routing outdoor tile on a concrete patio slab is a lot like grouting interior tile, with a few key differences. First, the grout must be rated for outdoor applications. Second, some of the grout lines should be filled with caulk rather than grout, to create expansion joints that allow the tile to expand and contract with temperature changes. And third, outdoor grout must be applied and fully cured in dry weather within the grout’s specified temperature range. If rain is expected, you may need to tent the area to keep the grout dry while it cures (sometimes for several days).

There are two basic types of grout that are suitable for outdoor use and are relatively DIY-friendly: standard cement-based grout and pre-mixed grout. Again, both types must be rated for outdoor use.

Cement-based grout is a dry powder that you mix with water (or a liquid additive) prior to application. Most outdoor applications call for a sanded grout, which is used for grout joints 1/8 inch or wider (for smaller joints, you can use unsanded grout). Cement-based grout used outdoors must be rated for “no efflorescence” to ensure that the grout won’t effloresce or lose moisture and develop a chalky white residue. Cement-based grouts also should be sealed periodically for weather- and stain-resistance.

Pre-mixed grout is made with a polymer, such as acrylic, and does not contain cement. It is sold in tubs and is simply applied right out of the container without mixing. Pre-mixed grouts tend to be more flexible than cement grouts, which may help resist cracking in adverse conditions. But perhaps the best advantage of pre-mixed grout is that it does not need to be sealed.

Plan your grouting project for good weather, and be prepared to cover any exposed areas, if necessary. Both cement-based and pre-mixed grouts typically must be applied when the air and tile surface temperatures are at least 50 degrees and no more than 90 or 100 degrees Fahrenheit. Both also need about 72 hours for the initial cure and may need up to seven days to cure fully. During this time, the tile area must be fully protected from the rain. Rain protection must allow for adequate airflow to ensure proper curing. Grout sealer also must remain dry as it cures.

Working Time: 2 hours

Total Time: Up to 8 days; more if sealing grout

Material Cost: Between $60 and $80

Equipment/Tools

• Utility knife
• Cleaning sponge
• Bucket
• Caulking gun
• Electric drill and mixing paddle (cement-based grout only)
• Grout float
• Grout sponge
• Microfiber cloth
• Sponge paintbrush

Materials

• Grout
• Commercial silicone caulk or other sealant
• Grout sealer (as needed)

Seal the grout with an outdoor-rated grout sealer, using a sponge paintbrush or other applicator. You do not need to seal most pre-mixed grouts, but this may be an option. Let the grout sealer cure fully, as directed.

One of the main challenges beginners face when grouting is the working time of the grout, or the amount of time you have before the grout starts to harden. If the weather is over 80 degrees F and/or there is direct sun hitting the tile, the grout may set up more quickly than the product’s standard working time. For this reason, it’s best to grout when the sun is not hitting the tile. If the weather is hot, start grouting in a very small area to get a feel for the actual working time before grouting over larger areas.