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.