Stucco comprises fine aggregate (sand) and a cementitious binder, which creates a mortar-like material that’s applied to the exterior of a building. Stucco systems can be direct applied to a backup material like masonry, CMU, or concrete. They can also be applied to lath, and either integrated with building paper or a water-resistive barrier over a wall assembly or integrated with a water/air barrier and drainage mat over a wall assembly. Each system is designed to accommodate shrinkage, support the cladding system, and manage water with flashings.
Early 19th century uses of stucco binders were lime-based, including quick lime, lime putty, or hydrated lime. Later, natural cements were used. Around the early 20th century, with the widespread production of portland cements, the most common binder for stucco was portland cement with some lime. Gypsum has also been used in lieu of lime. Other admixtures for workability and curing properties have also been found in stucco mixtures, including fibrous materials, oils, waxes, water-reducers, and air-entrainments.
Stucco is often applied in 2 or 3 layers. The composition and material proportions of stucco layers differ to provide specific properties to the assembly of layers. Scratch coats help in neutralizing the moisture between the substrate and the stucco to limit shrinkage and improve bond. The middle layer(s) provide more stability with additional sand and fibers while limiting shrinkage. The finishing layer provides a mix that can create the desired texture and color. Total stucco thicknesses are generally between 3/4 inches and 1-1/4 inches.
For successful historic and existing projects, it’s important to prioritize repairs based on the structure’s conditions and project team goals. Project strategies differ per project and can be selected from a variety of repair options.
Refer to our restoration best practices page for additional information, details, and resources. Below, find stucco repair and restoration options for consideration on your project.
If damage to the stucco system is past the level of repair, then removal and reinstallation of a new system may be required. You can refer to our new construction stucco page for preliminary guidance.
Damaged stucco often exhibits cracking, spalling, and delamination. Root cause of damage can vary from steel mesh/lath corrosion, freezing-thawing, lack of bond, or intrinsic issues with the mix or construction. Investigations for stucco projects can be complex, but critical for determining the appropriate repairs. Repairs in general remove unsound material, prepare the substrate, add new lath, add new flashing or weather barrier sheeting, and replace stucco section(s) with a compatible repair mix. New lath, barriers, and flashings should be integrated appropriately with the existing systems.
The overarching goal of stucco repair mix is to develop a repair mix that has similar properties of strength, density, porosity, and water absorption to the existing stucco. If the repair mix is too strong and dense in a stucco system with weaker, more porous stucco, it can cause premature future failures of the historic or existing stucco. As new layers/coats of the stucco system are installed, they need to be cut into the existing stucco so that each succeeding layer is cut into the existing stucco at increasing amounts. In other words, the scratch coat is cut in, then the brown coat has a slightly larger area cut in, and the finish coat spans the largest in area.
Crack repair options depend on whether the crack is moving or stable, its width, and if it’s causing water infiltration. If still moving, as determined with a crack gauge, then a flexible material like sealant may be an appropriate solution. Fine sand can be broadcast into the sealant to assist with matching the crack repair to the remaining main field of stucco. If the crack is no longer moving, then a more rigid repair material, like a repair mortar, may be appropriate. Mortar repairs should be formulated and installed to match the exterior of the existing stucco.
IMI’s free project support, technical assistance, and education is here to help you at any stage in your building’s lifecycle.
Our multidisciplinary team draws on decades of experience developing solutions for high-performing masonry and tile projects.
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