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Present a Clean Face

11 May 2011

Pollution, algae, limescale and graffiti can all soil a building’s brickwork. Adam Stone’s rough guide to façade cleaning explains the different processes employed in cleaning brickwork to ensure FMs create the right first impression

FIRST IMPRESSIONS COUNT so it is important to ensure the exterior of buildings are maintained to create a good impression.
There are now a number of processes that FMs can chose from to clean their building’s stone and brickwork. However, it is essential to seek advice from an expert since aggressive cleaning could irreparably damage the fabric of a building resulting in costly restoration.
Before cleaning begins FMs need to consider a number of issues:
● the type of soiling
● the cleaning methods available
● the surrounding building fabric
● the location of the works.
● has it been previously cleaned and how?
During the 1970s and 1980s there were a number of aggressive methods used to clean building facades. As a result stone or brickwork that has previously been cleaned may be more delicate than expected and some modern cleaning methods may need to be ruled out to prevent further damage.
Some of the most common types of soiling or coating encountered on a façade range from algae, limescale, carbon soiling, masonry paint, textured coatings, lime-wash and distempers, oil-based paints and graffiti. Each one of these conditions will require a specific cleaning method, robust enough to be effective but applied with sufficient restraint to prevent damage. It is wise therefore for FMs to seek expert advice when selecting a cleaning method.
For removing algae it is important for FMs to select a cleaning method that not only removes the algae but also works to prevent re-growth. For delicate or historic facades nebulous sprays, also known as intermittent spray mist, can effectively remove algae, although a biocide is needed to kill the spores, a light scrubbing can also help.
Nebulous sprays produce a fine mist of water through nozzles set up at intervals across the building. Nebulous sprays are probably the least aggressive form of façade cleaning, however it does use significant amounts of water and water ingress can be a problem. Water applied to masonry can risk mobilising minerals and compounds within the stone resulting in staining, efflorescence and eventually exfoliation. Nebulous sprays are, therefore, better suited to facades where joints are less frequent, such as limestone ashlar work, rather than brickwork.
An alternative and straightforward method is steam cleaning. DOFF is a low pressure steam cleaning system that removes algae at high temperatures, killing any remaining spores and, therefore, inhibiting future growth. Repeated passes with the cleaning lance at low pressure softens and removes the algae gradually and avoids leaving streaks on the façade that high pressure single passes can produce.
Light scrubbing of the softened matter with a phosphor bronze brush further assists with algae removal. This is a fairly risk-free cleaning system and although some water ingress into the building structure is to be expected, the volume of water used is relatively low by comparison to other methods. The DOFF system is, therefore, better suited to cleaning brickwork where the joints represent a significant absorbent area.
Limescale build-up can be removed chemically using a hydrochloric based cleaner that effectively dissolves and breaks down the calcium. The challenge with this process is the timing. The chemicals need sufficient dwell time to remove the limescale but not so long that they damage the surrounding masonry. As a result this will often mean repeated applications
are needed, followed by low pressure rinsing.
Very heavy limescale deposits will need to be reduced mechanically before the chemicals are applied to ensure it works effectively. Furthermore, since hydrochloric based cleaners are corrosive, water run off must be managed carefully and to avoid damage to the surrounding masonry, preferably prevented from running down the building. Water run off must also be disposed of correctly to avoid contaminating the local water course.
Although acid cleaning is ideal for removing limescale, it was previously used widely on facades as a general cleaning method. However, alongside sand blasting, acid cleaning is now rarely specified because it has long-term effects on a buildings fabric. The acid not only breaks down the soiling, but slightly dissolves the face of the stone or brickwork making the façade much more susceptible to future soiling.
Carbon soiling affects the aesthetics of a building making it look dirty and badly maintained. FMs can tackle heavy carbon soiling effectively by using an airborne abrasive system such as the ‘Jos’ swirl abrasive system. This method works by an abrasive media rotating in a mixture of air and water being directed onto the building façade’s surface at pressures as low as 5
psi. This produces a scouring action on the surface, this is significantly gentler than the impact pattern produced by conventional blasting systems. The versatility of the system also allows operators to work closely with the façade surface enabling them to concentrate the cleaning action more precisely, avoiding gun shading or streaking that other higher pressure, less versatile methods can cause.
An alkali cleaning process is another option that is occasionally specified for removing stubborn carbon soiling. This method involves applying an alkali-based cleaning gel that has the ability to soften the carbon build-up, thus allowing the carbon to be removed by the use of phosphor bronze ‘lamb chop’ brushes and rinsing. Once the cleaning is completed it is essential to rinse the façade and test its PH to ensure the surface is neutralised, protecting it from contamination that could lead to accelerated deterioration of the stonework or brickwork and efflorescence.
Since the introduction of the Clean Air Act and the subsequent improvement in air quality, resoiling of building facades is mostly water soluble and so can be removed easily without causing damage to the stone or brickwork. Lighter soiling can be successfully removed by nebulous water cleaning, or the DOFF system.
Masonry paint stripping is challenging and often requires the removal of many tenacious coatings from delicate stone that can range from lime washes and distempers through to modern textured coatings, or even spray applied elastomeric weather coatings.
FMs should take a cautious approach when removing masonry paint. Before the project starts it is essential to establish with your specialist contractor what you hope to achieve.
A strategy must be discussed and agreed to ensure the right balance of cleanliness against potential for brick or stonework surface deterioration. Often the most successful chemical stripping process is the use of sodium hydroxide based poultices. These are safely applied and capable of dissolving, or at least softening, a wide range of coatings which when
peeled away from the surface, often then only require a DOFF clean to rinse down, leaving the surface intact.
Graffiti on building facades is an increasingly frustrating problem for FMs. Removing graffiti from stone and brickwork needs careful handling and expert knowledge since contractors need to use a raft of chemicals to effectively remove a vast array of mediums used by the ‘artist’. These mediums can range from metallic spray paints and marker pens to lipstick. Most of these require a different chemical to remove them.
The most common ingredient used for graffiti removal is methylene chloride, which, due to its hazardous nature, requires full COSHH compliance. This is of particular importance if used in a pedestrian thoroughfare and would require exclusion from the works area.
FMs have a number of cleaning methods at their disposal to clean stone and brickwork.
However, when cleaning historic buildings it is essential for FMs to seek advice from contractors with expert knowledge. Remember to consider the H&S implications at the planning stage.
● Adam Stone is technical director at historic buildings contractor Cathedral Works Organisation (CWO)

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