The Importance of Sufficient Mortar Coverage in Large Format Tile Installations

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 examines standards for mortar coverage, perhaps one of the most important criteria to achieving a good bond.

If a project’s architectural specifications reference ANSI A108 American National Standard Specifications for the Installation of Ceramic Tile, the tile contractor is obligated 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. 

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 GPT panels/slabs.

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.”

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, then 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 GPT panels 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.

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 TCAA Trowel of Excellence contractors and BAC 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 Scott Conwell's column in the Tile Contractors Association of America journal, 9300 Contractor, Winter 2019 Edition.