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 pre-wetted. 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 inch 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.