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The Importance of Sufficient Mortar Coverage in Large-Format Tile Installations

by Scott Conwell, FAIA, FCSI, CDT, LEED AP, director of industry development with the International Masonry Institute (IMI)

Photos courtesy of IMI

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There are many factors that contribute to the ability of a tile to permanently bond to the substrate, including substrate preparation, proper selection of setting material, environmental conditions during installation, application of bonding mortar and embedment of the tile. This article will examine standards for mortar coverage, perhaps one of the most important criteria to achieving a good bond.

For projects with specifications referring to ANSI A108 American National Standard Specifications for the Installation of Ceramic Tile, the tile contractor must meet the minimum mortar coverage requirements specified in the standard. Even if the specification does not refer to ANSI, or if there is no specification, it is widely acknowledged that the mortar coverage requirements stated in ANSI represent best industry practice.



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This figure represents a 12- x 12-inch tile with 95% coverage (5% voids), as required for wet areas and exterior tile installations. This diagram also shows no voids larger than 2 square inches and no voids within 2 inches of a corner or edge, which is not required of ANSI, but is representative of best industry practice.

This figure represents a 12- x 12-inch tile with 80% coverage (20% voids) as required for interior non-wet areas. This diagram also shows no voids larger than 2 square inches and no voids within 2 inches of a corner or edge, which is not required of ANSI, but is representative of best industry practice.

The coverage requirements stated in ANSI A108 apply to glazed wall tile, ceramic mosaic tile, quarry tile, paver tile and other unmounted vitreous and impervious tile to be installed on walls, ceilings or floors with modified or unmodified dry-set Portland cement mortar. The standard excludes requirements for glass tile, stone tile, gauged porcelain tile (GPT) and gauged porcelain tile panels/slabs (GPTP). Generally, large-format tiles (tiles with at least one side greater than or equal to 15 inches) present a greater challenge to the installer than their smaller counterparts when it comes to achieving required mortar coverage.

For the types of tile covered by the standard, the average mortar contact area shall be a minimum of 80% for interior non-wet areas (i.e. maximum 20% voids in the mortar) and a minimum of 95% for exterior areas and showers (i.e. maximum 5% voids in the mortar). Best practices extend the 95% requirement to all wet areas, not only exteriors and showers. ANSI defines wet areas as “tile surfaces that are either soaked, saturated, or subjected to moisture or liquids (usually water) such as in gang showers, tub enclosures, showers, laundries, saunas, steam rooms, swimming pools, or exterior areas.”

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Transparent Plexiglas simulates a tile laid in thinset mortar applied with a curving motion of the notched trowel. Note the inability of the ridges in the mortar to collapse due to non-uniformity of the troweling.

Transparent Plexiglas simulates a tile laid in thinset mortar applied with a one-directional motion of the notched trowel. Note the ideal embedment of the tile due to the ease of mortar ridge collapse.

As any skilled tile setter knows, it isn’t enough to merely spread mortar to the 80% or 95% requirement for contact area; the tiles must be fully and adequately embedded in the mortar to meet coverage requirements. ANSI describes successful tile embedment by keying mortar into the substrate with the flat side of the trowel, then combing the mortar with the notched side of the trowel in a single direction, and finally beating in the tile or pushing it in a direction perpendicular to the combed ridges (it should be noted that very large tiles like GPTP employ alternative embedment techniques).

The ANSI standard includes a provision to check for coverage in freshly laid tile by removing not less than three tiles for visual inspection. If the ridges in the mortar are collapsed, that’s an indication of good coverage.

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To verify proper coverage and embedment, lift a tile and examine the back of the tile for mortar coverage and examine the substrate for ridge collapse. This photo is an example of ideal coverage and embedment.

Architects and other design professionals want to ensure that the tile assemblies on their projects achieve sufficient mortar coverage and a good bond. ANSI A108 provides a thorough definition of what those coverage requirements are and how to achieve them. An additional measure that may be taken is to require tile installers who have been certified in the Advanced Certifications for Tile Installers (ACT) for Large Format Tile, an industry wide program that evaluates installers on mortar coverage and other critical points of installing large-format tile. Since ACT verifies tile setters’ knowledge and ability to install to the ANSI standards, the certification can be an effective means of ensuring a successful, lasting installation. Architects who specify qualified installers like union tile setters who are ACT-certified in Large Format Tile can be confident that the installers have been trained, tested and certified in providing tile assemblies that meet or exceed best industry practices in mortar coverage and other aspects of tile installation.

Note: This article is adapted from a technical column by the same author, previously published by the Tile Contractors’ Association of America in their journal, 093000 Contractor.

Scott Conwell is a director of industry development with the IMI. A leader in the tile and stone industry, Scott sits on the tile/marble/terrazzo labor-management craft committee of the IUBAC. He is a delegate to the TCNA Handbook for Ceramic, Glass, and Stone Tile Installation committee and sits on the ANSI A108 Committee for ceramic tile. In his 25+ years with the IMI, Scott has presented over 1,000 seminars to architects, designers, contractors and students all over the world. He is a registered architect, a Fellow of the American Institute of Architects and the Construction Specifications Institute.

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