DIRECT APPLIED OVER CEMENT BOARD
Cement board, such as Durock, is a tough and highly weatherproof material. As such, it is an acceptable substrate for a Direct Applied application of EIFS lamina.
Point 1. Reinforcing Mesh.
Although it is acceptable to only tape the cement board joints, full meshing is recommended because the mesh gauges the Base Coat thickness and creates a more even application of Base Coat. This produces a more uniform appearance and depth in lamina. If meshing only the cement board joints is elected, we recommend the use of 9-1/2" wide 4-ounce standard, polymer coated fiberglass reinforcing mesh. If full meshing is elected, we recommend 38" wide rolls of 4-ounce standard, polymer-coated fiberglass reinforcing mesh. Runs of reinforcing mesh should be lapped at least 2.5".
Point 2. Control joints, isolation joints, and expansion joints in the cement board.
When attaching the cement board, the control joints should be spaced in accordance with standard lath and plastering practicing, i.e., every 150 sq. ft., and be approximately 1/2" wide, or as noted in the job drawings. As a general guide, place vertical control joints at least every 16' and horizontally at floor lines. Control joints should be trimmed with UV rated PVC accessories, such as a V-joint from Plastic Components. Filing the plastic V-joint with sealant after the installation is optional. Isolation joints function as gaskets around penetrations such as windows and doors. The cement board should be trimmed with a UV rated PVC trim, such as a J-stop, and the 1/2" opening between a clad window and the PVC trim should be filled with a non-absorbing backer rod and low modules sealant (and primer as required).
Expansion joints will likely be through-wall joints designed into the structure to compensate for large wall stresses due to movement. These joints will be a minimum of 3/4" wide and the cement board will be terminated with a PVC stop trim at each side of the joint. The joint will be filled with a backer rod and sealant as described above.
Point 3. Sealant recommendations.
The backer rod should be a closed cell non-absorbing type. Total Wall recommends Pecora, Sika, Sonneborn, Dow and Tremco as primary approved sealant manufacturers. Total Wall further recommends use of either a 2-part urethane such as Dynatroll II by Pecora, Dymeric by Tremco, or single component ultra-low modules silicone such as Dow 790 series silicones or Pecora 890 silicone. As a minimum requirement, the sealant must meet ASTM C920, Type M and S, Grade NS, Class 25 specifications. Total Wall feels that the single component ultra-low modules silicone sealant and primer as recommended by either Dow or Pecora may have some application and performance advantages. For a list of approved sealants, please refer to the Total Wall literature.