Our Upper West Side cooperative is a 13-story, prewar brick building with appealing historic features, including sandstone window trim, a copper cornice, and terra-cotta moldings. Although the building is in reasonably good condition, years of city soot and grime have given the exterior a dingy appearance. The board would like to undertake a façade-cleaning program, but we’re worried about damaging the decorative elements that give the building its architectural character. What cleaning methods are strong enough to remove dirt and stains while safe enough to protect the delicate parts of the building? Can we use the same treatments throughout the façade, or is brick cleaned differently than stone and other parts of the exterior?
You’re right to be concerned about selecting the proper methods for cleaning your building, because the wrong treatments can scar the building and permanently harm its finer historical elements. Different types of masonry call for different types of applications, but even the right cleaning method can cause damage if incorrectly applied. Similarly, different types of grime, such as soot and smoke, require different cleaning agents than, say, oil or metallic stains. As a rule of thumb, the gentlest treatment should be used when possible, and whenever stronger methods are called for they must be used carefully.
There are three main methods for cleaning façades: water, chemical, and abrasive treatments. Abrasive treatments, such as grinding, sanding, and blasting, are not recommended, however, because they remove surface material along with dirt and paint and are therefore the most likely to damage masonry. That leaves water and chemical treatments, both of which are effective and safe when properly used on the materials and surfaces for which they are intended.
Cleaning with water. There are three kinds of water-based methods for cleaning façades: pressure-washing, soaking, and steam/hot-pressurized washing. Properly applied, water-based treatments are the least invasive types of façade cleaning and a conservative way to start the job, especially when dealing with typical dirt and grime.
The most common water-based method by far is pressure-washing, in which a low- to medium-pressure spray (100 to 400 pounds per square inch) is applied to the surface of the building. (As a point of reference, the spray from a garden hose is about 60 psi.) The water pressure usually starts out low and is increased as needed, followed by scrubbing with a natural or synthetic (but not metal) bristle brush for stubborn areas and detailed elements.
Sometimes a non-ionic detergent made from synthetic organic compounds is added to the water to remove oil-based dirt from surfaces. Soap and other household detergents should not be used in water-based cleaning because they can leave a visible residue on the masonry.
Soaking involves spraying or misting the masonry surface for an extended period, usually up to several days at a time, to loosen heavy accumulations of soot and crusts, particularly in parts of the building not exposed to rain. Soaking, used in conjunction with pressure and followed by a final water rinse, requires repeated applications that can take up to several weeks. But because it’s mild, it’s ideal for historic masonry.
The third method, steam or hot-pressurized cleaning, is not commonly used, but it can be effective for removing built-up soil deposits and plants, such as ivy. It’s also an option for cleaning stone that is sensitive to the acids used in some chemical cleaners.
Chemical agents. Chemical cleaners are effective for removing dirt, and unlike water-based treatments, they can also be used to remove paint, coatings, metallic stains, and graffiti. Acid-based cleaners are effective on unglazed brick and terra-cotta, cast stone, concrete, granite, and most sandstones. Alkaline cleaners are best used on acid-sensitive masonry, such as limestone, marble, polished granite, and calcareous (chalky) sandstone.
Both types of chemical cleaners are sprayed or brushed on a wet surface and allowed to sit for a period of time, which varies depending on the particular cleaner, typically longer for alkalines than acidic cleaners. Depending on the substance being removed, several applications of the cleaner may be necessary. Both types are rinsed off with water; alkalines are given a slightly acidic wash first to neutralize them.
Cleaning preparation. Before beginning a façade cleaning program, it is important to identify the types of masonry on the building and select the most appropriate cleaning method for each type. Distinguishing among different types of stone is especially crucial. Certain limestones, for example, can look like sandstone, and what looks like natural stone can be cast stone or concrete. In addition, some bricks may contain impurities, such as iron particles that can react with certain cleaning agents, resulting in staining. Choosing the wrong type of cleaning method can cause irreversible damage to the building materials, so when in doubt, it’s best to consult with a historic preservationist about the composition of the masonry.
Also, be aware that some chemicals (and even water) that are safe for masonry can corrode or damage other building elements, such as decorative metal elements, glass, wooden window sashes, iron window bars, and window air conditioner sleeves. Any parts of the building not subject to cleaning but susceptible to damage should be covered or otherwise protected.
If your building is planning major exterior repairs, the board should consider waiting until that work is completed before cleaning the façade. Water from sprays and hoses can enter the building through cracked or missing bricks and deteriorated mortar joints, eventually rusting lintel steel and metal supports. If the building is in mostly good condition, however, water washing the façade could help expose hairline cracks and other small defects, as well as remove loose mortar from joints, facilitating the repointing work.
It is best to schedule your building’s façade cleaning so that it takes place during warm weather. Water-based cleaning should not be done in temperatures below 45 degrees Fahrenheit because water trapped in cracks and joints could subsequently freeze, leading to spalling and further cracking. Chemical cleaning agents also work better in temperatures 50 degrees or warmer. A masonry wall can take a week or longer to dry, so it’s best to plan the cleaning when a cold spell isn’t likely to settle in soon after the application.
Testing sample patches. Before launching a full-scale cleaning of the façades, the cleaning method should be tested first on an out-of-the-way part of the building. Start with a small section, less than a square foot, trying the mildest treatment first and testing progressively stronger methods as needed.
Many chemical agents come in different strengths, and they can be diluted until the right mixture is found. The amount of time a chemical cleaner is left on the surface can also be adjusted, depending on the severity of the dirt or stain. Caution should be used when testing with chemical agents, however. Leaving certain hydrochloric-based agents on masonry too long, for example, or not adequately rinsing them from the surface, can leave a hazy, whitish residue. The cleaning contractor should follow the manufacturer’s guidelines in all cases and contact the product’s technical representative with any questions.
Because your building has different kinds of masonry, a sample patch should be conducted on each type. Water-based treatments shouldn’t automatically be dismissed on particularly grimy or stain-ridden areas: water can help loosen dirt and grit, enabling a milder chemical cleaner to be used than would otherwise be the case.
In a misguided attempt for a quick, thorough façade cleaning, some contractors blast the water at a very high pressure or increase the concentration of chemical cleaner beyond what is recommended. Using too strong a stream or too strong a cleaning agent can be abrasive and easily etch glass, marble, sandstone, and other soft stones, and certain types of bricks. Holding the nozzle of the spray applicator too close to the surface can also damage the masonry and strip away delicate decorative elements. If grains of stone or sand are found in the runoff cleaning water, that’s a sign the water pressure is too high, or the nozzle is being held too close.
Given your building’s age, a uniformly pristine look may not be attainable – or desirable. Different areas of the façade have probably been repaired or replaced at different times, so it may not be possible to achieve the same degree of luster on all types of masonry. For example, paint may have been used to protect soft brick or hide defects, so stripping it throughout the building may not produce the best visual effect. Or some crusts may have become so embedded within stone surfaces that it may be impossible to remove them without also removing part of the stone.
Keep in mind that if your building is in a designated historic district, the board will need to obtain prior approval from the New York City Landmarks Preservation Commission, which will want to make sure that the planned cleaning methods and agents will not damage the masonry.
By carefully analyzing the building’s exterior elements and the special treatment needs of each, a safe and effective cleaning program can refresh the look of your building while preserving its historical character.