Reinventing Cement Stucco
Cement stucco can be thought of as the finish coat of a three-coat cement plaster or the entire assembly as defined by the building codes. Three-coat cement plaster or “stucco” has been in the modern codes since its inception. Over the last century, the plastering industry has had many experts on lath, cement basecoats and the stucco or acrylic finish applied for final decoration. Names like Walt Pruter, Dick Gorman, John Bucholtz, Tom Geary, Clint Fladland, Bruce Pottle, Bob Drury and Jim Gulde are but a few of the famous names that helped shape the plaster industry for half a century. That is changing and the history they left is being re-written by others.
The current bureau experts regularly attend meetings on lath and cement plaster, and while the preferred method of installation can and should be discussed openly as projects, regions and practices can vary; it is the re-writing of lath and plaster history that should concern us the most. We sit and listen to others pontificate the history of lath and plaster on why certain practices became standard, codified or simply recognized. What is concerning is how the history is being re-written to fill agendas or provide a bumper sticker explanation. The Portland Cement Association puts out great information, and a paragraph I read while attending architecture school and working part time as a plaster patch man to pay school expenses had a profound impact on me and may be even more relevant than ever:
“Portland cement plastering is simple or sophisticated depending on the specific application. With experience gained over the years, certain materials in specific geographical areas have demonstrated their superiority in producing weather resistive facing for buildings.”
I soon found this not only to be true but could be expanded to include regional practices and nuances. This is what made lath and plaster unique from other claddings. Cement stucco is a blend of craft, science, art and experience; it is hard to think of another cladding that even comes close, except possibly stucco’s brother, EIFS.
The concern we should have is the re-writing of history may not be based on reality and more intent on creating an agenda. Some of us are fortunate to have worked at a plaster bureau with archives dating back almost one hundred years; allowing you to look up the real history. Below are some historical positions that have been altered. The list of gentleman mentioned earlier, most who have passed away, would agree many are getting the history about lath and plaster wrong.
Why Two Layers
The two layers of paper were initially introduced into the code in 1979 simply to provide additional moisture protection over wood based sheathing. Prior to adoption in the code, many plastering contractors were already adding a second layer over plywood and found that cracking was reduced by this practice. This was on the recommendations of Walt Pruter and Dick Gorman from the Plaster Bureau as early as 1969. They discovered incidental moisture finding its way behind the cement membrane could cause plywood sheathing to swell just enough to place stress on not fully cured cement plaster, resulting in map cracking. They tried the two-layers of paper and cracking was reduced.
Today, the industry seems to be moving toward the idea the top sheet is merely a sacrificial sheet serving no purpose or is just a bond breaker paper. This re-write of history has now led to code change that directs flashing to be between the layers of building paper or WRB.
Porous Cement Membrane
This is a myth the industry has fought for decades. The cement stucco membrane is highly water-resistant when properly mixed and applied. Dozens of independent tests have proven this, yet the rumor persists. Real plaster experts agree that water can and will tend to find its way behind the membrane, and the assembly should be designed on the premise of incidental moisture. Entry is typically at terminations, penetrations or cracks in the membrane. This leads to hairline cracks and how much water they allow to reach the WRB. The concrete industry has tested this issue many times over the last century. Hairline cracks are less than 1/32 of an inch wide and they do not allow enough bulk water to penetrate the membrane to be a concern to a framed wall assembly with a secondary moisture barrier and proper flashings. This explains why the code exempts cement stucco from water testing; they know it works, unless history can be re-written.
Even the Smallest Pinhole
If people erroneously believe cement plaster is porous and water is not even slowed, the next step is to ensure the WRB is sealed to absolute perfection. This makes sense, and adding negative pressure to the equation means water will be drawn through the tiniest of pin holes. The mistake with this thought is the WRB and lath is part of the whole assembly. In reality, the cement membrane is an integral part of the moisture management assembly and required to make cement stucco function properly. The cement provides UV protection, abuse resistance and resists the passage of air and water to reach the secondary moisture barrier. As an effective air barrier, the negative pressure becomes a moot point. This may be shocking to think of cement plaster as an air barrier but the 2012 International Energy Code section C402.4.1.2.1 under Materials lists Portland cement plaster as an air barrier not needing to be tested for compliance. Removing this air barrier during a water test is virtually promising failure.
This re-write has led some experts to recommend placing large daubs of sealant over fasteners used to attach the lath. This can result in thinner areas of cement in a straight line over the stud. Since the vertical stud is a source or conduit of stress for brittle cement, the thin score line is precisely on the source or transmitter of stress. Fasteners have been used to attach lath over building paper on framed walls for decades with no evidence of water intrusion. A water leak through fastener penetrations is when the air barrier (cement and drywall) is removed.
Belts and Suspenders
When in doubt, just add layers. Cement plaster has been applied over masonry for decades here in the United States and for centuries in historic Europe. Cement applied directly to the absorptive masonry creates a strong bond, improves the wall’s water resistance and aesthetics. Today, many experts believe the addition of a sheet good type water resistive barrier is needed for moisture protection. They fail to recognize the ramifications of such a move. A sheet good product will act as a bond breaker and typically not recommended by plaster bureaus, unless we can re-write history. The industry has many great trowel applied coatings from the EIFS industry to add water-resistance and increase the bond if extra protection is desired.
Hide the Flaws
Cement plaster is not a machined product. It is handmade, and while we marvel at the artist who makes a bowl by hand and love the imperfections as his or her unique signature, we do not give the plasterer the same tolerance. Many expect perfection, regardless of the unrealistic request. As an industry we encourage a final texture that will help us hide those minor “handmade” imperfections. We know the rougher the texture, the better the capability to hide minor unavoidable imperfections. Today the craze is for no texture, the smoother the better. The hide power of any smooth finished plaster is gone and when the sun hits the surface at just that right angle, those very minor imperfections can look just terrible.
The one-piece control joint has traditionally been applied over continuous lath for decades in the western U.S. The argument about if lath needs to be cut or left continuous has been debated to death and everyone has an opinion. When ASTM C 1063 became referenced by the code it changed history and perceptions. Cracking and cement plaster are inherently connected, that we all agree on. Investigating cracks and the source of stress was challenging and time consuming and most importantly hard to pin point. Those same great gentleman listed in the opening developed a lot of expertise and recommendations to minimize cracks and provide solutions against excessive cracking. Today, the answer is much easier. Various experts look at a control joint and can claim cracking is the result of the control joint being placed over the continuous lath. Even in the face of an adjacent two-piece where lath was cut and the same cracking is exhibited, seems to be ignored. Ironically, on a recent project the group of experts knew the problem was control joint over continuous lath before even opening the wall, as the contractor expressed he had done that. After a call to his lathing foremen, it was discovered the lath had actually been cut or was discontinuous.
Investigating cracking in cement plaster requires not jumping to fast, easy conclusions. Finding the real cause usually requires effort and skilled analysis.
The most troubling part of all this re-writing of history is that the future of plaster will suffer. The cost for installed cement plaster has climbed, but not enough to justify the host of new products required and extensive procedures being added. So who is absorbing these higher costs? Years ago, the plastering contractor and his crews were considered the experts; they were treated as such and trained to understand their assemblies, the materials they used and enough building science to make it work. Expertise and experience seems to be waning as we move into the future. Something changed over the last few decades and others have stepped in to take control of our industry, even if that means re-writing history to re-invent cement plaster.