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Damp staining on chimney breast?

Posted by: on 03/03/2022

Advice

I get a lot of calls from clients who have mysterious damp patches appearing on their chimney breasts and more often than not the cause of the damp staining is due to 'salt damp'. . This is a common defect associated with chimney breasts. The salts stem from the burning of fossil fuels, over many decades in the fireplace opening. Salt from the wet coal and smoke, migrate into the surrounding masonry.

The salts are hygroscopic (will readily absorb moisture from the atmosphere) and this results in persistent wetting and drying of the wall plaster depending on the relative humidity within the property. As the humidity rises the salts absorb the atmospheric moisture to the point where they reach saturation, and the salt will then dissolve into a liquid form. When the humidity decreases the water will evaporate and the salt reverts to its solid form. This process is known as ‘deliquescence’.

The damp staining can be intermittent and is very much dependent on the humidity within the property at a particular time. Trying to coat the affected areas with a damp or stain block is pointless as the salts will quickly bleed through such coatings. Remedial work often involves the removal of the salt contaminated plaster and the subsequent replacement with an appropriate remedial system for which I would recommend that you seek expert advice.

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