Stone cleaning: when is it needed, and what does it cost?

Grey patches on the paving, a slippery film on the patio, dark marks on the façade — sooner or later the question comes up of whether professional stone cleaning is due. Here is an overview of how to tell, and roughly what to budget for.

How do you know a clean is needed?

• A greenish-black film (algae, moss) — usually visible first in shady or damp spots.
• A growing risk of slipping when the surface is wet, particularly on patios and access paths.
• Discolouration on the façade below window sills or roof overhangs.
• Joints from which weeds or moss are spreading across the surface.

What does the price depend on?

• Area: the most important factor — pricing is usually worked out per square metre.
• Degree of soiling: a light green film comes off faster than moss that has been growing in for years.
• Material: natural stone, concrete paving and render each need a different pressure and, in some cases, a different method (hot water, cold-water high pressure).
• Travel and access: areas that are hard to reach add to the work involved.

How often is a clean worthwhile?

That depends a great deal on the position and the aspect: shady, damp areas under trees need freshening up more often (every 1–2 years) than sunny, well-ventilated spots, where every 3–5 years can be enough. Regular, preventive cleaning usually works out cheaper than occasionally removing heavily ingrained soiling.