Exterior Stucco as a Waterproofing Alternative
Stucco can reduce the moisture permeability of masonry-enhancing its waterproofing capabilities-and provides an aesthetic finish on concrete block or masonry. Cement stucco can also be colored or patterned to add a textured look to a building. Stucco applications are the most economically effective method of waterproofing deteriorated masonry surfaces that require extensive tuck-pointing. It has also been used as a waterproofing agent over the lower portions of a masonry surface.
Stucco exteriors have been used in United States construction since the early settlements and today they are prominently used in new construction throughout North America. The initial use of stucco was as a waterproofing covering over rough-cut stones or badly deteriorated masonry. Stucco was applied as a parge coat over the masonry in a thin coating. The practice of parge coating originated in Europe and is still in practice today.
MASONRY SURFACESCurrent stucco finishes are typically applied in two-coat application over masonry substrates. At times this may include a mesh and furring strip reinforcements. In these types of applications, galvanized metal laths are nailed to wood furring strips that are secured to the masonry surface-the desire is to create an airspace that prevents humidity from damaging the wall and to add drainage. Stucco can also be applied directly to the masonry surface. In these types of applications, the first coat of cement should be rough enough to provide better adherence of the following coats. A key factor in applying cement stucco directly to masonry is a good suction or bond between the masonry surface and stucco.
Many designers have thought that adding a building paper to a direct-applied system over masonry will aid in moisture resistance. This is not recommended by the plaster bureaus, as it creates a bond breaker between the masonry and the cement stucco. In addition, it is almost impossible to find the mortar joints for the proper attachment of the lath. Since attachments to the masonry unit itself can cause spalling, attachment into the mortar is preferred. For additional moisture protection prior to adding cement stucco, there are several trowel-applied coatings that are made by the EIFS manufacturers. These coatings have proven to work well. Contact your local EIFS dealer and follow manufacturers’ directions.
Stucco mixes differ depending on specific application circumstances. Determination of the existing stucco composition is critical in remedial and repair applications for material compatibility. Repair of existing stucco surfaces should be completed with similar chemical and physical properties to the existing stucco. Not all mixes are compatible and incompatible materials can lead to a premature failure. For example, a stucco mix of soft lime and sand should not be patched with Portland cement. The repaired or remedial stucco should match the existing in strength, composition, color and texture. It is not a good practice to repair stucco with a mix that is stronger and/or does not convey the same appearance.
A typical remedial or repair stucco mix is two parts of lime to one part of Portland cement and nine parts of sand, with a little animal hair. Stucco for new applications consists of Portland cement and sand, with lime added for plasticity. The lime content should not exceed 10 percent by weight or 25 percent by volume.
EXTERIOR STUCCO DEFORMATIONSStucco has many of the same characteristics as brick in the fact that it has a high tolerance level for moisture presence. Stucco is not necessarily damaged by the presence of moisture and can remain submerged for long periods of time (even decades) without negative effects. However, under certain conditions and over time, moisture presence may eventually lead to material delamination from the formation of salt crystals on the interior and exterior of the surface. Salt crystals form from the evaporation of moisture in the stucco and create two conditions: efflorescence and subflorescence.
Stucco is not damaged beyond appearance (blemishes) by efflorescence, which is the formation of salt crystals at the material surface. Subflorescence damage (salt crystals within the material) can result from expansive sources, such as continual exposure to cyclical changes (freeze/thaw), which overtime contributes to spalling and cracking. Stucco also contains calcium, which is susceptible to damage from continued contact with water. If the calcium leaches out of the material, it could lead to a weakened internal structure or surface corrosion, leading to deformations, such as spalling, cracking and bowing.
Repairs should be made as soon as cracks (larger than what would be considered typical hairline), bowing and other deformations occur in the stucco surface. This will eliminate large-scale deterioration.
REPAIR METHODS FOR EXISTING STUCCO SURFACESPrior to the stucco application, proper preparation is required. All loose, bowed and deteriorated stucco should be removed down to a sound surface or the masonry substrate. Loose stucco can be detected by tapping on the wall with a wooden mallet. Loose stucco will resonate a hollow sound. Existing cracks in the stucco surface should also be cut out and cleaned. If only repair patches are required, it is good practice to remove stucco to architectural breaks. Patches in the field can often be extremely evident and unpleasing to the building owners. The sound stucco around the removed areas should be undercut or have rough jagged edges to allow for proper adhesion of the new stucco to the existing surface. A bonding agent may also be used. Once the deformations have been removed, the area should be power washed and cleaned free of all dirt, dust and contaminants.
Stucco is typically applied in two coats to reduce cracks and to cover wall irregularities and deformations making the final coat appear uniform.
For remedial applications that desire a “rainscreen” approach, it is best practice to apply galvanized metal mesh over pressure treated furring strips that are secured to the masonry substrate. Many consultants believe this application can add reinforcement to the stucco and creates an air space between the stucco and the substrate that helps to prevent further moisture damage. In repair applications, the condition of the mesh, if applied, must be inspected and repaired accordingly if required.
The stucco application shall be applied over the prepared furring surface in a three-coat application, similar to framing, scratch, brown and finish. The scratch and brown coats are each applied at about 3/8 inch thick. The basecoat is recommended to be no less than 3/4 inch total thickness. The first coat or scratch coat is applied and a cure time of 48 hours is required by code prior to the second or brown coat application. The scratch coat shall be scored to improve the bond of the following brown coat. It is good practice to wet the area prior to the application of the layers when absorption is too fast to promote proper hydration.
The final (finish) coating is applied no sooner than seven days (per code) after the first two coats. This mix is typically made up of a higher lime content than the two previous coats. This is to allow the plasterer a more workable coating to achieve a desired texture not possible with the basecoat mix. The final coating is applied to produce a nominal thickness of 1/8 inch and may be a smooth or textured finish to match the rest of the wall.
Stucco applications are recommended to be completed within the temperature constraints as stated in ASTM or Portland Cement Association standards. Proprietary blends shall be per the material manufacturer and application and mixing shall be in accordance with the manufacturer’s latest printed specifications. Stucco application should not be completed in winter months where the ambient outside temperature is below freezing, unless proper precautions have been taken. Best results occur in temperatures between 40 and 70 degrees F. Application completed in hot weather requires special considerations.
Damp curing (hydration) of Portland cement basecoat is often overlooked and underrated. ASTM and PCA standards require damp curing during hot dry weather; failure to follow these requirements may lead to soft, weak stucco. Misting with a standard garden hose, as needed, will provide adequate hydration.
One of the primary benefits of stucco is that pigments can be added to the finish mix to produce different colors during application. Stucco manufacturers have blended finish coats with box color that can be added to maintain a consistent mix ratio for best results with color uniformity. If there is consideration of painting over the stucco surface, the manufacturer should be consulted for approvals and application procedures. Best practices and plaster bureaus recommend not applying paint or sealer that can act as a vapor barrier over the stucco. It is best to allow the surface to breathe.
Acrylic coatings, as manufactured by the EIFS industry, are typically vapor permeable and excellent for stucco. Be sure to follow manufacturers’ guidelines.
To remove existing paint from stucco surfaces it is best to use a chemical paint remover, such as an alkaline stripper. Apply the strip material in a painted fashion over the stucco surface and let it remain covered for one to two days. When the strip material is removed, the paint layers will pull away from the surface. Alkaline strippers with a base of sodium or potassium are the most economical and are generally effective. Cleanse the surface prior to re-coating.