2024-25_NTCA_Reference_Manual_dig_dwnld

Flat Back-troweling and Notched Back-troweling (Formerly Backbuttering and Buttering)

FLAT BACK-TROWELING Flat back-troweling tiles – as stated in ANSI A108.5 – is listed as an option to achieve the desired coverage. Flat back-trow eling may help with coverage, but it will not provide the neces sary coverage for an installation by itself. The other option listed in the standard to achieve the needed coverage is to “select a notched trowel sized to facilitate the proper coverage.” Select ing the properly sized notch trowel to achieve the needed cov erage for an installation is a key requirement for any properly installed installation. Flat back-troweling can, however, promote increased bonding strength of the bond coat material. By spreading the thin layer of bonding material to the back of the tile, the bonding material is now forced into any surface voids of the tile. The tile should be installed and embedded in a properly-prepared notched surface while the mortar is wet, ensuring the wet transfer of the mortar. This increases the bond strength as the flat back-troweling has increased the contact of the bonding coat material to the back of the tile versus a tile that was not flat back-troweled. Another suggested use of flat back-troweling tiles is to fill voids in the patterned backs of tiles that often occur in the manufac turing process. By filling in the voids, the back plane of the tile is now more uniformly flat, allowing the notched ridges of dry set cement mortar in a properly-prepared installation surface to fully collapse without any voids or air pockets. Flat back-troweling tiles can fill in the inherent warpage of tiles; however, one must pay attention to the dry-set cement mortar/ thin-set mortar manufacturer’s maximum thickness allowed for the bonding material that is being used. By filling the inherent warpage of a tile, the back plane of the tile is flatter and can be compressed uniformly into the properly-prepared notched ridges of the installation surface allowing ridges in the dry-set cement mortar to fully collapse without voids or air pockets, providing the mortar coverage and support required per ANSI A108.5.

The purpose of this document is to provide clarity to the new industry designations for applying adhesive to the backs of tiles and stone, commonly referred to as backbuttering or buttering. The terms backbuttering and buttering have taken on their own definitions in different parts of the country and have been mis takenly associated with spot bonding. When these methods are used properly, they assist with ensuring mortar contact to the tile for proper adhesion. In the case of tiles with recessed patterns or rough profiles, flat back-troweling also assists with achieving required mortar coverage provided troweled ridges on a properly-prepared substrate are completely collapsed. Flat Back-Troweling is defined as: “ The application – using the flat side of the trowel (keying in or burning in) – of a thin layer of modified/non-modified dry-set cement mortar to the backside of ceramic, porcelain, glass and stone tile just before the tile is embedded into the setting mor tar.” (Note: When done correctly, the flat side of the trowel will make a scraping sound on the tile back as pressure is applied to the surface.) This was formerly known as backbuttering. Notched Back-Troweling is defined as: “ After flat back-troweling has been completed, additional modi fied/non-modified dry-set cement mortar is applied to the back side of ceramic, porcelain, glass and stone tile and uniformly combed in one direction with an appropriate notched trowel just before the tile is embedded into the setting mortar. (Caution: When notched back-troweling is used, the embedded thickness of the dry-set mortar bed cannot exceed ANSI or manufacturer requirements.) Formerly known as Buttering. CAUTIONARY NOTES: • Using dry-set cement mortar to true or level the substrate is not recommended. Additional dry-set cement mortar when ap plied to the back of a tile is not a substitute for a properly-pre pared substrate meeting flatness requirements for the speci fied tile, whether it be ceramic, porcelain, stone, glass, etc. It is used only to add mortar to fill irregularities of the tile underside and compensate for thickness variations of the tiles and pro mote increased bond strength.

Flat back-troweling is also recommended when installing stone or large-format tiles.

• Notched back-troweling is not to be confused with the Box Screed Method of installation (see page 196).

NOTCHED BACK-TROWELING Notched back-troweling is required on both the substrate and the back of the tile panel for the installation of GPTP (Gauged Porcelain Tile Panels) per ANSI A108.19.13.6. Because the mo tion to slide a tile to collapse the ridges is virtually impossible with GPTP, notched back-troweling is the technique used to fill the ridges between the substrate and the tile when it is in

• Neither flat back-troweling or notched back-troweling should be construed in any way to allow spot bonding of tiles. When mortar is notched to the tile back, the ridges must be collapsed when placed to create adequate coverage.

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NTCA Reference Manual | 2024 / 2025

Chapter 8 | Thin-Bed Method Installations