By William Spilchen
Stucco has successfully been used for centuries and provides a resilient, unified look for residential and commercial facades. The variety of stucco finishes can complement both traditional and contemporary styles. Moreover, stucco is cost-effective as compared to various exterior coatings, it may be applied at a rapid rate and is extremely durable. Stucco has the lowest annual maintenance cost when compared to other claddings.
Stucco is a cementitious material so cracking can occur under certain conditions. Cracking presents aesthetic as well as water entry concerns. It is impossible to eliminate cracking entirely, but understanding what impacts stucco walls will lead to better stucco installations.
Stucco, similar to concrete, is very strong in compression but weak in tension. Stucco compression strengths range from 2000 to 3000 psi, but tensile strength is only 100 to 200 psi. Because of this, lathing is required which serves two functions: it provides a mechanical attachment path from the stucco to the structure and it provides reinforcement to the stucco to provide tensile strength, similar to reinforced concrete. All laths provide good reinforcing strength in the horizontal direction, but are weaker in the vertical direction. Some lath types are weaker than others in this direction, providing less reinforcing strength. Therefore, it is not uncommon to see linear horizontal cracking on stucco.
Stucco will crack when the imposed tensile stresses exceed the tensile strength of the stucco and lath combination. Tensile stresses are caused primarily by shrinkage of the stucco during the curing process. Shrinkage of stucco has been studied over the years and is well documented.
Hence the need to follow good work practices to minimize the stresses on the stucco and minimize cracking potential. Here are some of those fundamental factors:
With the advent of external insulation systems on walls, we are seeing more stucco cracking on these types of installations:
It is impossible to eliminate all stucco cracking. Hopefully all of these points are helpful and lead to improved stucco wall performance and greater satisfaction to architects, building owners and the lath and plaster trade.
William Spilchen is a Mechanical P.E. with more than 40 year experience in the construction and construction materials industry. During this period, he has been involved with the design, construction and inspection of various building systems throughout North America. He has created new product designs, some of which are patented, and designed automated manufacturing processes for their fabrication. In the 1990s, he became involved in the analysis of building failures as a result of moisture ingress, and subsequently with rainscreen technologies. He has been involved with all aspects of stucco construction and stucco materials. Further, he was involved with the R2000 energy code in Canada, and now with the changing energy codes in the U.S. He has continued research in the field of improved durability and energy efficient building elements, and improved seismic designs. Presently, he is a consultant to Structa Wire ULC.