Traditional masonry stucco is a cement-based plaster that is applied over walls and other surfaces inside and outside of buildings. It is made from cement, sand, and lime and hardens into a highly durable material that requires little maintenance. Like traditional decorative plaster, stucco can be troweled, brushed, or otherwise textured to create a variety of finish effects. Stucco typically is applied over a galvanized wire mesh called lath, which helps the stucco adhere to the supporting structure and strengthens the entire assembly. Stucco usually is mixed on-site and is applied in three coats.
Metal stucco lath is installed over a moisture membrane applied to the wall sheathing. The lath is a structural mesh with holes that are about 1/4-inch wide. The first coat of wet stucco squeezes through the holes to create a strong connection to the lath.
Traditional, or "3-coat," stucco is applied over the lath in three thin coats: the scratch coat, the brown coat, and the finish coat. The scratch coat is applied first and is embedded into the metal lath to provide a strong base for other two coats. It is called the scratch coat because it is often scarified with a small raking tool to create a grooved texture. The grooves improve the bond of the second coat.
The second coat, or brown coat, is applied next and is troweled smooth to create an even surface for the finish coat. The finish coat is applied last, creating the decorative finish on the wall surface. This is also the coat that is colored, if desired. The total thickness of the three coats is approximately 7/8 inch.
Building Code Standards on Stucco
The International Building Code (IBC) provides recommendations for stucco installation, including the minimum time delay between each coat. In addition, there are two standards from the American Society for Testing and Materials (ASTM) that regulate the application of stucco:
- ASTM C 926 Standard Specification for Application of Portland Cement-Based Plaster
- ASTM C 1063 Standard Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster
Because stucco is cement-based and hardens when mixed with water, it must be mixed just prior to application; it cannot be sold in pre-mixed form. "Pre-mixed" stucco is a dry mixture containing all of the necessary ingredients, and all you add is water. These commercially prepared stucco mixes can be an easy solution to many construction projects.
Traditionally, stucco is mixed on-site using bulk materials. The trick to creating your own stucco mix is getting every batch exactly the same. A basic stucco mix consists of:
- Three parts sand
- One part Portland cement
- Hydrated lime to 10% of the cement by volume
The slower stucco dries, the stronger it becomes. In sunny and hot locations, stucco must be shaded to prevent the dehydration of the stucco mix. When using pre-mixed stucco, it's best to follow the manufacturer’s instructions. Never apply stucco if the weather temperature is lower than 40 F.
Pre-Mixed Stucco Manufacturers
There are many major manufacturers that you can contact for more information on mixing and applying stucco:
- Parex USA
- Omega Products
- Mission Stucco
- Merlex Stucco
- Eagle Building Materials
- California Stucco
- BMI Products
Stucco can be mix with pigment to create a range of colors, or it can be used without pigment and simply bear the natural gray of its cement. Some contractors tend to use warm colors, such as yellow, pink, or orange, but beige and white are perhaps the most common colors used for stucco. Stucco can be painted, but it must cure for at least six weeks prior to painting. Coloring stucco with pigment is far preferable to painting it because paint creates a regular maintenance issue, while pre-mixed coloring is permanent.