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IBC Changes Regarding Exterior Cement Plaster

Q: Can you explain the recent changes in the International Building Code regarding exterior cement plaster?

A: The 2021 version of the IBC has significant changes relating to the application of cement plaster on the exterior of buildings. The wording in Chapter 25, “Gypsum Board, Gypsum Panel Products and Plaster,” section 2510.6.2, says that in moist or marine climate zones, a water-resistive barrier shall comply with one of the following:


“1. In addition to complying with Item 1 or 2 of Section 2510.6.1, a space or drainage material not less than 3/16 inch in depth shall be applied to the exterior side of the water resistive barrier.


“2. In addition to complying with Item 2 of Section 2510.6.1, drainage on the exterior side of the water resistive barrier shall have a minimum drainage efficiency of 90 percent as measured in accordance to ASTM E2273 or Annex A2 of ASTM E2925.”


Section 2510.6 on water-resistive barriers states that “water-resistive barriers shall be installed as required in 1403.2 and, where applied over wood-based sheathing, shall comply with Section 2510.6.1 or 25106.2.”


The important change is the addition of a 3/16-inch-thick drainage material or open plane between the water-resistive barrier and the cement plaster. This is new. The code does not prescribe how this open plane will be accomplished or maintained. It can be envisioned as an open space between the back of the lath and plaster including any continuous insulation and the face of the water-resistive barrier. This requirement is limited to exterior assemblies that employ “wood-based” sheathing, which in effect means that likely it will not apply to structures sheathed with gypsum sheathing or many buildings of Type I and II (non-combustible) construction. This new requirement then covers Types III, IV and V, which includes a significant portion of the IBC-built multifamily construction market. The wording in the International Residential Code is similar, so this change will impact in all forms of residential construction where wood-based sheathing is specified. Further, wood-based sheathing may be interpreted as including the engineered wood family of products.


This change will impact a large portion of the United States. The International Energy Conservation Code divides the United States into climate zones, and the result is that a very major portion of the United States that will fall under the new requirement.


But there are a few unknowns. The first is how will it be interpreted, and the second is who will enforce it. The last unknown is when it will be required. In all cases regarding building codes, their interpretation is left to the authorities having jurisdiction. Most commonly this is the local municipality, but it remains to be seen how the local authorities will view this new requirement. The second question can vary on how local jurisdictions will inspect and enforce the job site. In many locations a third-party inspection agency is contracted to handle inspections. The last unknown, the time frame, will be based on if and when the locality adopts the 2021 International Building Code. It is important to understand that local authorities having jurisdiction have the right to adopt or amend the model code as required to meet local needs. Therefore, it is vital that a contractor be aware of regional variations.


Multifamily construction typically has a more formal process leading up to the construction phase. This includes construction documents where the new requirement should be included. In this scenario, the contractor should become aware of the change prior to bid. In many cases, residential construction does not follow the same protocols, and the contractor may not be aware until inspection.


On any given project, the informed contractor should be cognizant of which code will be followed, and how that code will be interpreted. The second step is to determine how the installation should proceed, or how the space will be attained. Keep in mind that when a contractor has started physically working on the exterior wall, he/she has accepted all prior materials and work.


AWCI, as with other issues that impact the industry, will serve as a source of information. Also, your local plaster bureau may be invaluable in determining local code interpretation and enforcement. Contractors should be aware of the potential for this at the time of bid. This means that if the locality where a project is to be built adheres to the 2021 IBC, the contractor should seek formal clarification if there is nothing in the construction documents relating to the space in details or specifications.

Robert Grupe is AWCI’s director of technical services. Send your questions to, or call him directly at (703) 538.1611.

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