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Stoned: Problems with Thin Stone and Thin Manufactured Stone Veneer, aka, “Lick and Stick Stone,” aka “Lumpy Stucco”

By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE

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©2019 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 61, no. 10, October 2019.

About the Author
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Westford, Mass. Visit www.buildingscience.com.

The issues with stucco are pretty well understood and we have discussed them in detail before (“The Coming Stucco-Pocalypse,” ASHRAE Journal, January 2018 and “The Perfect Storm Over Stucco,” ASHRAE Journal, February 2008). Drainage, drainage and drainage are the key…Did I mention drainage? Also, inward vapor drive. Did I mention that as well? Can any cladding be worse? Ah, yes. Manufactured stone veneer or thin stone veneer also known as “lumpy stucco” is my candidate.

Take stucco and stick thin stones—both unreal (manufactured stone veneer, which is precast concrete veneer made to resemble natural stone) and real (thin cut stones) to the stucco surface. How can this be worse than plain stucco? Easy, with plain “smooth” stucco rainwater can drain off the surface. With “lumpy stucco” rainwater is held in the nooks and crannies so the surface stays wetter longer and because drainage is discontinuous in the nooks and crannies we get hydrostatic pressure….and we have covered hydrostatic pressure before (“Hockey Pucks & Hydrostatic Pressure,” ASHRAE Journal, January 2012). The results are often disastrous (Photo 1 and 2). Not only do we get hydrostatic pressure we also have a much bigger reservoir (stored and absorbed water) and that means inward vapor drive. Think bad stucco on steroids.

However, the problems are easy to avoid. Same approach as with stucco: provide a drainage layer behind the “lumpy stucco. Drainage, drainage and drainage. And, if the drainage gap is 3/16 inch or greater, we get to address inward vapor drive and provide some hygric redistribution—aka “drying.”

Easy, pretty much done here. So, what else is there to talk about? Not so fast Bucko. There is much, much more. The real stone and the fake stones are falling off stucco all over the place and have been for quite some time. The problems go back decades but have been attributed to issues that I believe are incorrect. Basically, the argument is that workmanship is the issue.

We are not dealing with the rot and damage shown in Photos 1 and 2. We are just dealing with the thin stones falling off. Yup, workmanship. Over and over I get the same argument. Workmanship. Apparently, over three decades and more, and in regions all over North America, but only where it rains, we can’t seem to get folks to install the stuff correctly. I bought into that argument at first. Sure, there are workmanship issues on some projects. But there have been countless failures where the workmanship was exceptional and supervised continuously by third parties. The failures are now so pronounced we have social media stirring up the pot.

So how is the stuff installed? The stucco layer is installed over a sheathing and water control layer…then the veneer is adhered with “thin set” mortar to the stucco. The thin set mortar is applied to the back of the veneer and then the veneer is “smooshed” into the stucco layer. Now it gets interesting. The patterns of failure are distinctive.


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