Cleaning Natural Stone

The cleaning of a building’s facade is often the most obvious aspect of building conservation, but is it always the best choice? If not done properly, or for the right reasons, it can cause significant damage either immediately during the cleaning, or over a period of time once the cleaning is complete.

Before deciding that you want to go ahead with cleaning your stonework, you need to consider the type of stone involved, what type of dirt you are going to remove, how that dirt is currently affecting the stone and how it will potentially affect the stone once removed.

Stone cleaning is not a decision to be taken lightly. In Scotland, since 1992, stone cleaning has been considered a building alteration. Therefore, any and all proposals to clean listed buildings require Listed Building Consent and for unlisted buildings within a conservation area, need planning permission. For England, it is recommended that you contact your District Council for advice on whether any form of permission is needed as it can vary from district to district and can also vary depending on the cleaning method proposed.

An old heritage building for example, may have had various repairs or extensions undertaken in the past and may therefore have different types of stone and mortar used; at the very least, the stone may come from a different quarry. This could result in the cleaning method not being universal for the entire facade.

Different stones all vary in resistance to cleaning. Some stones are very soft and porous – the softer the stone, the greater the risk of damage during the cleaning process. Some stones, like Limestone and Marble, are particularly vulnerable to etching and therefore any acidic cleaning products should be avoided when working with them.

Some stones are also more prone to suffering from staining than others, meaning that great care should be taken when deciding on how much water to use. If a material is more porous, the water can saturate the stone, which in turn can cause rotting to the internal joists. Additionally, it can even freeze during the winter months – creating a whole new plethora of issues.

Why do I need a Professional Stone Clean?

Some of the more common reasons for carrying out a stone clean include:

• To improve appearance
• To remove impervious paintwork
• To help blend in original stonework with new eg. following an extension or extensive repairs
• To slow down damage and decay due to deposits such as lichen and algae on the surface of the stone
• To help reveal the condition of the building so that cracks, missing pointing and other structural issues are more easily identified
• To allow the repair of structural issues such as cracks and missing pointing

The decision to clean and how to clean will largely depend on the materials present and the level of soiling. Whilst unsightly, lighter soiling doesn’t usually require any remedial action and usually doesn’t cause any harm. Heavily soiled stonework, however, can retain water and hold agents which are harmful to the stone.

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What can damage my Natural Stone?

Lichen

Lichen are symbiotic – a combination of fungi and algae. The fungi element seeks out the water and salts whilst the algae element manufactures the food in the form of photosynthesis. Lichen are not only unsightly, appearing as black, white and even orange spots on natural stone, they produce acids which can etch into stone, with Limestone, Marbles and finely carved stonework being at particular risk.

Algae

Algae prefers damp conditions which in itself could be indicative of a larger issue going on. Usually green, algae can also appear as red, brown or even blue. Just like lichen, algae produces acidic secretions which can damage stone. It also collects water and dirt – allowing for the potential to affect the stone’s transpiration rate (the water movement).

Moss

Moss retains water which runs the risk of freezing in the colder months, causing damage to the building envelope. Moss, like algae, is also indicative of excess dampness, which is a symptom of a much larger issue. Not only does moss secrete acid like lichen and algae, it can also cause rot within particularly soft stone.

Fungi

If they can find a food source, fungi will live on natural stone and produce acid, causing etching and decay in susceptible stone such as Limestone.

Bird Droppings

Not only can birds cause damage to the facade of a building by pecking at the salt, their droppings also release acid which can cause damage to certain natural stone. This can be significantly worse within urban environments, with the worst culprits being Starlings and Pigeons.

Acid Rain

Including mists and fogs, acid rain can directly dissolve stonework. The reaction between acid rain and the calcium carbonate in Limestone causes the Limestone to dissolve and crumble. A wet or dry deposition of sulfur dioxide significantly increases the rate of corrosion on Limestone, Sandstone, and Marble.

Professional Stone Cleaning

Before carrying out any stone cleaning, consider whether the dirt is actually harming the building. Is the soiling a symptom of another issue or the result of some form of decay or neglect? Once you are certain that you want to proceed with a clean, ensure you enlist the services of a professional stone cleaning company such as Ideal Response, who will carry out a site survey to ascertain the best method for cleaning your stonework.

Why Ideal Response?

Ideal Response are Stonehealth approved contractors and our methods include DOFF and TORC. Both methods use a minimal amount of water to complete the clean, which avoids saturation of the stone and the associated risks of damage; the stone will usually dry within a matter of minutes.

As no chemicals are used, the risk of a chemical reaction (such as etching) and decay are nulled. And as both DOFF and TORC are completely adjustable, it means that they can be controlled by our experienced technicians, mitigating the chance of accidental damage. Our technicians are able to start the clean using the lowest pressures and temperatures and gently increase either as needed.

To find out more about professional stone cleaning, call us today on 01622 926 505, alternatively you can request a call back by completing the enquiry form below.