The Case for Stucco: Learning from Failures, Planning for Success

1/24/2024

Stucco is one of the oldest natural types of cladding systems around, dating back to ancient Greece. Prized for its durability and versatility, the exterior cladding generally consists of cement, water and aggregate and is used in commercial construction throughout the United States. 

According to the 2019 Stucco Market Report, stucco has been a preferred cladding system due to several characteristics: 

Durability. the U.S. Department of Energy, using the Building Energy Efficiency Standard, reported that properly applied stucco has a useful service life of 100 years. 

Codified fire ratings. A 1-inch-thick coating of stucco provides a one-hour firewall rating, which means it will prevent the spread of fire from one side of the wall to the other side for at least one hour. 

Low life-cycle cost. In rating several cladding materials, ASTM STP 1269 published results showed cement stucco ranked the lowest cost in a 30-year lifespan. 

Low maintenance. Stucco is resistant to pests, mold, rot and mildew. 

An exterior insulation finishing system (EIFS) is sometimes mistaken for stucco because its appearance can be similar. The main difference is that EIFS has an acrylic finish with a cement-mixed base coat applied over an insulation panel providing a higher R value to the wall assembly. By comparison, stucco is a cementitious-based system that is applied directly to masonry or metal lath. Though not as durable as stucco, an EIFS system is sometimes selected for its ability to increase a building’s energy efficiency. 

Typically, the stucco cladding system consists of two or three coats. For a three-coat system: 

• The first coat is the scratch coat, which provides adhesion to the building. 
• The second coat, the brown coat, creates an even surface. 
• The third coat, the finish, provides the aesthetics of the cladding’s outward appearance. 

Investigating failures 

While stucco has been installed successfully throughout modern history, like any building enclosure application, if it is done wrong, problems will ensue. That is what occurred with a five-story landmark commercial property in the British Virgin Islands (BVI). Located in the business district of Road Town, the building’s Portland cement plaster façade failed shortly after its completion in 2010. 

Investigation revealed the deficiencies, the missteps that led to them, the way the façade could be repaired and the way to prevent such missteps from happening again. 

Initial failures were identified as white deposits – efflorescence – appearing as early as 2011, which led the owner to perform repairs due to the aesthetics. In 2015, the white deposits again were observed with subsequent repairs made to alleviate moisture intrusion, particularly at windows. 

Following another observed failure in the system, the owner enlisted support to help investigate the cause of the ongoing issues. A 2016 investigation revealed that deficiencies had spread and were documented throughout the building. Among the findings the team observed were white deposits leaching from cracks, as well as blistering, discoloration of surfaces and hairline cracks. 

Some of the worst areas of failure occurred at the precast exterior beam between the first and second floors. There also were large sections of stucco that de-bonded. Some as large as 4 feet square had fallen off the building. 

Telegraphing of the concrete masonry unit (CMU) mortar joints through an exterior cladding, often referred to as ghosting, was additionally noted, which can occur if the scratch coat is too thin or if the blocks are too wet, creating a differential in suction between the CMU and the mortar joints. 

Following documentation of the conditions, additional investigation was performed, including sounding of the stucco. At numerous locations throughout the facility, sounding revealed a hollow ringing, indicating the stucco likely was not well bonded. 

Areas identified as de-bonded were removed from the building and sent for laboratory analysis. To determine the approximate proportions of the stucco scratch coat and its role in the de-bonding, petrographic analysis – an examination by optical and scanning electron microscopy – was performed. 

Identifying missteps 

In investigating the property, the team looked into BVI’s code standards and found minimal governing requirements for the application of a stucco system. Further, the one-page original design specifications were limited, although they did reference ASTM C926, which specifies the standard requirements for the application of Portland-cement based plaster exteriors and manufacturer’s instructions.  

The investigation found that the Portland cement plaster – stucco – cracking, efflorescence and de-bonding distresses were caused by a combination of factors that included poor underlying substrate preparation, out-of-plane underlying layers, improper mix proportions and poor application. 

In cases such as these, recommended repairs include: 

• Replace existing stucco with a watertight cladding system. 
• Properly prepare the CMU and precast concrete for application and incorporate a water-resistant barrier as a secondary drainage plane. 
• Install accessories with flashing at terminations, reveals and intersections and incorporate weeps for water management at the drainage plane. 
• Install control joints to allow for differential movement and match existing locations to reduce the potential of stresses on the cladding. 

Preventive measures 

Stucco application is not complicated and is incredibly resilient and long-lasting when certain best practices are in place. Architectural plans can capture the design of the entire building but might not include detail relating to building-enclosure considerations. Building owners and managers should look for such information and request it to be added if absent, or they should consult a building-enclosure expert. 

Trades people retained to apply stucco should have experience with the product or demonstrate their training and supervision will deliver the quality result required. 

Further, guidance should be provided for various trades because their respective work intersects, such as with stucco and curtain wall window systems, so the transitions between them have the necessary integrity. There also should be oversight of the project by a knowledgeable contractor or building enclosure consultant. 

While these aspects might vary, best practices for the application of stucco are consistent everywhere, with exceptions based on climate. 

Vu The Nguyen is a senior project manager with Terracon, a consulting engineering firm.