Cleaning can be a challenging portion of a facade repair project. It often requires preliminary trials and an iterative process to select safe but effective processes and materials. Facade cleaning is also performed as housekeeping or in conjunction with minor repair work. In these cases, power washing a facade with water is often considered a simple, low-cost way to remove minor soiling. Although this is an effective cleaning method on many substrates, using a water pressure that is not properly controlled and limited to levels appropriate for the materials present can cause damage to a facade. Several recent projects demonstrate the risks of excessively high pressure washing.
In the first example, a limestone commercial building was cleaned as part of a municipal facade beautification campaign. The work was properly specified to use water at low pressure, defined as less than 2750 kPa (400 psi), but the contractor misunderstood the intent of the specification. In developing the cleaning protocol, complicated—and mistaken—calculations were prepared purporting to show that to deliver 2750 kPa pressure at the surface of the wall, the pressure washer should be set to 27,500 kPa (4000 psi). The specification, and the preservation guidelines from which it was derived, did not intend such complexity; the specified pressure limit was simply the pressure as measured by the gauges on the pressure washer. Unfortunately, the discrepancy was overlooked during submittal reviews, and the contractor washed the facade using the higher pressure. This damaged the fragile stone substrate, creating stone spalls and eroding mortar joints (Figure 1).
In other recent projects, the damage to the facade substrate was less extreme, but inappropriate pressure washing resulted in a noticeable alteration that will be difficult—if not impossible—to address. In one example, the workers occasionally held the spray wand too close to the surface or lingered too long in one location. This resulted in etching of the polished stone surface. The permanently altered texture of the stone now makes it impossible to restore the original aesthetic consistency of the facade (Figure 2). In another similar example, building maintenance staff pressure washed a coated concrete facade using on-hand equipment for which pressures could not be reduced to the appropriate levels. As a result, the excessive water pressure irregularly eroded the facade’s intact coating system, creating a pattern of spray marks across the wall (Figure 3). What was intended to be a quick clean-up instead resulted in the need to recoat the facade.
Similar to damage caused by sandblasting of facades in previous decades, pressure washing with a water pressure that is inappropriate for the substrate can be challenging to control, resulting in permanent damage to the facade. Therefore, it is critically important to limit the pressure to a level that is safe for all the materials that are present.
Kenneth Itle, AIA, is an architect and associate principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation. He can be reached at firstname.lastname@example.org.
Mike Ford is an architect and senior associate with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois. He specializes in facade assessment and cleaning. He can be reached at email@example.com.
The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect that of The Construction Specifier