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Standards for Installation of Plaster
There are two main standards organizations in North
America, the American Society for Testing and Materials (ASTM) and
the Canadian Standards Association (CSA). ASTM generates consensus
documents with the input of many people, including contractors,
engineers, architects, and material producers/suppliers. They agree
on what constitutes “best practice” and develop standards
to reflect that. These documents cover the ingredients and the application
of portland cement plaster, or stucco.
- ASTM C 926, Standard Specification for Application of Portland
Cement-Based Plaster
- ASTM C 1063, Standard Specification for Installation of Lathing
and Furring to Receive Interior and Exterior Portland Cement-Based
Plaster
Solid
Base for Stucco
People often ask us if stucco, or portland cement
plaster, will adhere to concrete or concrete masonry. This question
probably arises because plaster is often attached to wall surfaces
that have metal lath affixed to them. Metal lath is regularly used
over stud wall construction with or without sheathing materials.
With a substrate of concrete or concrete masonry, is it necessary
to use lath?
Stucco is made from the same materials as concrete and concrete
masonry. As such, they have a great affinity for each other. Portland
cement adheres well to lots of materials, especially to other materials
made from the same type of ingredients. ASTM Standard C 926, Specification
for Application of Portland Cement-Based Plaster, permits direct
application of stucco to solid surfaces like concrete and masonry
as long as bond is sufficient. Concrete masonry surfaces are both
absorbent and textured, two things necessary for bond.
If contamination is present on the substrate surface, good bond
is inhibited. This is generally not a concern with new walls. Older
walls may have bond-inhibiting characteristics, in the form of paint,
sealer, or some other coating or dirt on the surface. The entire
surface must have uniform bonding potential or you can run into
problems. Partial bond will create undesirable stresses and can
lead to delamination and cracking of the stucco layer.
 Potential
for bond can be quickly checked by a simple test: sprinkle the wall
with some water to see how it absorbs. If it is readily absorbed,
then the surface is expected to bond well with stucco. Immediately
prior to plastering, the wall should be prewetted. This prepares
the unit surface to absorb paste from plaster. It dampens the unit,
reducing its water demand and the potential of premature dryout.
Unless it’s very hot, dry and/or windy, moisture on the wall
together with that in the plaster is usually sufficient for curing.
Generally, stucco that is applied directly to solid surfaces is
placed in two coats that together total 5/8 in. of thickness.
If water sprinkled on the wall is not readily absorbed, then the
surface must be handled differently. Depending on the contamination,
you might only need to wash with water. With worse conditions, you
need more aggressive techniques. These include sandblasting or acid
etching. Alternately, you might apply a dash-bond coat or a bonding
agent to allow for direct application of stucco. If bond can’t
be attained by cleaning or bonding agents, you will have to use
paper-backed lath to assure mechanical anchoring to the wall. This
completely isolates the stucco from the wall surface but supports
its weight so it doesn’t fall off. Then the application of
plaster over concrete or concrete masonry becomes very similar to
application over open stud or sheathed frame construction.
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