The same type of performance can be achieved with a thinset mortar over a poured slab. Most of the considerations to be made remain the same with exception to the setting materials. Let’s look at the secrets of success from my perspective, which closely mirrors that of industry recommendations.
Exterior tile needs to shed water as rapidly as possible. While there is no official “rule” on how much slope is required, the ¼” in 10’ used for showers is widely accepted as the bare minimum. This slope needs to be in the slab and should not be made of fill material such as mortar, which is much more porous than concrete and will actually hold the water rather than allowing it to drain. If the slab area must be properly sloped with a fill material, the use of an appropriate properly installed waterproof membrane should be considered mandatory to avoid moisture retention in the fill material. If the area to be tiled abuts a structure, that membrane must be appropriately flashed into the structure and a proper movement joint constructed at the wall line. Not all membranes are suitable for this type of application so choose carefully. In general, avoid using waterproofing in exterior applications unless there is a specific reason to do so. They can cause nearly as many problems as they solve, particularly when it comes to developing efflorensce issues. When the opportunity presents itself, use of a drainage mat under a mortar bed can be very helpful in promoting rapid drainage of the floor system, greatly reducing efflorescence issues; but remember, the water has to drain to someplace to be effective.
Selection and Application of Setting Materials
Not all thinsets are suitable for prolonged exposure to moisture. In exterior applications it is wise to spend a few extra dollars to insure you are using a good premium bonding material suitable for the application. When using a polymer or latex modified product you must also make sure there are provisions to protect the newly set tile from direct exposure to the sun, which can cause rapid drying and thus low bond strengths. Protection must also be provided to insure the polymers have opportunity to coalesce or dry. This is not the typical tile setter’s 12-hour overnight cure; we are talking days, not hours for many thinsets. Recommendations will vary according to formulations but three to seven days protection should be considered a minimum time frame. If you don’t have the time to wait consider using a rapid-set type material, but even with rapid-set, a simple overnight cure may not prove adequate; check with your manufacturer.
Thinset coverage is everything when it comes to exterior tile work. I could fill page after page with photographs of what happens when coverage is not near 100 percent. The industry recommendation is 95 percent for exterior and wet areas. Failure to achieve good coverage is often responsible for the efflorescence that seems to never go away. Moisture left in voids (i.e. trowel ridges) under the tile will dry out to a gaseous form and pass through the grout joints. As soon as that alkaline laden moisture hits the air, the salts will dry leaving their crystalline deposits behind. Exterior tile work is all about good thinset and coverage. No matter what your troweling technique, you’re likely going to have to flat trowel the backs of the tile if it is of any appreciable size or has one of those moon crater backs. I am a cement grout fan on all exterior applications; let it breathe. A good grout job, one that is nice and dense, full to the top of the tile or edge of the bevel, sheds water much better than one that is low and soft. Elsewhere in this article is a picture of my front porch in which neither the grout nor the sealant has been touched in 19 years despite use of ice melting chemicals or being under ice 4 to 5 months a year.
With sun exposure there are certain to be numerous cycles of pushing (as the tile expands) and pulling (retracting as it cools down). This will likely be over damp thinset which means the bonding strength is less than it would be when dry, as typical in all cement products. With all installations, properly placed and constructed movement joints are critical to long-term performance of exterior or any other type of tile work. I have been trying to drill that point in everywhere I go for probably 15 or more years now. I did not always feel that way. As a matter of fact, I paid them little heed my first 15 to 20 years in the trade. It was only after architects beat on me and a few of my floors blew up that I ended up buying that I really started paying attention to the movement needs of the tile installation.
Answering technical calls for TCNA and CTEF for 10 years, where nobody calls for payment of a claim (hence they don’t lie as much) has positively demonstrated their importance to me. In my current role as a consultant, I run lab tests all the time on thermal expansion and occasionally on moisture expansion. I have never tested anything that did not move when exposed to heat and/or moisture. I doubt that in all but the rarest of instances, exterior tile work dries out completely, and even then, it is only temporary. Moisture growth in tile is a long-term, one-way growth; it is bound to get bigger. We know that when tile gets copious amounts of sun and thus becomes hot, it grows. During the evenings or periods of non-sun exposure, it contracts. Hence the reasons for more frequent joint spacing in exterior tile work. Over the years the recommendations contained in the Tile Council of America Installation Handbook under EJ 171 have greatly expanded and I feel are very clear. For additional reading, the American Society for Testing of Materials publishes ASTM C-1193 Standard Guide for Use of Joint Sealants and ASTM C-1472 Standard Guide for Calculating Movement and Other Effects When Establishing Sealant Joint Width. Lot of boring reading to be sure but, you will be the smartest guy around on the need and design of movement joints if you read them a few times.