Cleaning stone facades properly

While the beauty of natural stone facades has been celebrated for centuries, the art of cleaning them can be somewhat tricky. Before cleaning facades or patios of natural stone, you must first identify the material you are dealing with as well as the overall condition of the facade or patio. It’s also important to assess the degree of soiling and the different types of soiling. Based on this analysis, you can determine which measures need to be taken and adapt cleaning methods accordingly. While one type of stone might benefit from pressure washing, low-pressure grit blasting may be more suitable for another.

Natural stone facades: advantages and disadvantages

Natural stone is one of the oldest and most durable building materials in architecture. It has been used throughout history to construct prestigious buildings such as castles and cathedrals and can also be seen in the traditional stone cottages that dot the British countryside. However, weathering and environmental pollution leave their mark over the years. Often, stone surfaces can take on a grey or black colour due to dust deposits and exhaust fumes. Over time, they can also become covered with patches or a patina of moss, algae, or lichens.

When cleaning a natural stone facade, one must distinguish between acid-sensitive variants, such as marble, dolomite, travertine, Jura and other calcareous natural stones, and acid-resistant variants, such as granite and slate. The type of dirt and the severity of the soiling also play a major role in choosing the appropriate cleaning technique.

A natural stone facade is being cleaned with a pressure washer.

What to pay attention to when cleaning natural stone

In preparation for cleaning, the stone facade must first be checked for existing damage. If holes are visible, loose parts must be removed before filling the holes with a lime-based mortar. If there is significant damage to a larger area, damaged stones may need to be replaced with new stones. Once stones have been replaced and holes have been patched, the remaining dirt and grime should be removed by employing the gentlest cleaning method possible.

Use low-pressure grit blasting for heavily encrusted natural stones

If stubborn dirt or crust formations need to be removed from surfaces in a gentle way, low-pressure grit blasting is an established and effective method. This method is also well suited for the removal of graffiti.

For low-pressure abrasive blasting, a blasting gun is supplied with compressed air using a construction compressor. A fine, soft abrasive is added to the air in the mixing chamber, and, in some cases, water is also added to suppress dust. To clean soft stone, a particularly fine-grained abrasive should be used with low pressure.

Industrial vacuums can be used to ensure that as few blasting particles as possible remain in the cracks or on the surface of a wall after cleaning. Such vacuums are also able to efficiently and thoroughly remove large quantities of blasting media residue and dirt from the floor.

A person wearing protective eyewear, a mask and ear defenders cleans a natural stone wall using the low-pressure grit blasting cleaning method.

Tip 1 – Make sure to choose the appropriate blasting medium and grit size:

There are over 2,000 blasting media available worldwide. If you want to clean a sensitive open-pored stone facade, for example, you must first determine which type of abrasive and which grit size are most appropriate for cleaning such a surface.

Tip 2 – Test a sample area:

To ensure that the chosen method will not cause any damage to the surface of the natural stone, test a small sample area first. When sectioning off the sample area, make sure to cover the area directly surrounding the sample area.

A person in protective clothing removes graffiti from the inside of a house wall

Effectively remove graffiti

There are many methods for graffiti removal. From hot water high-pressure cleaning to various blasting methods, the appropriate method is selected according to things such as the type of graffiti paint, the material of the substrate, and local and legal regulations. In addition, preventative options exist so that unwanted graffiti can be removed more conveniently and more gently next time.

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A person wearing protective clothing and a helmet cleans a natural stone facade with a Kärcher Pressure Washer.

Use a pressure washer with hot water and steam for natural stone that is lightly soiled or overgrown with moss and lichens

Pressure washing with cold water is common practice in commercial cleaning. With the right nozzle, water flow rates of up to 1,000 l/h and a suitable impact pressure that doesn’t exceed 2 bar, good results can be achieved with this method when cleaning natural stones. Often pressure washing with hot water or steam is regarded as nothing more than a way to expedite the cleaning process, however using hot water and steam offers other advantages as well.

Pressure washing with hot water, in particular when using the steam setting, gently removes animal soiling and biogenic growth such as algae, lichens and moss from natural stones. The fine droplets can penetrate the porous spaces of stones and with a temperature of approximately 95 °C, the steam is able to kill residual biological particles and plant spores. This thermal denaturation ensures a deep clean and delays new growth, extending the period of time between cleans.

Tip – Half the water output is all it takes:

You can achieve great results with only half the water output (approx. 650 l/h) by using the steam setting. By holding the nozzle at a distance of 10 to 20 centimetres from the surface you want to clean, the surface pressure won’t exceed 0.5 bar.

If you are dealing with acid-sensitive stones that require a cleaning agent, the cleaning product must be alkaline or neutral. Before starting to clean, you’ll need to determine whether the stones are open-pored, i.e. whether they are absorbent. If they are, make sure to pre-rinse the stone facade with water before applying the cleaning detergent, otherwise the cleaner may penetrate the stone and cause damage.

Sandstone surfaces should be handled with extreme care

A special case in the cleaning of natural stone facades is work done on sandstone facades. Sandstone is a sedimentary rock and is not as sturdy as its igneous and metamorphic counterparts. As a result, it requires special care when cleaning. It’s important to note that many listed buildings have a sandstone facade.

Use the right cleaning technique to avoid damage when cleaning sandstone facades

Due to the nature of its formation, sandstone is very soft and doesn't have a stable structure. If improper cleaning techniques are used, individual grains can become detached from the facade. A thorough inspection and a testing of sample surfaces should be carried out together with the owner and restorers of the building to determine which cleaning method should be applied. In the case of listed buildings or buildings in conservation areas, consent must first be obtained in order to clean the building.

Over the course of decades, dust can accumulate from environmental pollution and emissions, especially in rain shadow areas, i.e. in places where the rain cannot wash away dirt. These contaminants, as well as biological soiling, can be gently removed by using the steam setting of a hot-water pressure washer. If hardened crusts have formed over time, low-pressure grit blasting can be used after first testing a smaller sample area. Alternatively, poultices can be used for desalination or to remove various types of residue.

Monument cleaning and restoration

Cleaning and restoring monuments is a task that presents a unique set of challenges. One of the key considerations is the risk of damaging these precious and historic objects with incorrect procedures. In order to determine the appropriate cleaning techniques, a thorough analysis of the materials, type and level of dirt, as well as the shapes and structures involved is necessary. This often involves testing possible cleaning methods on sample surfaces to ensure their effectiveness and safety. Some of the available options for restoration cleaning include low-pressure particle blasting, hot-water high-pressure cleaning with a steam stage, dry ice blasting, compresses, or even laser technology.

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