpendicular to the combed ridges, or other means to achieve the specified coverage. This method should produce maximum coverage with the corners and edges fully supported. Periodically remove and check a tile to assure that proper coverage is attained. (When coverage is not specified, go for 95% minimum coverage.)
The term “back butter” has been used for decades as one of the steps to assure proper contact and coverage between the tile and substrate. To back butter means spreading the bond coat to the back side of the ceramic, glass, stone or specialty tile prior to placing it into the substrate.
Flat backed troweling and notched back troweling are both terms used in lieu of back buttering to further define the process. Flat back troweling uses the flat side of the trowel to burn in/key in a thin layer of thin-set modified/non-modified dry set mortar to the back of the tile before it is embedded into the setting mortar on the substrate.
Flat back troweling is recommended for the installation of stone and large-format tiles. This troweling method assists in filling in voids in the patterned backs of tiles and may create a flatter surface to correct for warpage.
Required for the installation of gauged porcelain tile panels (GPTP), notched back troweling is the application of an additional thin layer of thin-set modified/non-modified dry set mortar to the back of the tile, in one direction with an appropriate notched trowel just before it is embedded into the setting mortar on the substrate.
The embedded thickness of the mortar bed cannot exceed mortar manufacturer’s maximum thickness requirements (commonly 1/2-inch thick). The notches must be in one direction, aligned (parallel) with the notches on the substrate creating a zipper effect and fully collapsed during installation to achieve the necessary contact, coverage and support.
Flat back troweling and notched back troweling should meet ANSI A108.5 requirements. Application of spots of thin-set without complete contact and coverage between the tile and substrate is not acceptable.
In order to achieve a uniform thickness of the tile and combined mortar when working with large tile that may vary in thickness or flatness, consider using the flat back troweling and notch back troweling methods with a box screed.
Application of thin-set mortar is only one step in the process of proper tile installation. Every step done correctly increases the chances of a successful, long-lasting and durable tile installation. Always work toward successful tile installations.