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Installing ceramic tile or stone outside is an effective way to transform any outdoor living space. When installed properly, outdoor tile offers great visual appeal with lasting durability. However, extra mindfulness must be applied to outdoor installations. Factors like site location, yearly weather cycles, tile choices and installation methods set exterior projects apart from interior ones. Let’s look at three areas of consideration that must be addressed for successful outdoor installations.

Surface Preparation

Concrete substrate requirements for interior application carry over to exterior applications with one vital variation: slope. Installers must still stay within the 1/4 inch in 10 feet maximum allowable variation in the substrate before installing tiles (and 1/8 inch in 10 feet for tiles with an edge longer than 15 inches). However, an outdoor substrate connected to or against a building requires adequate slope for proper drainage. Typically, 1/4 inch of slope away from the building is prescribed. There are no drainage requirements when installing over a concrete substrate that is open on all sides, but best practice would be to have slope in one direction to allow water to run off the surface.

These slope and flatness demands are easily met with high-quality levelers rated for exterior applications. Products like Merkrete’s Pro Patch Plus are quick-setting, polymer-modified Portland-cement-based skim coating compounds for finishing and smoothing interior and exterior subfloors. They can be applied up to 1/2–inch-thick in a single coat to effortlessly build up needed drainage slope and flatten substrates to withing the allowable variation.

If your installation is in an environment where it contends with extreme heat, direct sunlight, rain or intemperate cold, the amount of expansion and contraction at play is significant. It is essential to follow TCNA Handbook guideline EJ171 for movement joints to allow for substrate fluctuations the elements will cause. Expansion joints, saw cuts and perimeter soft joints are indispensable steps that will help prevent substrate failure for these outdoor environments.

You will want to take one more surface prep precaution to protect your exterior installation. Even the best laid slab with thorough movement joint accommodation can develop cracks over time, especially in demanding outdoor conditions. Crack-isolation membranes like Merkrete’s Fracture Guard work to prevent in-plane faults that can develop in the substrate from affecting the finished tile surface. Liquid-applied membranes deliver optimal strength and elongation that keep eventual substrate cracks from transferring through to damage tile and grout. These membranes act as an insurance policy that can save the considerable time and monetary costs needed to replace damaged tiles or, if unable to adequately match, replace the entire installation.

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With outdoor installations, extra caution must be exercised to ensure the product is designed to handle challenging outside conditions.

Mortar Selection

With a properly prepared substrate in place, you are about ready to begin laying tile. However, for outdoor installations, extra caution must be exercised to ensure your product is designed to handle challenging outdoor conditions. This is not a process to take lightly.

Mortars developed for outdoor use are highly modified to handle severe changes in environmental circumstances. You will also need to consider the size of the tile to be installed. For instance, a great number of outdoor installations involve tile with one side longer than 15 inches. These installations call for a large and heavy tile (LHT) mortar that meets ANSI A118.15 standards for bond strength. Products like Merkrete’s 855 DUSTLESS XXL provide exceptional support for large and heavy tiles in interior and exterior applications. Non-slip attributes keep tile from sliding down vertical applications, while non-slump properties help prevent tile lippage on horizontal applications. These outdoor-rated mortars are also, importantly, freeze-thaw stable to bear up against repeated temperature cycling. All these factors, combined with industry application best practices, will ensure a lasting bond.

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Ideally, a high-performance cement grout meeting ANSI A118.7 will be used for exterior applications due to its ability to maintain its strength and appearance against punishing exposure.


Like mortars, choosing the correct grout for outdoor applications can be tricky. In short, any standard Portland-cement-based grout can be used outdoors as long as it is polymer modified. Optimally, a high-performance cement grout meeting ANSI A118.7 will be used due to its ability to maintain its strength and appearance against punishing exposure. Grouts like Merkrete’s ProGrout are formulated to withstand extreme environments while resisting unsightly efflorescence and holding color when properly maintained.

When tiling outdoors, it is crucial to find a reliable manufacturer with a line of products rated for exterior use. Equally important is verifying that the manufacturer offers a thorough system warranty to back their products’ performance throughout the life of the installation. Knowing your job environment, adhering to industry specifications and carefully selecting high-quality exterior-rated products will greatly simplify your next outdoor tile project.