Stucco or render is a construction material made of aggregates, a binder, and water. Stucco is applied wet and hardens to a very dense solid. It is used as a decorative coating for walls and ceilings, external building siding, and as a sculptural and artistic material in architecture. Stucco may be used to cover less visually appealing construction materials, such as metal, concrete, cinder block, or clay brick and adobe.
Traditional stucco is made of lime, sand, and water. Modern stucco is made of Portland cement, sand, and water. Lime is added to increase the permeability and workability of modern stucco. Sometimes additives such as acrylics and glass fibers are added to improve the structural properties of the stucco. This is usually done with what is considered a one-coat stucco system, as opposed to the traditional three-coat method.
As a building material, stucco is a durable, attractive, and weather-resistant wall covering. It was traditionally used as both an interior and exterior finish applied in one or two thin layers directly over a solid masonry, brick, or stone surface. The finish coat usually contained an integral color and was typically textured for appearance.
Modern stucco is used as an exterior cement plaster wall covering. It is usually a mix of sand, Portland cement, lime and water, but may also consist of a proprietary mix of additives including fibers and synthetic acrylics that add strength and flexibility. Modern synthetic stucco can be applied as one base layer and a finish layer, which is thinner and faster to apply, compared to the traditional application of three-coat stucco.
The repair of historic stucco should begin by identifying the cause of the damage to the stucco finish. Historically, the application of stucco was quite similar to the process of applying lime plaster. Repairs should be carried out as soon as problems become visible, as the damage will only become worse over time. Cracks may form in the stucco due to building settling or direct damage to the exterior coating. Once water is able to breach the coating, whether through an opening in the stucco itself or from beneath its surface, fragile stucco can begin to buckle and crumble. Wood is a common structural material that is often used as substrate beneath stucco. It can absorb moisture from at or below ground level and draw it away from the original source of the problem. Stucco can also be applied to masonry such as brick or stone, which can also be damaged by moisture infiltration.
Rising damp from groundwater or drainage issues is especially problematic. The stucco can delaminate from damp wood lath beneath and as the wood rots, the stucco may begin to deteriorate and separate from it and the building. Damage to the stucco itself leads to further moisture infiltration that exacerbates the deterioration of the finish as well as the substrate. Downspouts, gutters, flashing and other means of managing directing water away from the building will prevent damage from getting worse. Without proper guttering, water may splash up onto stuccoed surfaces, staining and accelerating the deterioration of the finish. Grading of the soil around the building may also be necessary to redirect moisture away from the structure and foundation.
Depending on the extent of the damage to the finish, varying degrees of repairs may be carried out. Small hairline cracks may be sealed with an additional layer of finish coat or even simply a coat of paint. Modern caulking materials are not ideal mediums of repair. Making the choice to patch or completely repair a stuccoed surface depends on the texture of the finish coat. Repairs, especially numerous ones, made to a smoothly finished surface will be more noticeable and recovering the entire surface with a new layer of finish coat may be more appropriate. Conversely, it may be easier to conceal patches of repair work to a textured surface and complete refinishing may not be necessary.