What are appropriate sheathing materials for plaster construction?

Rigid sheathing materials are commonly used behind plaster finishes. They are directly attached to support studs then covered with building paper or other weather resistant barrier (WRB). Metal lath attached over the sheathing and into the supports carries the plaster. The weather resistant barrier is intended to resist water penetration, so the sheathing is protected from moisture. That means that many materials are suitable for this application, but the common ones remain plywood, oriented strand board (OSB), cement board, and exterior grade gypsum sheathing.

In the early 1910s, basic research on stucco systems looked at the then-current practice of using board lumber (not panels) as sheathing. These were commonly 6- to 8-inch wide boards attached to support studs at 45 degree angles. During that period, diagonal placement of the boards transitioned to horizontal placement, and was followed by a move to 4-by-8-foot panels.

Wooden lath, small, narrow boards (1/4-by-1 ½ inches), were also common at that time. Although it’s not exactly clear when metal lath was first used in plaster applications, it appears that both metal and wood lath were available at least as early as 1910. Both wood and metal lath were common substrates for plaster at least through the teen years of the 1900s.

It should also be noted that it is possible to place stucco over open frame construction. This is accomplished by stretching line wires between studs and adjusting the wire to support paper as the backup to plaster. The paper is supported by the wire creating the backstop for plaster as it is applied to create the wall face. Because this is not as rigid as a board, the open frame method can allow for more variation in the thickness of the plaster. In addition it is important to understand that the open frame must be adequately rigid to resist deformation due to wind or other forces—that often means bracing the frame. The stucco layer may not be considered as part of the stiffening system.

Common sheathing materials today come in 4-by-8-foot boards. These are readily available building materials of consistent quality and are easy to install over wood or steel frames. The boards assure a more uniform thickness of the plaster layer and add structural rigidity to the building frame. For a variety of reasons, then, the currently preferred practice in many regions is to use sheathing boards for frame construction. As noted above, sheets of plywood, oriented strand board (OSB), cement board, and exterior gypsum sheathing are the most common materials for independent veneer plaster applications.