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Sheathing materials
What are appropriate sheathing materials for plaster construction?
A: 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 WRB 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 in. to 8 in. 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 x 8 ft panels.
Wooden lath, small, narrow boards (1/4 in. x 1-1/2 in.), 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 x 8 ft 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.
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