High-performance epoxy adhesives applied by the “spot bonding” or “dab” method are used on approximately 10% of the veneer surface, 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. The two main advantages of high-strength epoxy adhesives over the traditional method of mechanical anchoring are described below.

1. Speed

The spot bond method, incorporating high-strength epoxies, is up to five times faster than mechanical fastening. Traditionally, a three-man crew installing a 2- x 2-foot granite veneer panel would be expected to install a maximum of 150 square feet per day. Using the spot bond method, a smaller crew of two people can easily and realistically install 350 square feet per day. This number will dramatically increase to over 500 square feet per day via the use of a cordless mixer that dispenses the adhesive at an even faster rate, thus eliminating the need for hand mixing. In addition to the time savings gained during the installation itself, the set-up and clean-up time take only a couple of minutes; therefore saving the contractor even more time.

2. Ease of installation

Adhering stone with mechanical anchors is not an easy process. There are a myriad of sizes, shapes and techniques used to adhere stone to the substrate, along with the need for many other supporting products used in conjunction with the anchor. From drilling holes into the substrate, setting pins or bolts, assembling anchors to even having to cut kerfs in the stone, it's easy to understand how an average three-man crew can only set 8 to 12 stones per day. Often, the installation of just one stone requires a few “dry sets” and a lot of lifting until the proper position is obtained. In addition, breakage of stone is common, due to drilling actually making the structure weaker, and can add cost as well as slow down the installation.

The spot bond method via high-strength epoxy adhesives is not only easier with which to work, but it also provides jobsite advantages. First, the contractor only needs to scarify the areas on the back of the stone to accept the bonding adhesive, which takes literally only seconds to prep. Then, the adhesive can be applied either by hand mixing or via a cordless mixing gun to those prepped areas.

The veneer can then be installed over the substrate; this is where the real benefit is noticed. Many substrates are not perfectly smooth or plumb and, in addition, stone thickness varies quite a bit. Both of these situations are easily adjusted for with the use of the high-strength epoxy adhesive. With a non-sag epoxy on the stone, it is then placed against the substrate and then simply adjusted until plumb. The contractor can pull out or push in one side or corner until the desired position is achieved. The non-sag performance and high strength of the adhesive allows the veneer to stay in place until fully set in just about 30 minutes.

Application factors

When comparing the two systems in terms of installation speed and ease, a high-strength epoxy adhesive has considerable advantages that translate directly into cost savings to the contractor, architect and customer. However, the use of high-strength epoxy adhesives must meet all building code requirements and norms. It is important to use only those products with a track record of performance and those that are approved by the building community.

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

• Thermal Movement - The entire system must be able to accommodate the expansion/contraction occurring over wide temperature shifts. An example of rapid change 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, there are also routine stresses placed on the cladding system - such as wind loads, building settlement, moisture shrinkage/expansion and “creep.”

• 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.

In addition to the points above, the substrate's condition is just as important. 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; therefore causing 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.

Similarly, the natural stone must be appropriate for the intended application. For example, certain types of fissile stone (such as slate) can delaminate within their 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.

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 spot bond or chemical anchor method of installing stone is a recognized and proven method that has been painstakingly developed to perform under all the conditions described above. Certainly, building code compliance is an important consideration as well. Look for only those installation products that are approved by the various code bodies as acceptable installation methods for exterior or interior applications. It is imperative that the building owner and the specifier check with all manufacturers involved and get as much information as possible.