Installer Update: Mosaics – Beautiful, Practical and Sometimes Challenging
March 1st, 2006

By Rachel Gibbons

March-April 2006

Mosaic tile patterns are a beautiful, practical art form that can be installed in commercial and residential applications ranging from kitchens to swimming pools.

Although mosaics can pose installation challenges, installers shouldn’t be intimidated. The path to successful mosaic projects takes education, practice and knowing where to find help.

A bit of background

An art form that dates back thousands of years, mosaics are intricate designs or patterns made up of small tiles or pieces of material that are thinner than conventional tiles. Commonly used materials include ceramic, porcelain, glass and stone.

While the traditional piece-by-piece fabrication of mosaic designs still occurs, most designs are mounted on paper or mesh fabric sheets (often 12-by-12-inches or 24-by-12-inches) for faster installation. The mosaic material may be face-mounted (often paper), clear film-faced (plastic adhesive film) or back- and edge-mounted (mesh fabric, perforated paper, resin, polyurethane or other mounting material).

Installation challenges largely stem from the nature of mosaics. Because the individual tiles are so small, it doesn’t take much to interfere with the adhesive bond between the mosaic sheet and the substrate. For example, mesh fabric back-mounted material can prevent the mortar from properly adhering to the substrate. The small tiles can also be pulled loose from the wet mortar as the face-mounted material is removed.

Understanding installation materials

Each type of material used in mosaics has different properties. Therefore, it’s important to remember how each mosaic material reacts with installation materials. Here are some examples:

Porcelain: Because it’s an impervious material, porcelain ceramic tile requires a high degree of bond strength. A latex-modified thinset mortar (that meets or exceeds ANSI A118.4) or a new mortar type, called performance mortar, provides the necessary bond strength to successfully install porcelain mosaics. Unglazed ceramic tile is more porous and can be installed with several types of mortars, including performance mortar or latex-modified mortar.

Glass: Also an impervious material, glass requires a high degree of bond strength. A latex-modified thinset mortar that meets or exceeds ANSI A118.4 is recommended to bond glass mosaics. However, it’s up to the project specifier and the installer to confirm with the mortar manufacturer and the glass mosaic manufacturer that the installation product being considered is right for the job.

Another point to keep in mind: White mortar provides a consistent appearance and is generally recommended for glass tile mosaics. Gray mortar can darken the look of glass and might not be suitable for some glass tiles.

Natural stone: Higher density stone varieties, such as granite or marble, are often used in mosaic designs because it’s important for the small tiles not to flake or chip after installation.

While a portland cement mortar may be used to install stone mosaics, latex-modified thinset mortars provide flexibility that allows a secure bond with higher density stone varieties. All stone types require 100 percent coverage of the bonding material to achieve a successful bond.

Performance mortars are formulated to adequately cover many stone types while delivering bond strength. For mosaic wall applications, some performance mortars have non-sag characteristics that support sheets weighing up to 6 lbs. per square foot without the use of bracing. Performance mortars are not recommended for installing mosaics made with green marble or other types of moisture sensitive stone.

A word about grout

Unsanded grout is generally recommended for the narrow (1/8 inch or less) joints found in most mosaics. Unsanded grout also won’t scratch the surface of glass, stone or porcelain tiles.

An acrylic grout additive should be considered for grout used in mosaic installations that may be subject to expansion and contraction from exposure to the sun and/or freeze-thaw conditions. Grout additive is not recommended for natural stone tile.

Installation methods

A key consideration to successfully installing mosaics is to match the installation method with the type of mosaic being installed and the substrate.

Three basic installation methods—direct bond, backbuttering and conventional wet set—are described here. It’s best to consult with the mosaic manufacturer for specific installation recommendations.

Direct bond: In the direct bond method, the mosaic is bonded directly to the substrate (such as cured concrete) with a bonding material, like a latex-modified mortar or a performance mortar.

Extra care should be taken to apply a uniform amount of mortar under the tiles and then flatten the ridges with the smooth edge of the trowel. This will help prevent the mortar from filling the grout joints. After the mosaic sheet is applied, a beating block and hammer are used to bed the mosaic into the fresh mortar.

If the mosaic sheet is face-mounted, the paper facing material can then be dampened with a sponge and peeled away. Any loose tiles should be immediately pressed back into place. The mosaic can be grouted after the mortar has cured (approximately 24 hours). If the facing is plastic, it’s recommended to wait at least 24 hours before removal. The mosaic can then be grouted right away.

Backbuttering: Backbuttering involves applying a thin coating of mortar to the back of the mosaic sheet to help ensure adequate coverage. This is done in addition to spreading a layer of mortar on the substrate. Backbuttering may be necessary when installing back-mounted mosaic sheets. This is because the mesh fabric that holds the tiles together can interfere with the mosaic sheet achieving a good bond with the substrate.

Conventional wet set: In the conventional wet set method, a slurry, or bond coat, consisting of sand, cement and lime, is applied to an uncured, still-workable concrete substrate. The mosaic sheets are then installed on top of the bond coat. Any face-mounted paper can be dampened and removed. The installation can be grouted after the bond coat has cured.

In addition to the three common installation methods, some types of glass mosaics may be installed by using a “one-step” technique. This calls for mixing a premium unsanded grout with an acrylic mortar additive, which provides adhesion and flexibility. By backbuttering the mixture to the mosaic sheet, the installer pre-fills the grout joints. The paper facing on the glass mosaic sheets is removed after the mixture cures (typically 24 hours).

Because sanded grout will scratch the glass, unsanded grout must be used in the “one-step” technique. Colored grout may be used to add variety to the installed mosaic.

Avoiding problems

Common installation problems with mosaics include bond failure and unsightly mortar ridges. Here is a brief overview of these problems and solutions:

Bond failure. Most bond failure of mosaic sheets stems from one or both of these factors: 1.) The bond strength is inadequate for the type of tile; 2.) The mortar coverage is insufficient to bond the mosaic sheet to the substrate.

To ensure that a mortar has adequate bond strength (particularly for impervious materials, such as glass and porcelain), consult with the mosaic manufacturer before beginning installation. To ensure sufficient mortar coverage, consider using the backbuttering method, particularly for back-mounted mosaics with unique mounting materials, such as mesh fiber, resin, polyurethane, etc. Consult with the mosaic manufacturer regarding specific installation recommendations.

Also be aware that contaminants on the substrate, such as dust, paint or seals can cause bond failure if not removed prior to installation and that substrate movement can affect the bonding of mosaics.

Unsightly mortar ridges. This problem occurs when troweled mortar ridges are visible through the clear or translucent mosaics. It can be avoided by flattening out the ridges before setting the mosaic sheets or by using the backbuttering method.

In addition, some glass tile used in mosaics is sensitive to the high alkalinity of mortar. This may result in the glass becoming discolored and/or the installation losing its adhesive bond.To prevent this problem, consult with the mosaic manufacturer to learn which mortars are compatible with the glass.

Using resources

Mosaic tile manufacturers and installation product manufacturers are excellent starting points for mosaic installation advice. Other industry resources include the Marble Institute of America, the Tile Council of North America and the Ceramic Tile Institute of America.

Rachel Gibbons manages the TEC brand of tile and stone installation systems. For more information, see

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