If your historic house or building has a stucco exterior, the stucco is most likely applied over a wood frame structure. Stucco is sometimes applied over brick, stone or concrete block structures. Stucco is a Portland cement product installed over a wood lath or a metal lath that is nailed to the wood or masonry walls. Sometimes stucco is applied directly to a masonry wall.

Your maintenance routine should include an annual inspection to evaluate the condition of your stucco. This annual evaluation you can identify potential issues that could turn into significant problems if not addressed. The most common problems you may encounter with your stucco exterior are described below.

Cracks in Your Stucco Exterior

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Dane County. Cracking on a stucco wall is repairable, but this is not a do-it-yourself project. Hiring a trained mason is recommended for stucco repairs. Source: WHS - State Historic Preservation Office.

Your annual inspection might reveal cracks in your stucco exterior. These cracks are caused primarily by foundation settling or upward heaving. Old houses can settle a bit over many years, and often the cracks do not get any worse. If no other moisture or structural problems exist, these cracks can be repaired by an experienced mason. If a crack does get worse, you will have to address the structural foundation problems with a preservation mason under the guidance of a preservation-oriented structural engineer.

Sections of Stucco Bulging or Falling Off

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Racine County. In this instance the stucco wall has not only cracked but has also bulged and shifted out from the house framing. This type of deterioration is often seen with major water infiltration within the wall cavity. Source: WHS - State Historic Preservation Office.

You might find serious failures in your stucco exterior, such as whole sections that are bulging or falling off your house or building. These failures can be caused by excessive foundation settling or heaving, or massive amounts of water that enter the wood framed wall or masonry behind the stucco. To remedy this damage to your stucco, you must first address the underlying problem or problems. Once the underlying problem is repaired, you will have to hire a professional to remove the damaged stucco and apply new stucco.

Stucco Surface Erosion

You might find problems with surface erosion on your stucco exterior. Stucco surfaces can erode if they are exposed to moisture for extended periods of time. Sand blasting and pressure washing can also cause severe erosion of stucco surfaces. As stucco cures, the surface develops a hard protective layer. Sandblasting and high-pressure washing can remove this hard layer and expose the softer inner layer to moisture, causing erosion and failure. These abrasive cleaning techniques are so damaging to masonry that the practice has been banned in all preservation guidelines as well as by Wisconsin state statutes (WI Statute 101.1215) for all historic properties.

Although you may not like the look of your eroded stucco exterior when you are close to your house or building, you are better off to not seal your stucco and just leave it alone. If massive erosion or previous sand blasting is causing excess moisture penetration or further erosion, you can protect your unpainted stucco by applying a water-repellent coating. These products include modified siloxanes, silanes and other alkoxysilanes, or metallic stearates. These specialized products are available from historic preservation masonry supply companies. You should only apply these products to the affected areas under the advice of a preservation masonry conservator. These products will penetrate into the stucco and impede the give and take of moisture through the stucco.

TIP: Never apply a sealer to sound stucco or any other masonry material.

Never use "water proof" sealants on unpainted stucco. These sealants stop the give and take of moisture and can trap water vapor, allowing it to pool into standing water inside the wall. This situation is a prescription for catastrophic stucco and paint failure.

If your stucco is painted with an oil-based paint, you can repaint it using a special vapor-permeable paint designed for masonry. These specialized paints are available from historic preservation masonry supply companies. However, if your stucco has latex paint on it, you cannot use these products. The latex paint creates a vapor barrier that will prevent these paints from working properly.

Stucco Containing Surface Debris: Dirt, Mold and Mildew

You might find areas of your stucco exterior that are dirty or have mold or mildew. Dirty stucco is usually the result of exposure to airborne pollution and particulates in the air that attach to the stucco. Dirt can also accumulate in areas that are protected from the rain. You should never remove dirt with high-pressure washing or any kind of high-pressure blasting, such as sand blasting. These practices will erode the harder surface of the stucco and cause permanent damage.

If you would like to give your stucco a fresher (but not new) look, you should clean your stucco using the mildest approach possible. Use a non-ionic detergent and a stiff nylon or natural-bristled scrub brush to remove harmful pollution and dirt on the surface. Never use acid-based cleaners on stucco. The acid will erode the surface and cause permanent damage.

Mold and mildew are the result of excess moisture in your stucco wall. If your stucco wall cannot dry out, excessive moisture could linger inside your wall. Even a large tree that constantly shades your stucco wall could prevent it from drying out. Excess moisture will allow mold, mildew and moss to grow on the stucco surface and even inside your walls. You should consult a preservation professional to help you fix the moisture problems and then clean your stucco. Preservation masonry supply companies sell a variety of products designed to kill and clean mold, mildew and moss.

Stucco Surface that Will Not Hold Paint

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Dane County. Painting stucco walls is not recommended, this photo illustrates the result of painting a stucco building. Source: WHS - State Historic Preservation Office.

Original stucco was never intended to be painted. However, most stucco houses have been painted multiple times. Stucco naturally takes in and lets out moisture, but excessive moisture that enters the stucco from both the exterior and interior can cause paint failure. The excessive moisture can enter through cracks due to cascading water from ineffective gutters, or the use of sealants that do not allow normal water vapor to escape.

Dampness rising from the saturated ground can also wick up into the stucco and prevent paint from adhering to the stucco surface. You should prevent moisture from entering your stucco by repairing cracks, replacing your gutters and caulking in appropriate locations.