Pressure washing looks effective. You can see it working. The soiling comes off in real time, the stone looks pale and clean, and the equipment is readily available. It's also fast, which matters when contractors are pricing against each other on day rates. For all these reasons, pressure washing is used on period stone constantly, by contractors who don't know better and sometimes by contractors who do.
The damage it causes doesn't always show immediately. This is part of the problem. A building that has been pressure-washed might look clean for months or a few years before the consequences become visible. By then, the contractor is long gone and the building owner is left with a facade that is in worse condition than before the cleaning.
What pressure washing does to Portland stone
Surface erosion. Portland stone is an oolitic limestone — formed from tiny calcium carbonate spheres cemented together. High-pressure water erodes the weakest parts of this structure: the cement between the oolites, the softer beds within the stone, and any friable areas where weathering has already begun. The result is a roughened surface with an opened porous structure. You can't see this clearly from street level, but at close range the change in surface texture is visible. The tooling marks from the original dressing of the stone are often partially erased.
Mortar joint damage. The lime mortar joints in Victorian and Georgian masonry are softer than the stone or brick. High-pressure water strips mortar from joints faster than it strips soiling from the masonry. After pressure washing, joint depths are often visibly greater than before. This allows more water penetration, accelerates freeze-thaw cycling in the joint, and eventually requires repointing — work that could have been avoided.
Water saturation. The quantities of water introduced by pressure washing are far greater than any heritage cleaning method. This matters because historic masonry was built with lime mortar, which is vapour-permeable: it allows moisture to move through the wall and evaporate. Saturating the wall introduces more water than the masonry can evaporate quickly, especially if the internal conditions don't allow drying. Salts in the masonry — from ground contact, from atmospheric deposition, from previous cement pointing — are mobilised by the water and carried to the surface as it evaporates, causing efforescence and sometimes spalling.
Opened surface structure accelerates resoiling. The roughened, opened oolitic surface that results from pressure washing provides much better substrate for biological colonisation than the original smooth face. Algae, lichen, and black fungi establish faster and more aggressively on a pressure-washed surface. Buildings that have been pressure-washed often reSoil faster than they did before — which leads to more pressure washing, more damage, and an accelerating cycle.
The damage that takes years to appear
Not all pressure washing damage is immediately visible. The water that saturates the masonry moves through it over months and years, carrying dissolved salts to new locations. Soluble salts that crystallise within the stone structure rather than at the surface cause internal damage — the expansion of salt crystals as they form breaks the stone from inside. This typically becomes visible as spalling (the stone face detaching in flakes or sheets) years after the washing. By then, the cause is rarely identified correctly.
We survey buildings where the spalling has been attributed to frost damage, water ingress, or just age. In some cases it is those things. In others, the pattern and location of damage is consistent with post-washing salt migration. We've learned to ask when the building was last pressure-washed.
What to use instead
For atmospheric soiling and biological growth on Portland stone: DOFF superheated steam cleaning. The 150°C steam kills biological matter and lifts carbon soiling without significant water volume, without abrasion, and without chemical agents. It doesn't open the surface structure. It doesn't damage mortar joints. Conservation officers specify it for a reason.
For heavily calcified soiling on flat surfaces: TORC vortex cleaning. The fine aggregate in the TORC stream shifts thick deposits that steam alone won't move, but at a much lower impact than a pressure washer and with controlled aggregate selection that protects the stone face.
The correct answer requires a survey. The correct method depends on the stone, the soiling, and the condition of the masonry. It cannot be assumed from the street and it cannot be the same for every building. A contractor who quotes for cleaning without visiting the building is pricing the wrong job.