3. Suitability of the stone for the intended application

4. Performance of the adhesive bonding system

1. Exterior applications are certainly the most critical as they involve numerous compatibility factors. These include:

Thermal Movement -- The entire system must be able to accommodate the expansion/contraction occurring over wide temperature shifts. An example of rapid changes in temperature is a hot sunny day hit by a sudden rainstorm that affects the cladding. A temperature swing of over 80 degrees Fahrenheit can happen in a matter of seconds, putting tremendous stress on the system.

Shock and Vibration Resistance -- All buildings move. While an earthquake provides a dramatic example of building movement, wind loads, building settlement, moisture shrinkage/expansion, and creep all are more routine stresses placed on the cladding system.

Freeze/Thaw Durability -- In cold climates, the ability to withstand freeze/thaw cycles is important. Water expands approximately 9% of its volume when it freezes, so the building materials must be able to withstand this stress.

2. The substrate must be clean and sound for a direct adhered system. Clean means that all bond breakers or contaminants must be removed. This includes dirt, dust, curing compounds, oils and sealers -- all of which are best removed by mechanical scarification. "Sound" refers to the ability of the substrate to support the weight of the veneer and to perform for the job conditions specified. For example, gypsum wallboard is a common substrate for tiles for interior, dry area applications. However, that same gypsum wallboard should never be used for exteriors because moisture would deteriorate the board and cause a failure. When specifying a manufactured substrate, it is most important to check with the manufacturer as to the suitability of their product for the intended purpose.

3. Similarly, the natural stone must be appropriate for the intended app-lication. For example, certain types of fissile stone (such as slate) can delaminate within its layered structure due to moisture/freezing expansion. For exteriors especially, it is critical to address the absorption rate, thermal movement compatibility with the adhesive, breaking strength, dimensional stability and frost resistance.

4. The direct adhered or adhesive method of installation provides advantages over the use of mechanical anchors. Perhaps the most significant is economy. The direct adhered method allows for the use of thinner, lightweight modules, which reduces cost and allows for greater productivity. In addition, there is less waste, since stones do not require drilling or cutting in the field, a process that often results in breakage.

The most widely used and successful direct adhered mortar system uses a liquid latex additive mixed with a Portland cement based filler powder. The resulting mixture, called a thin bed mortar, is applied to the substrate and to the back of the stone to ensure 100% coverage. This latex modified mortar (performance standard ANSI A118.4) meets the compatibility requirements for exterior applications noted in Section 1 above. Note that the shear bond strength of these latex modified mortars can achieve 400-500 psi (3.5 Mpa), more than sufficient to hold thick, heavy stone.

Another system in the direct adhered category utilizes a high-performance epoxy adhesive applied by the "spot bonding" or "dab" method. Here, the epoxy adhesive is used on approximately 10% of the surface area, thus providing a ventilation gap behind the stone. This is an important advantage because it will reduce the potential for water staining and efflorescence. These specialized epoxies are strong (shear bond strengths in excess of 1,100 psi), and yet flexible enough to handle building movement and stress. For certain applications, this system has been used in conjunction with mechanical anchors to provide a "belts and braces" approach to successful wall cladding.

In summary, the direct adhered method of installing stone is a most reliable, proven method. In the space of this short article, please do not conclude that I have addressed all the issues that are involved with a successful installation. Certainly, building code compliance is an important con-sideration as well. A most important point to the building owner and specifier is to check with all manufacturers involved and get as much information as you can. Lean on the manufacturers with a successful track record with installations, and use the best system possible for your application. The consequences of improper materials or poor workmanship are too severe to cut corners.