Ceramic and stone tiles are ideal surface coverings for exterior applications, despite the perception that installing these materials outdoors is risky. Roof decks and balconies are excellent opportunities for ceramic and stone tile installation, provided that proper installation systems are used.
One of the primary areas to be addressed in designing such applications is water management. Water management extends beyond simply waterproofing and plays a role in many aspects of the assembly’s design.
Unlike patio and walkway tile applications that are located on grade, roof decks and many balconies are located over "occupied space." The definition of occupied space varies depending on who you talk to, but in general it is meant to indicate interior space or other sensitive areas of the structure that must be protected with structural waterproofing. Structural waterproofing requires familiarity and experience with specialized products (e.g., roof membranes) and flashing details and brings liability outside the scope of the tile installation. Unless the tile setter is experienced and insured for this type of work, it is best for all involved to engage the services of a qualified roofing contractor to install a roof membrane. Regardless of who installs the roof membrane, it should be sloped to allow water to evacuate the system.
There are various types of roof membranes, such as built-up roofing using asphalt binders, modified bituminous systems, and single-ply EPDM or PVC sheets. Most roof membranes are not suitable for the direct application of thin-set ceramic and stone tile installations. First, it can be difficult or even impossible to achieve a bond to these membranes using thin-set mortar. Second, many roof membranes are relatively thick and compressible, which would leave a thin-set installation prone to vertical movement and damage under load. In such cases, it is necessary to provide a load-distribution layer, typically a wire reinforced mortar bed, over the roof membrane before setting tile.
The mortar bed is installed after placing a drainage layer over the roof membrane. The drainage layer has a higher permeability than the mortar bed, thus improving the water flow along the sloped roof membrane to the system exit. This helps prevent any buildup of water pressure on the membrane and reduces the potential for freeze/thaw damage to the mortar bed. Crushed stone has been used as a drainage layer, though various manufacturers offer lightweight drainage membranes that are configured to allow water to flow under gravitational force along the roof membrane. These drainage membranes also provide superior protection for the roof membrane.
The ceramic or stone tile covering is installed over the mortar bed using a sealed system or a water in/water out system. If the tile is gauged, meaning it features uniform thickness from tile to tile, it can be installed using the thin-set method. In this case, the tile setter needs to incorporate a bonded waterproof membrane meeting ANSI A118.10 directly below the tile. By creating this sealed system, moisture is not allowed to infiltrate the mortar bed, thus eliminating the potential for freeze/thaw damage or efflorescence due to a saturated mortar bed. Ideally, a bonded waterproofing membrane is chosen that also provides a forgiving shear interface to allow the tile layer and substrate to move independently. Incorporating such a membrane will protect the tile layer from damage due to differential movement stresses that would otherwise result from temperature gradients within the assembly.
If the tile to be installed is ungauged (e.g., some natural stones), meaning the thickness varies from tile to tile, it must be beaten into the mortar bed while it is still fresh to achieve a flat surface. In such a case, a bonded waterproof membrane cannot be incorporated and a water in/water out system must be employed. Thus, moisture is allowed to infiltrate the mortar bed, but can evacuate the system efficiently through the drainage layer.
Regardless of the installation method used, movement joints must be provided in the tile layer to accommodate the expansion and contraction associated with temperature fluctuations. For more information, please refer to the appropriate industry standard guidelines, i.e., the Tile Council of North America’s (TCNA) Handbook for Ceramic Tile Installation, and the Specification Guide 09300 Tile Installation Manual published by the Terrazzo, Tile and Marble Association of Canada (TTMAC).
The perimeter of the assembly is another area that warrants discussion with regard to water management. The importance of proper edge details is often overlooked, which can mean the difference between success and failure. For example, railing systems are a common source of leaks when not installed properly. It is advisable to choose under or side mounted railing systems when practical. If a top mounted railing system is the only option available, care must be taken to ensure that protrusions through the waterproofing layer(s) are properly sealed.
When the roof deck or balcony is sloped to the perimeter, water must be directed away from the structure using a drip edge of some sort. The drip edge must be placed such that it prevents moisture from wicking underneath the roof membrane, where it could otherwise damage the soffit or fascia. The drip edge should also be stilted from the structure to prevent the run off from leaving deposits on the walls that could stain the siding. For complete water management at the perimeter, a gutter system can be installed.
Ceramic and stone tiles offer a myriad of design options and represent the premium surface coverings for homeowners. When proper installation techniques are used, including the appropriate provisions for water management, these materials can provide beautiful and durable surfaces in various exterior applications. Thus, exterior applications represent a great opportunity for the tile industry.
The setting and hardening of unmodified thin-set mortars is directly related to hydration of the Portland cement in the mixture. Hydration is the chemical reaction by which water combines with the cement to produce new compounds, the most important of which is calcium silicate hydrate (CSH). The CSH forms a dense matrix around the other hydration products, remaining unhydrated cement, and sand particles, creating the final hardened bond coat.
Like most chemical reactions, the rate of hydration is dependent upon temperature. As the temperature of the materials decreases, so does the rate of hydration and resulting rate of strength gain. Thus, it is important that the tile setter be aware of the working temperature and provide adequate time for the mortar to cure. Further, the mortar should not be exposed to freezing at early ages, as it may not have the strength to resist the resulting moisture expansion and can be permanently damaged as a result.
In addition to Portland cement hydration, latex/polymer modified thin-set mortars must also be allowed to dry out to ensure that the latex/polymers coalesce as part of the curing process to gain strength and become stable. This is especially important in areas where the installation will be exposed to moisture.
According to the Tile Council of North America Handbook for Ceramic Tile Installation, such mortars must be allowed to "dry out thoroughly" before exposure to water. Drying times can vary with temperature, humidity, substrate, and tile type/format. For example, when porcelain tile is installed over a waterproof membrane, drying can only take place through the grout joints and longer curing times will be required.