Within the masonry and tile trades, there can be confusion surrounding the terms “mortar” and “grout.” This confusion is not surprising, because these terms take on entirely different meanings depending on whether they are used in the context of masonry or tile. This article will shed light on the correct usage of the terms mortar and grout in both applications.

For the purposes of this article, a masonry assembly is defined as a wall comprising one or more wythes (vertical layers) of stacked and fully bedded bricks, concrete masonry units (CMU), or full-depth stones; in each case, the units would be the full thickness of the wythe, generally at least 4 inches thick nominally. Each course (horizontal row), of masonry rests, or bears, on the course below.

And for the purposes of this article, a tile assembly is defined as a wall or floor with ceramic, glass, or stone tiles (facing units that are relatively thin in relation to facial area) that are adhered to the substrate, or backing surface, with an adhesive bonding material.

Mortar and Grout in the Masonry Assembly

Masonry Mortar

“Mortar: A mixture, in either the unhardened or hardened state, consisting of cementitious materials, fine aggregate, water, with or without admixtures, that is used to construct unit masonry assemblies.”

The above definition is from ASTM C1232, Standard Terminology for Masonry. In familiar terms, mortar in a masonry wall is the material between the masonry units. An effective mortar will have sufficient compressive strength to hold the units apart, and sufficient bond strength to adhere to the units. The typical width of a mortar joint is about 3/8 inch, and the mortar generally extends the full thickness of the wythe, nominally 4 inches for modular brick masonry.

Referring to masonry mortar as “grout” or referring to mortar joints in finished masonry as “grout joints” would be technically incorrect and should be avoided.

Performance characteristics and other properties of masonry mortar can be found in ASTM C270 Standard Specification for Mortar for Unit Masonry, and ASTM C1714 Standard Specification for Preblended Dry Mortar Mix for Unit Masonry.

Masonry Grout

“Grout: A plastic mixture of cementitious materials, aggregates, and water, with or without admixtures, initially produced to pouring consistency without segregation of the constituents during placement; or the hardened equivalent of such mixtures.

The above definition is also from ASTM C1232. In familiar terms, masonry grout is the material used to fill vertical cells within hollow masonry units, horizontal voids in bond beams and reinforced lintels, and sometimes collar joints (the open space between masonry wythes). Masonry grout is commonly placed in a wall using a grout pump or poured from buckets.

Often, masonry grout is used to encapsulate vertical or horizontal reinforcing steel to add shear and bending strength to the wall. In this case, a structural engineer or architect would design and call out the spacing of the reinforcement and the grouted cells and/or bond beams on the design drawings.

Referring to masonry grout as “mortar” or “concrete” would be technically incorrect and should be avoided. Although they look similar and share some characteristics, neither masonry mortar nor concrete should be substituted for masonry grout, and the terms should not be used interchangeably.

Performance characteristics and requirements for masonry grout can be found in ASTM C476 Standard Specification for Grout for Masonry.

Mortar and Grout in the Tile Assembly

Tile Mortar

“Mortar: A setting material used to adhere tile, meeting the specifications for dry-set mortar, modified dry-set mortar, EGP (exterior glue plywood) modified dry-set mortar, improved modified dry-set mortar, chemical resistant water cleanable tile-setting epoxy, chemical resistant furan mortar, or modified epoxy emulsion mortar.”

The above definition is from ANSI A108.T American National Standard Terminology of Tile Assemblies. In familiar terms, tile mortar is a thin layer of adhesive compound, usually applied with a notched trowel, that adheres a tile to its backing surface.

“Mortar” is the preferred term for this material recognized in the tile specifications and standards, but other colloquial terms sometimes used to describe this same material include “thinset mortar,” “adhesive mortar,” “bonding mortar,” “bond coat,” “setting material,” and other regional terms.

Performance characteristics and requirements for the different types of tile mortars listed can be found in ANSI A118.1, ANSI A118.4, ANSI A118.11, ANSI A118.15, ANSI A118.3, ANSI A118.5, and ANSI A118.8 respectively.

Mortar Bed

Certain methods for installing tile on floors and walls call for a “mortar bed” to be installed prior to the tile finish. A mortar bed is separate and distinct from the previously discussed “mortar.” The definition from ANSI A108.T reads:

“Mortar Bed: A mixture of sand, cement, and sometimes other additives that is floated, screeded, or otherwise applied over a horizontal or vertical substrate often used as a scratch coat or leveling coat for walls, and a leveling bed for floors. A mortar bed may be bonded or unbonded, and it may be reinforced with embedded wire reinforcement or lath.”

Whereas tile mortar is generally troweled to a relatively thin application, a mortar bed may be 1 to 2 inches or more in thickness.

“Mortar bed” is the preferred term for this material recognized in the tile specifications and standards, but other colloquial terms sometimes used to describe this same material include “mud bed,” “mud,” “dry-pack,” “float coat,” “scratch coat,” and other regional terms.

Once the mortar bed cures, tile may then be adhered to it using mortar, as in the method described in ANSI A108.1B Installation of Ceramic Tile on a Cured Portland Cement Mortar Setting Bed with Dry-Set, Modified Dry-Set, or Improved Modified Dry-Set Cement Mortar. Installing tile on a cured mortar bed may also be known colloquially as installing tile using the “pre-float” method, the “hardball” method, and other regional terms.

Alternatively, tile may be installed in the mortar bed before it has cured, as described in ANSI A108.1A Installation of Ceramic Tile in the Wet Set Method, with Portland Cement Mortar. “Wet-set” is the preferred term for this method recognized in the tile specifications and standards, but other colloquial terms include “fresh-set,” “float-and-cover,” and other regional terms.

Tile Grout

“Grout: A joint filling material for tile, meeting the specification for chemical resistant, water cleanable tile-grouting epoxy, chemical resistant furan grouts, standard cement grouts, high performance cement grouts, modified epoxy emulsion grouts, or organic premixed grouts.”

The above definition is also from ANSI A108.T. In familiar terms, tile grout is the material used to fill in the joints between tiles.

Unlike the finished joints between bricks or other masonry units which are formed as the units are laid, tile grout is applied after the tile has been set and after the tile mortar has cured. Tile grout is typically forced into the spaces between the previously laid tiles with the use of a grout float or in some cases a grout bag, sometimes known as a baker’s bag due to its resemblance to a pastry piping bag.

Performance characteristics and requirements for the different types of tile grouts listed can be found in ANSI A118.3, ANSI A118.5, ANSI A118.6, ANSI A118.7, ANSI A118.8, and ANSI A118.19 respectively.

Referring to tile grout as “mortar” or referring to the grout joints in finished tile work as “mortar joints” would be technically incorrect and should be avoided.

Conclusion

While the crafts of masonry and tile share some similarities, the vocabulary for each craft takes on unique meaning, despite having identical terms. Design and construction professionals are encouraged to be familiar with the craft-specific terminology and usage.

International Masonry Institute (IMI) contributes to U.S. and international standards, industry development, and technical service for both the brick and tile sectors, and delivers apprenticeship and training for all the trowel trades, jointly supported by the International Union of Bricklayers and Allied Craftworkers (BAC) and their signatory contractors.