Are there deflection limits for the structural backup for portland cement plaster?
A structural frame must be strong enough to resist bending so that the finish attached to it is not subjected to more flexural stress than it can handle. This helps maintain structural integrity and control performance of the finish. For plaster, deflection limits and proper sizing of panels are helpful in restraining unwanted cracking. An in-plane load that acts on buildings and wall panels is the force of gravity (acting on the building’s weight). Out-of-plane loads include wind and seismic loads. All of these forces can make panels bend, or deflect. Concrete and masonry substrates are very stiff, so when plaster is directly applied to either of these surfaces, deflection criteria of the backup is assumed to be within allowable limits. Frame systems, such as wood or steel framing, are less rigid. They will deflect due to loads, so limitations are needed to maintain function and appearance of the plaster panels. In ASTM C926, in Annex A2 on design considerations, it states: “Maximum a
