Outdoor work presents special challenges. It is easy to create award-winning spaces but only attention to installation details required for long-term performance will keep them there year after year. Photo courtesy of Phil Kean Designs.


Freeze thaw climates pose special concerns for exterior tile work. This slab next to its much larger counterpart has raised 3/4” due to frost. With spring it will once again be perfectly flush. Movement is a fact of life in all tile installations, even more so when they are exterior.

Sounds a little like a popular fairy tale or perhaps a significant other, but for this article we are talking about environmental conditions. Temperature and humidity play an important role in each and every installation. Unfortunately the value is rarely apparent and often goes unnoticed unless problems occur at a later date; then everyone scrambles to find a place to point the finger. Bad thinset, bad membrane, not enough nails in the underlayment or any one of innumerable potential reasons including, and I actually heard this once, a full moon! Yes, a full moon was alleged to have changed the gravitational orientation of the sand causing an imbalance that resulted in the sand rising to the top of the grout joint, leaving the cement at the bottom. So next time you get a grout complaint, check the lunar cycle and who knows, maybe you will have a new excuse or perhaps hear a new excuse. It is a well know fact among technical minds that both temperature and humidity or moisture levels can and do have wide ranging affects on both building products and setting materials. Installers on the other hand tend not to give much thought to those effects as we must play with the deck of cards we are dealt often under varying circumstances.

I would venture to say most of you feel as I always have, that there is no such thing as a job at 70 degrees and 50% relative humidity, which are the typical published levels given for things such as drying time. It is assumed by the technical minds that develop products the average installer understands this and compensates accordingly. Why 70 degrees and 50% relative humidity? To establish basic performance levels for testing purposes. Under the American Society of Testing of Materials (ASTM) there are very specific parameters established for materials and methods used in testing to verify performance properties. This allows you to effectively shop for comparable products knowing they have all been tested under the same temperature and humidity. The published parameter under standards is actually 70-77degrees F and 45-55% RH, or relative humidity. Like products tested at higher or lower levels of temperature and humidity would likely have different values. At extreme variations they would be greatly different.

So just what happens when an installation falls outside the range as above? With cement, which is only one ingredient of many in cement based setting materials, the answer is relatively simple. For every 18 degree temperature change below 70degrees F there is a 100% decrease in the minimum cure time. For every 18 degrees above there is also a 100 % increase. This is not just air temperature; this is both ambient and surface temperature. In general, cement based products should not be used when the temperature is lower than 50 degrees or higher than 95 degrees. That eliminates tile work quite a bit of the work year doesn’t it? That is where specialty products come into play. Some manufacturers make “hot mix” products to be used in higher temperature ranges. There are also thinsets that may be used in lower temperature ranges. I have heard the comment numerous times that they continue to pour concrete through-out the winter so there is no reason why you should not be able to thinset tile. The problem with that logic is the cement in concrete has a very high profile thickness, in road construction it can be from 6 to 12 inches. During the hydration process when the cement grows its crystalline structure, heat is created. With very little additional assistance this heat is able to keep the mix warm enough to properly hydrate both using and evaporating water as it develops strength. With standard thinset mortars we have a very thin profile or thickness of cement. It can be as little as 3/32 or with specialty mortars as thick as ?”. All thinset mortars, polymer/latex modified or not have additional chemicals which allow them to be used for direct bonding. It is the combination of the thin profile thickness and accompanying lack of heat it created by the addition of these other chemicals that prevent thinset from achieving bond in cold applications. At the opposite extreme, hot conditions, no cement-based product can develop good bonding ability unless moisture is present. Heat in both the substrate and ambient air will cause a rapid loss of water preventing the hydration needed for the bonding process. Anytime cement gets hot and/or dry it will rapidly stop the hydration process.

Probably the least-thought-of environmental condition is traffic or in-service use of an installation. The recent tile work at Hartsfield Airport poses numerous challenges. All work must be done at night and open to traffic the following morning. With proper product selection, there should be no problem.

While basic cement is unaffected (and quite possibly enhanced) by high moisture vapor from the substrate or air, some modified cement products may have extended curing times in damp or wet conditions. Additionally, there are modified products can be very sensitive to extended periods of high moisture. While there is no standard moisture vapor emission (MVE) limits for thinset mortars, some specialty products may be sensitive to excessive MVE. Under industry standards it is actually recommended that you moisten all dry surfaces prior to application of cement-based products. This is a quite different scenario than a sustained moisture environment. The purpose of damping the surface is to allow for maximum development of strength. This temporary moisture is quite different than sustained moisture such as that present in a slab with high MVR. There are a few additional common sense precautions relative to moisture and cement. In most areas of the country unacclimated slabs reach dewpoint on occasion such as changing of the seasons. When a slab reaches dewpoint, thinset mortars will not bond as the amount of water on the slab is greater than the microscopic reach and bonding abilities of cement. This is also the reason that excessive water used in the mixing process results in weak thinset or grout. When the water evaporates there is nothing left but air. This results in large voids, which make the cement product appear weak and powdery. The denser the mix, the greater the strength with cement products. Warm surfaces often cause rapid loss of moisture which effectively stops the hydration process required to gain strength which also appears powdery.

The George A. Purefoy Municipal Center chose natural unglazed materials and used premium cement based setting materials to ensure both long life and easy maintenance.

To prevent rapid cement hydration the ideally the installation of tile should never be done under direct sun light in exterior applications during warm periods. Some type of temporary shelter should be provided and the installation should be covered during the initial curing process. Plastic is not a good choice for temporary cover as rivulets of water collect on the underside and when they drop on the tile or grout surface they can cause staining that is difficult if not impossible to remove. Breathable non-staining building paper is a better choice of protection and very inexpensive. If you are covering a grouted installation you must be sure to completely cover the installation to prevent shading caused by the differing evaporation rates. Anyone who has had an anxious homeowner put a flower pot or area rug back in its position after a floor was freshly grouted can probably attest to the dramatic difference in colored grouts that covering or damp curing the surface makes. During the warm summer months, fans are very nice to have around. Unfortunately, tilesetters should not own fans as they also can cause premature drying, thinset skin-over and shading during the grouting process.

Cement itself is a simple and versatile product. Modern day chemistry used with cement bonding materials is increasingly complex. Even the most skilled and dedicated professionals are challenged to keep up with the latest developments and chemical enhancements. We now have thinsets for submerged applications, intermittent moisture applications and dry areas only. This is all quite different than formulations of years past. Basic cement is only one part of the chemical equation in thinset mortars. With the enhanced products needed and in use today there must be considerations well beyond the basic requirements of cement. The plethora of bonding products available today range from hot weather to underwater thinsets and all manner of epoxies. If you have challenging environmental conditions hot or cold, wet or dry, consult with your favorite manufacturer and he just may have a product that will make life simple, quick, and easy. Given the challenges every installer faces daily, I will take all three anytime I can get them.