Traditional Masonry Archives — Managing Moisture

Defining Stucco

Although some believe stucco is only found on quaint South­western-style homes in California, Arizona and New Mexico, those involved with historic masonry know otherwise.

Stucco (the word is said to have come from the Old High German word “stukki,” meaning crust, piece, or fragment) has been in use worldwide for centuries. In common vernacular, stucco is described as any type of exterior plaster applied in two or more coatings over masonry, wood, metal lath, or another substrate. Coats are typically referred to in order of application as scratch coat, brown coat and finish coat. Stucco may be finished smooth, in a wide variety of textures, or scored to imitate stone.

The material came into wide use in the United States in the 18th and 19th centuries, primarily in residential and relatively small-scale buildings, according to Anne Grimmer’s pamphlet “Preservation and Repair of Historic Stucco,” prepared for the National Park Service’s Heritage Preservation Services Division. As the U.S. strove to emulate more-developed European nations, stucco found its way into Federal, Greek, and Gothic Revival architecture, mainly “on residential buildings and relatively small-scale commercial structures,” writes Grimmer.

Although stucco is considered by many to be a form of sacrificial coating, early examples remain visible to the public today, including Monumental Church in Richmond, Va. (1812), St. John’s Church in Washington, D.C. (1816), and Reformed Church of Saugerties in Saugerties, N.Y. (1852).

Traditional stucco composition

Traditional stuccos were often composed of locally available material. A prototypical mixture might include a slaked-lime binder, sand or other aggregate, and inorganic or organic additives, according to “Focus: Historic Stucco,” published in New York Landmarks Conservancy’s Common Bonds. Inorganic additives such as brick dust or natural cement were used to quicken set. Organic additives might include animal hair, blood, urine, linseed oil or eggs to reduce shrinkage, increase strength, improve resistance to the elements and extend set time.

Portland cement, first manufactured in the United States in 1871, began replacing natural cements in stucco. By the early 1900s, portland-cement-based stucco was shaping the way buildings were constructed.

“No longer used just as a coating for a substantial material like masonry or log, stucco could now be applied over wood or metal lath attached to a light wood frame,” Grimmer writes. “With this increased strength, stucco ceased to be just a veneer and became a more integral part of the building structure.”

Repairing traditional stucco

While communicating all that goes into a traditional stucco repair is well beyond the scope of this article, numerous written references abound. Anne Grimmer’s encyclopedic “Preservation and Repair of Historic Stucco” is a great starting point.

Experts agree that several factors are key to repairing traditional stucco: learning the cause of damage (most often water infiltration), understanding the stucco’s composition (for which lab analysis may be required), and matching the building’s original appearance as best as you can (which will often involve more than just matching the finish coat).

There is more to stucco repair and remediation than replicating a particular look, according to Mame Cohalan, president of Cohalan Co. Inc., which trades as Keim Mineral Systems.

Keim supplied materials and expertise for the recent stucco remediation on Richmond’s Monumental Church, originally erected on a site where a movie theater fire claimed 72 lives in 1811.

“It’s important in working on many historic buildings to allow them to breathe,” Cohalan says. “Some contemporary products prohibit proper breathing — but others may allow too much vapor transfer.”
Monumental Church was repaired with Keim’s Universal Render, which Cohalan describes as the closest commercially available thing to ancient Roman stucco. “It is somewhat updated because it has very small fibers and mineral content that allows an inherit flexibility.”

Cohalan also describes a common problem that traditional stucco craftspeople face: Many historical structures contain a combination of traditional and modern repairs.

“Traditional masons, whenever possible, try to steer clients away from using contemporary patching or repair materials on historic buildings,” Cohalan says. “But masons and clients have (used them) in the past and probably will continue to do so in the future.”

Sadly and too often, repair methods on traditional structures are driven by economic considerations rather than maintaining historic integrity.

“I know that these decisions can be monetary,” says Cohalan, who was involved in stucco restoration at New York’s Metropolitan Museum of Art. “We had some sections (at the Met) where they decided to leave a few rather well-done portland-cement-based patches in limestone. It would have been much better to do a historical repair from the beginning.”

While higher raw costs associated with traditional materials may factor into a client’s decision-making process, the often painstaking process of getting it right falls on the shoulders of traditional masons. Even the material selection process can be time-consuming — which tends to ramp up costs.

“Choosing a stucco mix that is durable and compatible with the historic stucco on the building is likely to involve considerable trial and error, and probably will require a number of test samples, and even more if it is necessary to match the color,” Grimmer writes. “It is best to let the stucco test samples weather as long as possible — ideally one year, or at least through a change of seasons, in order to study the durability of the mix and its compatibility with the existing stucco, as well as the weathering of the tint if the building will not be painted and color match is an important factor.”

Beyond monetary value

Portland cement repair seems to have become constituent to historic stucco over the years, and it seems that at least one of its undesirable qualities has gained public acceptance. “I do see quite a bit of micro-cracking,” says Cohalan. “It’s accepted, and people seem to be saying, ‘That’s what happens with stucco.’”

Philosophically, Cohalan can accept that some brown-coat and scratch-coat repair formulations will be a little harder than traditional, but the finish coat should match the original structure as closely as possible. “When you get to the outer layer, we believe it should be soft, flexible and vapor-permeable,” she says.

Traditional masons and clients both typically toe the line together when it comes to refinishing large sections of building facades.

Micro-cracking — which may occur when modern materials are used to create smaller repairs on traditional stucco — seems to have gained at least partial acceptance in the public eye. But affecting repairs on larger surfaces with anything other than traditional methods may involve something aesthetically distasteful.

“If anyone is considering using a portland cement-based material and did research, they’d see that control joints (similar to those seen on poured concrete sidewalks) are required at regular intervals because of the cracking problem,” Cohalan says. “If you have a historic structure, you don’t want the historic facade to be broken up in panels.”

Indeed, the only way to ensure a seam-free, monolithic, historical treatment that is flexible, durable, and breathable is to use traditional masonry materials and techniques.

TM