How to clean your heritage building?

Mark Styles, Managing Director of Thomann-Hanry® the façade cleaning experts, offers an expert opinion on the different methods of cleaning the facades of historic and listed buildings.

HISTORIC & LISTED BUILDING FAÇADE CLEANING

Historic and listed buildings, no matter the size, have become a key record of the expressions of culture across time within the built environment. The need to maintain and preserve these structures is crucial to ensure they can be enjoyed by future generations.

While interiors are normally the first thought, when it comes to upkeep and preservation, the building’s façade is an area which will also need regular, specialist conservation and cleaning to prevent the expense and inconvenience of major structural work as the result of neglect and decay.

WHY CLEAN

The modern urban environment in which many historic and listed buildings are situated, are havens of dirt and grime, much of which, while perceived a benign, can, over time, cause significant discolouration and surface damage to the façade’s substrate. Even in more rural areas, long term build-up of biofilm (a thin layer of algae and phytobacteria) and water damage can be just as harmful.

Regular cleaning and maintenance checks can spot areas of concern and quickly remedy them before they turn into more structural, time-consuming, and most importantly costly problems.

WHICH METHOD IS RIGHT FOR YOUR BUILDING?

Before you undertake any façade cleaning, you will need to understand the structure and materials which are used on your building’s façade. This simple starting point will allow you to form a considered approach to the types of cleaning which can be undertaken for your historic or listed building. Using the wrong method, can cause both short- and long-term problems which can add unexpected time and cost considerations.

METHODS OF CLEANING

 

With historic and listed buildings, the cleaning methods used to maintain the building’s façade has several constraints which will need to be considered. The most important is that the cleaning technique must not cause direct or indirect harm to the building’s surface. Any cleaning method used should also preserve any masonry patina, not deposit or generate by-products which affect future preservation works, and the method should not initiate new decay processes or exacerbate old ones.

WET CLEANING METHODS

There are three main water-based options when looking at cleaning the façade of a historic or listed building, systems which use water at variable pressures with high temperature such as DOFF, systems which use water and abrasives at a low pressure such as TORC, and systems which use constant water misting.

With these water-based systems a good surface clean can be achieved, but key areas of concern include high usage rates of palatable water, over-saturation which can mobilise salts within the façade causing corrosion of metal fixings and efflorescence, the risk of frost-damage during the colder months (which can lead of to intricate carvings cracking and braking). With the constant water misting systems unsightly, time-consuming, and expensive scaffolding will be required, more persistent staining will require scrubbing or mechanical removal, can cause staining on limestone/sandstone, creates conditions favourable for rot in timber structures, and is ineffective on tightly bonded soiling.

DRY CLEANING METHODS

There are two key types of dry-cleaning methods, abrasive cleaning and façade gommage cleaning. With abrasive cleaning methods a medium to high pressure is used with an abrasive medium such as sand, shot, or ground-up nut shells to chip away at the surface of the dirt and grime. With this type of cleaning a good surface clean can be achieved but it can cause damage to the façade’s surface especially on softer stones such as sandstones and limestone, repeated use can, over time, cause highly decorative and intricate areas to be damaged or worn away by this method.

With the façade gommage cleaning method, ultra-fine Aluminium silicate powder is blown under low pressure over the surface of the façade, gently rubbing away the surface dirt and grime. Unlike most wet and dry options, scaffolding isn’t required so reducing the time taken to perform the clean and no temporary structures obscuring the building (an ideal choice for hotels and retail businesses). The system also offers one of the best end results, taking a building back to the original façade colour while preserving any patina which may be present.

CHEMICAL & SPOT SURFACE CLEANING METHODS

When smaller more specific areas need cleaning, such as graffiti removal or more stubborn or encrusted stains the use of specific cleaning methods such as chemical treatments, which are more commonly used for paint and marker graffiti on the sides of buildings. These need to be patch tested to ensure that no reaction to any of the materials on the façade.

Poultice application can also be used, but once again only on small areas after patch testing has been carried out. These topical pastes are applied and left to remove stains, but with typical ‘dwell’ periods of 24 to 72 hours, these are slow and may require additional applications for them to be effective.

Lazer and cryogenic cleaning methods use ablation as a way of removing encrusted dirt and grime. Once again this is not suitable for larger areas of soiling.

FINAL CONSIDERATIONS

It’s not just the method which needs to be considered when looking to clean and refresh a historic or listed building’s façade. The company you are looking to employ will also need to show that they understand the need for sympathetic cleaning for these types of buildings.

For those buildings which have listed or scheduled monument status, working with a company who understands the constrains which will need to be applied to any cleaning methods is vital. Ask to see examples of the types of buildings which they have previously worked on and how they were able to execute a sympathetic façade clean.

Versions of this article have previously been published in FMJ (April 2025) and Heritage & Conservation Journal (June/July 2025)

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