Hardcoat Stucco basics
Hardcoat stucco is an ancient building material that was historically applied over masonry buildings and was often referred to as plaster. Modern stucco is made of Portland cement, lime and sand and is often installed over wood-framed buildings in North American residential construction. Quality stucco siding systems are three or more coats of Portland Cement applied over a lath system that helps attach the plaster to the house. The resulting plaster siding system should be 3/4 of an inch or more in thickness. There are also less expensive and thinner one and two coat stucco systems. These are proprietary siding systems made by different manufacturers and as a general rule, they can be cheaper to install and less reliable.
You may also run into stucco systems that are not really stucco at all but rigid foam insulation with a coating of plaster over the top. You could hear these referred to as fake stucco, synthetic stucco, exterior insulated finish system (EIFS), or even by various brand names such as Dryvit. Although these systems look like stucco, they really could not be more different. These siding systems are barrier systems and if water gets behind this type of siding the water can become trapped and cause significant damage to the wood structure below.
Reservoir siding system
From a building science perspective, traditional hardcoat stucco is referred to as a reservoir siding system. This is quite different than a traditional barrier siding system which is designed to keep water out. Stucco is concrete and when it gets wet it soaks up water like a sponge. As it dries, the moisture will go from concentrations of more to concentrations of less, meaning it will dry in two directions, both away from the building and in towards the building. For a stucco system to perform well, the weather barriers below the stucco are critical. There should be two layers of weather barrier and all penetrations in the stucco, such as around windows and doors, require proper flashing techniques to prevent too much water from getting behind the siding.
During a home inspection, the flashings on the outside are visible, but the critical weather barriers and drainage plain below the stucco are not visible. Home inspectors must rely on visual clues on the outside of the siding system to determine how worried to be about the execution of details below the siding. As you can imagine this is a bit of a guessing game.
With quality comes risk
Quality hardcoat stucco is one of the nicest siding systems available. On the right style of house, the siding looks great and can go for decades without any painting or maintenance. However, stucco poses an inherent risk of concealed water damage, especially where the building is exposed to the weather. Stucco is installation-sensitive and even small installation mistakes can lead to significant siding failures and damage to the building. If too much water is getting behind the siding or the weather barrier and causing water damage, it is possible that this will not be visible, even to a well-trained and experienced home inspector.
Red flags and further evaluation
Home inspectors are trained to look for red flags in stucco systems such as cracking, discoloration, window and door leakage and poor flashing details. When discovered, these red flags should be further investigated by a qualified stucco installer or an exterior envelope engineer so you can determine the urgency and cost to repair. This additional inspection by a specialist may involve destructive testing where moisture probes are used to discover if wood framing below the house is getting wet or staying dry. You would need permission from a seller before this type of investigation is done because testing can damage the finishes of the siding system.
Painting and Refinishing Stucco
Because stucco is designed to hold moisture and then let it go, painting stucco risks fundamentally changing how the stucco was intended to perform. Most stucco installers will advise against painting stucco; there is a risk that water could get trapped behind the paint. If you desire a new look or color to the stucco, the best advice is to resurface the stucco with a new finish coat. While this is the best advice, it is more expensive, so you will see stucco that has been painted and I have run into contractors who recommend this approach. This presents a conundrum; it can be difficult to determine how well the siding is protecting the building once the stucco has been painted.
Overview
In general, hardcoat stucco is one of the nicest siding systems you can install to protect a house. However, this type of siding is installation-sensitive and inherently more risky than other siding systems especially where it is installed over wood framing and exposed to rain and moisture. This makes stucco better-suited to dry climates and houses that are protected by generous roof overhangs. When repairs to stucco are needed they are typically expensive and complex and repairs may come with concealed water damage that can lead to unanticipated repair costs.