First and foremost in our quest for longevity is the proper selection of products. We in the industry are fond of saying all tile installations are part of a system that is only as strong as the weakest link. This cannot be truer when selecting components for an exterior deck assembly. The component we must build the system around is first and foremost the tile product to be installed. It would be a very accurate assessment to say not all tile and stone products lend themselves to exterior installations. The biggest consideration in freeze/thaw applications is the porosity of the product in both the body and the surface. Many wrongly assume that all porcelain tile would be a suitable product because the body has a very low absorption rate. While this is true, the surface porosity is also a consideration. The test used to establish the porosity of a tile body uses absorption of moisture by the tile, and is known as the American Society for Testing of Materials (ASTM) C-373 or International Standards Organization (ISO) 10545-3. That test does not examine the role of surface porosity, if any, in a freeze/thaw situation. Suitability of the tile body and surface can be established by a separate freeze/thaw test, known as ASTM C-1026 or ISO-10545-12.
Another often-heard but inaccurate statement is that only porcelain tile may be used for an outside environment. Many tiles have been used in exterior applications long before the current popularity of porcelain floor tile. The industry-accepted recommendation is that tile with a porosity of greater than 5% should never be used in an exterior floor application; 3% or less is preferred. This would include most quarry tiles, many mosaic tiles and numerous other popular floor tiles besides porcelain. The more moisture and temperature cycles the floor is exposed to, the more appropriate a tile with lower porosity becomes. You must be very careful when selecting stone. Even stone products within their own generic family can perform differently depending on where they were quarried. A word to the wise: do not assume anything when it comes to stone applications.
Waterproofing is the next area of concern. This is where my experience tells me things go bad most often. Many tile-related product manufacturers tend to be a little gun shy about their waterproofing products being used in exterior applications. History provides many good reasons for their aversion. Most of us would agree that next to decks, showers are the most problematic installation. The reasons are the same: poor selection and application of waterproofing products. Exterior patios and decks, in particular over living spaces, are not the place to experiment with your personal hybrid waterproofing system. That has been the cause of most failures I have seen this year: a component or selection of one tile system borrowed and forced to work with another system. Not all common roofing membranes are accepting of mortar beds. Even less likely are roofing membranes that will accommodate direct bonding of ceramic tile or stone. Then there is the occasional double membrane scenario where a tile membrane is bonded to a roofing membrane to facilitate direct bond application. That often creates a system too compressible for any grout, stone, and most tile. Not all tile-related membranes are rated for exterior submerged applications, which would include any exterior deck or patio. The combinations of membrane systems are seemingly endless when it comes to this type of application. My recommendation is to thoroughly research the system you or the person specifying it has selected. Unless it comes with written instructions and a warranty, move on to another system. Once selected, follow ALL the instructions. Under most codes, while prudent, a water test is not required for enclosed exterior decks in applications such as balconies. There is a move afoot, which I support, to require flood testing of horizontal waterproof installations. There is a recommended testing procedure under ASTM D-5957 that while cumbersome in its current format, would work for tile installations.
Something that is also common in both slab-on-grade and mortar applications of exterior tile work is lack of drainage. We know we should pre-pitch a shower membrane below a mortar bed. Decks and patios are no different. Whether the water is shed at the surface or below it, proper drainage is critical to avoiding damage from freeze/thaw, moisture expansion, and efflorescence. A ¼” per foot or 2% slope is really the minimum that should be considered for any exterior waterproofing application exposed to the elements. When using exterior wire reinforced mortar beds, a drainage mat will also aid the reduction in moisture retention dramatically. Tile and cement-based products perform very well in outside applications as long as the moisture retention remains low. Anything that assists in reducing the amount of water held in the installation will aid both its aesthetics, by reducing the formation of efflorescence, and its longevity, by reducing moisture-induced expansion. It should go without saying that the proper drain assembly is required to allow for drainage in mortar bed applications. Evidently that need is not as apparent as one would think. In several recently reviewed projects in different areas of the country, both exterior mortar bed decks, one in a freeze/thaw area, the other not, the decks were serving as reservoirs with surface-installed drains. Several more had the proper drain assembly but lacked slope under the membrane and had plugged weep holes creating the same effect. When using waterproofing, drainage of the waterproofed surface is an important consideration in any application.
The selection of setting materials and grout for exterior applications is not as simple as it may seem. There is the obvious, that the thinset should be rated for a wet application. However, not so obvious is the definition of wet application. A vertically tiled surface is very different than a horizontal surface when it comes to waterproofing and thinset. We can generally be assured that a vertical surface is going to drain unless it is the wall of a swimming pool or fountain. Floor areas on the other hand function as submerged surfaces. Once they become completely saturated a different set of issues arise. Submersion creates a very different performance requirement for thinset as opposed to an installation where the surface simply sheds water. Not all polymer modified thinsets are suitable for submersion. A few contain polymers that will re-emulsify and others that will re-emulsify only when exposed to moisture for prolonged periods. Any manufacturer’s technical department will tell you that polymer or latex modified thinsets should be protected from exposure to the elements until they cure. This includes not only rain but direct exposure to sunlight. Premature exposure to rain will affect the performance of any thinset and possibly render any latex or polymer thinset useless. Heat on the other hand causes rapid hydration that will negatively impact the bonding abilities of the cement. There are many who may want to argue but my position on exterior applications, supported by scientific fact, is that use of a liquid latex will always offer superior performance to a dry polymer thinset, all other things being equal.
One thing is certain. People love tile decks, patios, and balconies. However, those types of projects require a very critical installation process where no shortcuts are acceptable without having a negative effect on the tile installation. Exterior applications of ceramic tile require experienced tile installers and quality products. Many otherwise good installers and some materials are simply not up to the task. Proper product selection and application are going to require more information than you will receive from your typical sales representative. Study all the components thoroughly and choose wisely before you venture outdoors.