Specifying the type and thickness of sheathing to be used for building construction is typically the responsibility of the building designer or architect. Sheathing’s principle purpose is to provide restraint from racking of the structure.

Sheathing that has been specified by the designer becomes of interest to the Masonry Stone Veneer installation. Indeed, masonry stone can be installed over a wood or metal stud wall even if no sheathing is required for building design purposes. However, the presence of sheathing provides significant benefit to the completed installation as well as the installation process. For example, sheathing provides a firm surface or back‐up when applying scratch‐coat or pressing stone into place. Once the stone is installed, the wall can withstand impact much better if sheathing is present.

Fortunately, buildings are infrequently designed with no sheathing requirement. The most frequent sheathings are OSB and plywood. Again, the choice and the thickness are in the hands of the designer. It is important to note that the sheets of sheathing should never be butted together when installing. Instead, a gap of 1/16” to 1/8” is preferred to accommodate the typical expansion wood products experience. Construction crews commonly use a 16‐penny nail as a gap device when installing wood sheathing.

The presence of a wood sheathing simplifies the installation of WRB since stapling to attach WRB can be performed directly into the sheathing. NOTE: It is important to note at this point that lath MUST NOT be fastened into sheathing… lath must be fastened into studs beneath the sheathing.

Other sheathings are also acceptable for use when installing Masonry Stone Veneer. Among these alternatives are cement board type of sheathings. The same installation guidelines apply… install WRB, lath and scratch coat as usual. For EXTERIOR applications, it is not acceptable in the view of MVMA to attach Masonry Stone Veneer directly to the cement board. Some cement board producers promote this direct bond approach for exterior applications, however, MVMA has not tested nor has MVMA received test results that prove such an installation is not subject to cracking at the cement board sheathing joints. There is concern that as a result of temperature fluctuations, the sheathing will shrink/expand that thereby created stress at the sheeting joints. This is not a concern when WRB, lath and scratch are employed since the stone is applied to the scratch coat which is not intimately attached to the sheathing directly.

Regarding direct attach of stone to cement board, the ONLY exception that is allotted is for INTERIOR applications. For interior installations involving cement board type sheathing, direct attachment of the stone is allowed. Since the wide temperature fluctuations will not be experienced with interior situations, direct attach to cement board is acceptable.

For those applications that include no sheathing, paper backed lath is heartily endorsed. Although installations of the stone for jobs involving sheathing can begin after 24 hours of scratch coat cure, it is advisable to allow the scratch to cure for at least 48 hours when no sheathing is present. The scratch must be stronger/stiffer to withstand the vertical forces exerted when the stones are installed. Other “sheathing” surfaces to be considered are insulation board, for example. The introduction of a foam insulation layer provides unique challenges but these are addressed depending on the thickness or foam and the type of design involved. Each of these potential scenarios can be handled, but it is best to discuss each design with your Eldorado Stone technical representative. Surely, there is no direct‐attach possible with a foam surface. In general, the type of fastener used to install that lath is affected based on the thickness of the foam insulation layer. Eldorado Stone technical support can direct you to the best information for fastener selection. When dealing with foam layers, there are opportunities for WRB changes, as well.

We updated our Technical Data Sheet with new test results.  Eldorado Stone is designed to meet or exceed building code requirements. Independent testing confirms compliance with ICC-ES Acceptance Criteria 51 for Precast Stone Veneer.  

See our technical data sheet for more information.

By Rick Garagliano

When installing Adhered Concrete Masonry Veneer, the issue of Expansion Joints and Control Joints must be taken into consideration.

First, we must understand the purpose of a joint and difference between an Expansion Joint and a Control Joint. In either case, the purpose of the Joint is to accommodate relative movement of the large, homogenous areas. Expansion and Control Joints provide for the unavoidable movement in a wall that will occur. These designed splits provide a location for the inevitable cracks to form such that they will be less noticeable. After all, cementitious materials are subject to dimensional change caused by thermal changes. In addition to thermal changes, shifting and settling effects can cause movement in the vertical surfaces/walls.

An Expansion Joint is a split or joint that is designed into the structure and extends through the entire width of the wall. Great examples are the joints that form when two tilt-up panels are butted together. In this case, the joint extends completely through the wall. A Control Joint is a separation that is designed into the outer layer only of the wall such as the façade layer. Control Joints are required by code for stucco coatings so that the location of the inevitable cracks is controlled, hence the name “Control Joints”. The code and the designer say “crack here and not anywhere”. Since much of the code requirements for Adhered Concrete Masonry Veneer (ACMV) were initially based on codes for stucco, the Control Joint issue would seem to apply to ACMV. However, recently, a study was commissioned that investigated the need for Control Joints when installing ACMV. The study and report (published on the MVMA website-masonryveneer.org) maintains that when it comes to thermally induced movement, stucco and ACMV do not behave alike. Stucco is a monolithic, vertical panel whereas an ACMV wall is a vertical array of individual stones. Whether grouted or not, the stone wall contains a control joint between every stone. If thermal or mechanical movement is induced, the stone won’t crack but the grout joint of dry joint would “absorb” the movement. Of course, stucco will show the crack if movement occurs. In short, Control Joints are NOT needed for ACMV.

Although Control Joints are not needed for ACMV, Expansion Joints are designed into the job by architects and are needed for the building, not the facade selection. Do not install stone across Expansion joints such that they bridge the Expansion Joint (even if the joint is filled flush to the surface). Eventually, there will be movement at this joint (as designed) and any stones which bridge the joint will crack. The best solution is to terminate stone at each site of the joint, thereby leaving the joint open and uncovered. Sometimes, the stone is installed across the joint and a saw is used to cut the stone on each side of the joint. This method provides a straight, clean cut on each side of the joint. Otherwise, the stones are cut and installed individually along each side of the joint.

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.