Cleaning an Older Louisville House Without Damaging It

Why older masonry is different

The National Park Service’s guide to cleaning historic masonry, Preservation Brief 1, opens with a warning: inappropriate cleaning and coating treatments are a major cause of damage to historic masonry buildings (NPS Preservation Brief 1). The brief warns that mortar joints, especially lime mortar, can be eroded by harsh cleaning, and that some brick and soft stone can be etched by high pressure water.

Decide what you are removing

The brief says to first identify what is on the wall, because each kind of soiling needs a different approach. It names dirt and soot, oil and metal stains, paint, and biological growth such as mold and mildew, and the tendrils ivy leaves behind. The goal is the gentlest means possible: the most effective method that does the least harm.

Start gentle, and test first

• Water: the brief calls low or medium pressure water washing one of the most common methods for historic masonry. Start at 100 psi or below, even a garden hose, and move up only as needed, generally no higher than 300 to 400 psi.
• Brushes: natural or synthetic bristle, never metal, which can scratch the surface and leave particles that stain.
• Test areas: the brief recommends testing methods on small areas first, and shows a test patch that revealed a mismatched brick repair hiding under the dirt.

What the brief warns against

• High pressure. Water at too high a pressure, which the brief ties to “power washing” and “water blasting”, is very abrasive and can etch marble, soft stones and some types of brick.
• Abrasive cleaning. Grit blasting and grinding are generally not appropriate for historic masonry. They remove the hard outer face of brick and erode mortar joints.
• Muriatic acid. The brief says hydrochloric (muriatic) acid should not be used on historic masonry because it can dissolve lime based mortar and damage brick and some stones.
• Cold weather. No water cleaning in cold weather or when frost is likely, because water inside the masonry can freeze and crack it. A masonry wall can take over a week to dry, so the brief advises stopping several days before the first average frost date. Chemical cleaning should never be done below 40 degrees F, and generally not below 50.
• Unsound walls. Before water cleaning, make sure the mortar joints are sound and the building is watertight, or water can get into the walls.

How we clean older houses

We do not blast siding or masonry. Our house wash is a soft wash: low pressure and a professional grade cleaning solution, given time to work, then rinsed, on vinyl, aluminum, brick, stucco, wood and fiber cement. For mold, mildew and algae, that is the same direction the brief points: let the cleaning solution do the work, not the pressure.

Preservation districts in Louisville

Louisville Metro has locally designated landmarks and preservation districts, and its Landmarks Commission design guidelines apply to properties in them. The Old Louisville Neighborhood Council advises contacting Landmarks staff in the Planning and Design Services office if a project might need a Certificate of Appropriateness (Old Louisville Neighborhood Council, 2020). Whether exterior cleaning needs one is a question for that office, so ask before any work on a house in a district.