Cleaning Limestone Trim, Steps and Foundations
Limestone sills, steps and foundations are common on older brick houses, and limestone reacts to acid. What the National Park Service says about cleaning it, and the gentle method that works best.
The short answer
Never use acid cleaners on limestone. The National Park Service lists limestone as acid sensitive and warns that acidic cleaners can etch and dissolve it, and that high pressure water has scarred limestone on historic buildings. Gentle water, including long, slow misting, is one of the best ways to clean it.
Know what the stone is first
The National Park Service’s guide to cleaning historic masonry starts with identifying the material, and warns that it is not always obvious (NPS Preservation Brief 1):
- Some sandstones are easily confused with limestone.
- What looks like natural stone may be cast stone or concrete, which were often used for trim and upper floors because they looked like stone from a distance.
- Some “stone” cornices and window hoods are not masonry at all, but painted metal.
The brief shows a brick building with a real limestone foundation and decorative trim above it that is terra cotta made to look like stone. Each of those takes a different cleaner.
Limestone and acid do not mix
Brief 1 sorts masonry into acid tolerant and acid sensitive:
- Acid sensitive, use alkaline cleaners: limestone, marble, calcareous sandstone, glazed brick and terra cotta, polished granite.
- Can take acid cleaners: granite, most sandstones, slate, unglazed brick and terra cotta, cast stone and concrete.
It warns that acidic cleaners can be extremely damaging to acid sensitive stones such as limestone, etching and dissolving them, and that muriatic acid in particular should not be used on historic masonry. Some masonry and concrete cleaners are acid based, so read the label before using one near limestone steps or sills.
Gentle water works on limestone
Brief 1 calls prolonged spraying or misting with water particularly effective for cleaning limestone and marble, and good for heavy soot and crusts in protected areas that rain does not wash. It also shows a limestone building scarred by high pressure water held too close to the surface.
How we handle limestone
Our house wash is low pressure, and we use high pressure only on flatwork: concrete, asphalt and paver drives and walks. Limestone steps, sills and foundations are part of the house, not the driveway, and belong with the gentle approach. If you are not sure what your trim or steps are made of, tell us when you ask for a quote.
More on older houses: cleaning an older Louisville house.
What cleaning will not fix
Cleaning will not repair stone that is cracked, flaking or worn away, and some dark staining in limestone is not dirt at all. The National Park Service describes a limestone stain that water washing could not move because it was an old water repellent coating. Stone repair is work for a mason.
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