Green Planet has three main methods for cleaning facades: Water treatments, chemical treatments, 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 but only 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 facades: pressure washing, soaking, and steam/hot pressurized washing. Properly applied, water-based treatments are the least invasive types of facade 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 psi) is applied to the surface of the building. (As a point of reference, the spray from a garden hose is approximately 60 psi.) The water pressure usually starts out low and is increased as needed, followed by a 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 masonry.
Soaking involves spraying or misting the masonry surface for an extended period of time, 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 water pressure and followed by a final water rinse, requires repeated applications that can take up to several weeks. But because it’s a mild method, 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’s sensitive to the acids used in some chemical cleaners.
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-base 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.
Before beginning a facade cleaning program, it’s important to identify the types of masonry on the building and select the most appropriate cleaning method for each type. Distinguishing between different types of stone is especially crucial. Certain limestones, for example, can look like sandstone, and what looks like natural stone could 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 facade. 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 facade could help expose hairline cracks and other small defects as well as remove loose mortar from joints, facilitating the repointing work.
It’s best to schedule your building’s facade cleaning so that it takes place during the 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 facades, it’s strongly recommended that the cleaning method be tested first on an out-of-the-way part of the building. Start with a small section, approximately 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.
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