Waterproofing for CMUs: What About Stucco?

In the July 2014 issue of The Construction Specifier, we published the article, “Durable Waterproofing for Concrete Masonry Walls: Redundancy Required,” by Robert M. Chamra, EIT and Beth Anne Feero. A month later, we received the following e-mail from G. Michael Starks, president of the Florida Lath & Plaster Bureau (FLAPB):

I thought this article offers sage advice should your plans call for struck and painted concrete masonry unit (CMU) walls. Unfortunately, the predominance of walls today are not struck and painted, but rather have some other exterior finish applied, such as stucco. In this case, recommending a waterproofing admixture or surface sealer without the caveat that doing so will greatly impair the bond of the stucco to the wall is a bit disingenuous. Applying sealers to the CMU prior to plastering requires further bond augmentation where stucco is specified.

The bond of the stucco to the CMU is achieved by the concrete masonry absorbing the water from the fresh stucco. This absorbed water carries the cement paste into the voids in the CMU (a process commonly known as ‘wetting out’), where it cures and locks the two together. Sealing the CMU before stucco application renders this process null, and results in the future debonding of the stucco from the CMU.

ASTM C926, Standard Specification for Application of Portland Cement-based Plaster, includes a Section 5.2, which discusses remedies when bond cannot be achieved over solid bases. Sealing the CMU voids all but one of the possible augmentation procedures and leaves only the last resort of applying a lath and accessories system to the CMU. As stated in C926:

5.2.3. Where bond cannot be obtained by one or more of the methods in 5.2.2, a furred or self-furring metal plaster base shall be installed in accordance with Specification C1063.

In effect, the other six recommendations required before lathing is approved are disregarded by the recommendations provided in the article. The seven means by which to achieve bond provided in ASTM C926 are listed in decreasing order of effectiveness. As such, there is intent for the application of lath to be last in that order. The addition of lath and accessories creates a huge differential in movement characteristics resulting in much higher cracking potential.

Additionally, the installation of lath over a solid substrate, such as CMU or concrete, introduces another challenge in the anchorage and fastening system required to attach the lath and accessories. Finally, from an economic standpoint, stucco applied over lath systems costs three to four times that of stucco direct-applied to the CMU.

Testing performed by the National Concrete Masonry Association (NCMA) indicates the stucco renders the wall as waterproof as (if not more than) those where admixtures and sealers are applied.

Early in this article, there is a statement about shrinkage cracks that occur in unit masonry. These are a result of the same absorption process discussed earlier though with the mortar and CMU. Many of these can easily be prevented by specifying a leaner mortar, such as Type N. In the circumstance where a cement-rich Type M or Type S mortar is specified, proper masonry workmanship can also mitigate the shrinkage effects, such as requiring the masons to fog their walls after initial set and prior to leaving the site in the afternoon much as the plasterers are required to do (ASTM C926) for the stucco. This process averts rapid water loss (volume) during the hydration process, thus minimizing the propensity for shrinkage crack formation.

In conclusion, if you want to waterproof your CMU and you plan to stucco it—seal the stucco, not the CMU. This at least puts the waterproofing on the positive pressure side of the wall.

One of the article’s co-authors— Mr. Chamra—reached out to Mr. Starks, and allowed us to share his response here.

We appreciate the feedback; we concur with your recommendations with stucco applied over CMU. Our article focused on CMU walls because single-wythe without stucco is used in Texas, and there is a lack of knowledge on how to treat them. There is more knowledge on how to waterproof stucco, but this was outside the scope of this article. We also did not discuss other finishes over CMU for the same reason—that topic could be another article in and of itself. Your letter is a good clarifier if our intention was not clear within the article. Thank you for your time and feedback.

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3 comments on “Waterproofing for CMUs: What About Stucco?”

  1. Wow. Thank you ! Your article just answered a big question and saved me a lot of headaches in years to come. My new cmu’s are absorbing rainwater during this rainy season. A potential stucco contractor suggested sealing the surface prior to applying stucco to prevent further water intrusion.. Now I know what a bad idea that is.

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