There are considerations that must be addressed when installing MSV (masonry stone veneer) in a dry stack mode. Dry Stack installations are subject to de-bonding problems when the installer does not fully understand his actions and decisions when installing. Grouted installs do not experience de-bond failures because of three main reasons:

1. The installer is not motivated to adopt a shortcut process which leads to failure in two modes.
2. The grout serves as an additional bonding area which also tends to lock the stone in place.
3. The grout serves as a secondary barrier to water infiltration to the stone back.

When installing any MSV in a Dry Stack design (non-grouted), it is important that the mortar be “loose” enough or flowable such that the mortar paste can flow into the fine pores and capillaries of the stone and scratch coat. It is the flow of the mortar into the microscopic pores and capillaries that creates that immediate “stick” of the stone to the wall. If the mortar is too dry, that bond won’t develop because the mortar can’t flow into these pores.

Secondly, the stone must be fully back-buttered.

Thirdly, the stone must be firmly pressed into place. When all three of these proper activities are followed, mortar will flow out and beyond the perimeter of the stone. On a dry stack install, the installer is then compelled to remove this SQUEEZE-OUT before setting the adjacent stone. Typically, a quick swipe with the trowel tip on each half or on all sides of the stone does the trick. This takes time, however.

Consequently, installers try to eliminate SQUEEZE-OUT. They will:

1. Prepare a dry mortar which doesn’t flow well
2. Fail to fully back-butter the stone, often leaving a non-mortared path around the stone perimeter.
3. Fail to press the stone in place and create mortar flow.
4. All of the above.

The result will be a mortar that is NOT FULLY HYDRATED and which does not make intimate contact with the stone back. When summer conditions prevail, a hot, dry stone will exacerbate this problem since water in the mortar will be immediately pulled to the stone.

When mortar does not fully coat the entire stone perimeter or if there is a mortar gap in the middle of the back of the stone rainwater can collect and reside in these spaces. If the water is then exposed to freezing temperatures, the expansive forces can cause the stone to de-bond. We cannot leave gaps in the mortar such that water can get between the stone and the substrate! Some installers talk about leaving a void in the middle of the stone to create “suction”—this is a poor practice. If anything, this void creates an air bubble or pressure, not suction. What does happen is that a void is left that is not acceptable.

Type S mortar is more cement-rich than Type N and should be encouraged (but not required) when performing dry stack. With a higher ratio of cement, a rich paste will develop with the proper addition of moisture. Again, this rich paste will flow into the pores and capillaries to create a great bond. If the cement is reduced, as in Type N, the process bonding will still work but the “window” of success is reduced. Type N is approved by MVMA for dry stack applications, but it is our recommendation to improve the opportunity for success by opening the “window” by using Type S. Type S is a bit more costly (less sand, more cement), but if the geographic area experiences high incidences of freeze thaw cycling or if the installation is occurring on a hot, breezy day, Type S might be the cheapest solution in the long run.

Of course, polymer-modified mortars are the BEST bet. These proprietary mortar mixes have agents to absorb and hold the moisture such that drying-out is less of a concern. There are other additives that provide smoothness and flowability. Despite the cost, many masons like these polymer modified products because they tend apply easier and require less work to mix—just add water. Polymer modified mortars provided very high bond strengths – up to 3 times standard mortars.