ASHRAE Journal - October 2019 - 70

COLUMN BUILDING SCIENCES
Joseph W. Lstiburek

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

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" (Figure 1). Drainage, drainage
*I

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 (Photos 3 and 4). 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

am taking credit for coining this phrase; OK, I am really taking blame for coining this phrase.

†Wolf,

J. and B. Lahnert. 1990. "Performance of latex-modiﬁed thin-set mortars in applications of thin-brick veneer." Serviceability and
Durability of Construction Materials, Volume 1, B. Suprenant, American Society of Civil Engineers.

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