Technical Focus: Exterior Tile Installations on Walls
An incredible opportunity for growth, with challenges
The ceramic tile industry has grown substantially in recent years and a big percentage of the growth is seen on exterior installations. This can be for multiple reasons, including a cleaner, more contemporary look, better mortars and grouts, and large-format and gauged porcelain tiles. Large-format and gauged porcelain tiles today really lend themselves to be an exterior cladding material competing with wood siding, stucco, EIFS, glass and aluminum composite panels because this installation has a flatter appearance, a large variety of sizes, as well as ease of maintenance and cleanability. There are a lot of important decisions that must made when selecting tile for exteriors.
When selecting for walls, properly prepared and building code-approved substrates are essential. Common exterior substrates include masonry, concrete, mortar bed and cement backer board over framing. There is also existing cement stucco over a cement basecoat, etc. The current ICC building code in Chapter 14, under paragraph 1405.10.2 Exterior Adhered Masonry Veneers-Porcelain Tile, states: “Porcelain tile shall be adhered to an approved backing system.” The ICC building code also states in Chapter 14 that adhered units shall not exceed 5/8-inch (15.8 mm) thickness and 24 inches (610 mm) in any face dimension nor more than 3 square feet (0.28 m2) in total face area and shall not weigh more than 9 pounds psf (0.43 kN/m2). I am aware of a successful effort to get these sizes in the building code updated in future printing and adoption of the ICC. This effort will allow for the size of tile in the case of Gauged Porcelain Tile (GPT) to be up to five times larger. Until the building code is changed, however, the project owner, builder, architect or engineer should seek local code approval for the use of a GPT and/or larger format porcelain tile finish material for the exterior veneer.
There are many other things to consider when installing exterior tiles — surface preparation to bring the substrate within tolerance is one of the most critical. The industry requirement for the substrate preparation for tiles larger than 15 inches on one edge is 1/8 inch in 10 feet (6 mm in 3 meters) and 1/16 inch in 2 feet (1.6 mm in 0.6 m) from the required plane when measured from the high points in the surface. You must reach out to your setting material manufacturer to find a suitable render or repair material for exterior walls. It should have the ability to float out minor imperfections, including bird baths or big bows, in the framing or substrate. Thinset mortar should not be used for truing of substrates. If you are using an exterior mortar bed on the walls, whether bonded or metal lath and building paper, with properly placed casing beads and expansion joints, you should be able to screed the base flat enough to bring the walls within tolerance. Once the walls are flat, it is important to understand how you will manage water. In framed construction with cement board (CBU), it is common to apply a waterproof membrane over the top of the cement board after it has been installed and properly taped. In the case of a mortar bed, it probably will not require additional waterproofing, but you may choose to add it for additional protection. When applying building paper, an air barrier or waterproofing to the structure, it is critical that you understand the necessary flashing and how you properly tie into it. Ideally, the flashing is already installed; head flashing in windows or doors, kickout flashing where one roof may meet another wall and flashing around all openings.
Select a ceramic tile that is suitable for an exterior installation. With ceramic tile, the most common recommendation is porcelain tile due to its low absorption, wide variety of colors and finishes. With less than 5% absorption, most porcelain tiles are suitable for exterior walls, however, always confirm with your supplier. Testing the tile for suitability, consider ASTM C1026, Standard Test Method for Measuring the Resistance of Ceramic and Glass Tile to Freeze-Thaw Cycling. Other decisions for exterior tile selection can be color selection. Light-colored tiles can lower a site’s heat absorption or heat island effect. Tiles with a Solar Reflectance Index (SRI) of 29 or greater can contribute toward compliance in Leadership in Energy and Environmental Design (LEED) by complying with New Construction (NC) Sustainable Sites (SS) Credit - Heat Island Effect, Option 1 — non-roof. Exterior walls of dark-colored tiles can become very hot, creating a hot wall, which is also known as a heat island effect. In addition to the effect on people and the environment, heat islands can increase cooling loads in the summer, necessitating larger, more powerful air conditioners that use more electricity, in turn increasing cooling costs, producing more greenhouse gases and generating pollution.
The best tile mortars for exteriors combine bond strength with flexibility to allow for movement in the substrate caused by fluctuating changes in moisture and temperature levels. Today’s better thinset mortars are superior to mortar standards in the past. As an example, ANSI A118.4 modified mortars and better go through freeze-thaw cycling, and are ideal for exterior installations. For ANSI A118.15 Improved Modified High-Performance mortars, in addition to freeze-thaw cycling, an additional testing requirement is heat aging. The heat aging test uses two 2- x 2-inch tiles bonded face-to-face with ANSI A118.15 Improved Modified High-Performance mortar with a 1/8-inch offset. These samples are cured at lab temperatures for 14 days, then placed in an oven at 158°F for 14 days. Afterwards, the samples are put into the shear jig and sheared apart. In the case of porcelain tile, the minimum shear strength is 400 psi. This is an excellent example of a lab test for these high-performance thinset mortars that are expected to perform best in the exterior environment. When installing exterior mortars, always consult with the mortar manufacturer, but I believe depending on your temperature swings, most will recommend an ANSI A118.15 Improved Modified High-Performance mortar.
Some avoidable issues we see with the installation of exterior tile can be loss of bond due to lack of coverage, spot bonding and poor troweling techniques. Spot bonding comes from these larger tiles requiring better surface preparation to stay flat. Rather than provide surface preparation, some installers will apply five or more dots or blobs of mortar to the back of the tile and squish the tile into the substrate until it becomes level with the tile surrounding it. This is a bad trend that is causing failures at an alarming rate today. Exterior tile thinset mortar should have a minimum of 95% coverage or contact between the substrate or waterproofing and the tile. The thinset mortar works best when it is contacting what we expect it to bond together. Lack of mortar coverage due to improper trowel selection, spot bonding and poor troweling techniques cause bond loss, moisture issues, poor impact resistance and load resistance.
Grout is also an integral part of an exterior installation’s longevity. High-performance grouts offer increased bond strengths, flexural strengths and lower water absorption to resist freeze-thaw damage.
In especially demanding environments, cement grouts are factory blended or mixed with polymer additives to create better stain resistance and color brilliance, with higher bond and compressive strengths. Polymer-modified cement grouts are more resistant to freeze-thaw damage, lower water penetration and can also increase grout flexibility, providing increased crack resistance.
Products complying with ANSI A118.7, High-Performance Cement Grouts for Tile Installations, are ideal for exterior applications. High-Performance Cement Grouts are generally available in wide variety of colors and frequently have color-coordinated sealants for movement joints. Designers’ aesthetic goals determine the color of the grout used. Color coordinating the grout to the tile color will usually give a larger, more continuous appearance, while contrasting colors highlight the tile, drawing more attention to tiles.
Some potential grout issues on exteriors can be efflorescence or latex leaching. Efflorescence is more common in cement grout meeting ANSI A118.6 Specification for Standard Cement Grouts. Efflorescence is due to water soluble alkali found in Portland cement; it looks like a whitish crystalline or powdery deposit on grout lines and tile surfaces, which is not aesthetically pleasing. This can be prevented by using an ANSI A118.7, High-Performance Cement Grout, for exterior tile. There is not typically Portland cement in ANSI A118.7 grouts, so there is no potential for efflorescence.
On exterior installations, another issue can be latex leaching, which is often the result of the introduction of rain or water below the tile before the polymer-modified thinset and grout has had an opportunity to cure.
Protecting an installation during application from weather is critical not just during installation but while it is curing. This would require tenting. After the tile is installed, it should be protected from weather. A good rain on an open tile installation before grout is installed can take on a lot of water. Then installers show up the next day, grout the wall and are surprised when the grout has efflorescence or latex running down the tile. This can also occur due to bad sequencing of work; the tile is installed, but the roofer doesn’t install the metal cap on the parapet until weeks later. The framed cement board has topical waterproofing and a low-absorption porcelain tile is installed. It rains a couple of days and the wall takes on a lot of water. The metal cap is installed on the roof weeks later and all of the water in the assembly only has one path out, through the grout. This can also occur due to a poor installation or lack of flashing, lack of sealants, etc. For these reasons, carefully review all potential areas water may enter the installation before beginning. Protect the installation before, during and while the assembly is curing. It should also be noted cooler temperatures increase cure times.
Due to gauged porcelain tile panels being as large as 5 x 10 feet, it is not uncommon to see all of the grout joints filled with sealant on exterior installations to follow the movement joint requirements.
All tile installations require movement joints, but due increased moisture and temperature swings, it is critical that they are installed on exterior projects. Consult Tile Council of America (TCNA) method #EJ171 and ANSI A108.01 for the proper selection of sealant and the frequency of the movement joints. On exterior installations, the frequency is increased from 8 feet to 12 feet in each direction. More frequent joint placement may be required depending on materials and environmental conditions. For movement joints to work effectively, movement joint cavities must be open and free of grout, mortar and setting materials. Install movement joints per ASTM C1193 Standard Guide for Use of Joint Sealants. Tile edges to which the sealant will bond must be clean and dry. Some sealants may require tile edge priming. For proper use, always consult the sealant manufacturer’s specifications.
No two exterior tile installations are the same, so a clear understanding of the project conditions is necessary. If you are planning an exterior installation, it is important you consider protection of the installation. Make sure you are selecting quality mortars like those meeting ANSI A118.15 and verify your coverage by removing tile on occasion. Before grouting, be sure the installation is kept dry and rain is not expected later that same day. During installation and at a minimum for at least three days after completion, protect the completed installation from rain and freezing conditions. Keep excessive water out of the installation by verifying sealants are installed, flashing is done properly and roofing or metal caps are directing water away from the wall installation. If you keep these points in mind, select quality tile, waterproofing, mortars, grouts and sealants that work together as a complete system, you should have a beautiful exterior installation with great durability.