Salt damp

Salt damp is perhaps one of the commonest causes of dampness. Many sources of moisture are not pure but contain minerals and other contaminants. For example, ground water is often contaminated with chloride and nitrate salts. As water rises up into the wall and subsequently evaporates away these salts are deposited in the masonry and, over time, can build up to very high concentrations.

Chlorides and nitrates are not efflorescent so they do not appear as fluffy white crystals but, instead, are hygroscopic, readily absorbing moisture directly from the air and causing damp staining. The distinct wetness that can be seen on a wall affected by rising damp typically results from high concentrations of hygroscopic salts. In fact, the highest concentration of these salts is usually found to correspond with the highest point of the damp rise – often a height of between 500mm to 1000mm above floor level – this region referred to as the ‘salt band’.

Hygroscopic salts are not just associated with ground water or rising damp. High levels of these salts can be found in chimney flues and fireplace areas. In addition, nitrates often affect the masonry of barns, stables, and other agricultural buildings.

This wall has been affected by rising damp.  The 'salt band', which corresponds with the maximum height of the dampness, can be clearly seen.  Typically, the only cure for this problem is to remove the affected area of plaster and to replace it with a salt resistant plaster.  It is also essential that the original cause of the dampness is corrected.

The combustion of fossil fuels and wood releases hygroscopic ammonium, chloride, and nitrates salts. The salts migrate through the plaster, causing damp patches to become visible on the chimney breast, as shown in this image.

Former agricultural buildings can be affected by very high levels of salts, particularly nitrates that derive from animal waste and farming activities.  The plasterboards seen in this image have been fixed to the face of an old stone wall of a farm building during conversion works.  Plasterboards, and the adhesive that is used to secure them in position, are very porous.  The hygroscopic salts present in the old masonry migrated through to the surface of the plasterboards where they caused significant damage to internal finishes.

This image shows a property extension.  To the left is an old farm building, with the new extension featured on the right.  The salt damaged dry lined wall in the picture above is the former external wall of the barn.

Here a fireplace has been opened up and lined with plasterboard dry lining. Hygroscopic salts present in the brickwork have migrated through to the surface of the boards. The adhesive 'dabs' securing the plasterboards can be clearly seen as circular damp patches.

This wall was treated with a chemical injection damp proof course.  The plaster had been replaced up to a height of about a metre.  The presence of hygroscopic salts has caused the old plaster to deteriorate.  This situation can arise because either the old contaminated plaster was not fully removed or because the newer moisture resistant plaster   forces evaporation to take place higher up the wall.

Salts on this wall are clearly visible on the external brickwork and form a distinct band 1.0 - 1.5m above the outside path level.  The white deposits are efflorescent salts - most probably sulphates - which are not hygroscopic.  However, the darker staining on the same area of brickwork will be a result of hygroscopic salt contamination.

In this internal image of the property pictured above, the efflorescent salts can be seen on both the brickwork and the face of the plasterboard lining. The yellow discolouration is caused by hygroscopic salt contamination - in this case nitrates - and corresponds with the position of the salt band at the highest point of the damp rise.

Hygroscopic salts do not form crystals.  The white, fluffy salt crystals seen on the base of external walls are often sulphates.  Sulphates are not hygroscopic and do not absorb moisture from the air or cause the wall to be damp.  However, as the sulphates crystallise they take up a greater volume than they did when in liquid form.  This process can disrupt the surface of masonry and can cause plaster and renders to debond from the wall surface.  In addition, sulphates attack cement based materials and cause them to break down.  This can lead to structural issues and other damp related problems.

Sulphates affecting this wall have crystallised on the masonry behind the plaster causing it to debond and fall away from the wall surface.  When the plaster is reinstated, it is essential that precautions be taken to control the effect of sulphates.

This property also shows sulphate contamination which has caused the plaster to deteriorate and fall away from the underlying brickwork.

A typical damp survey does not include salts analysis.  In many instances, a skilled surveyor can ascertain if hygroscopic salts are likely to be present.  However, in some situations, this is not straightforward, and Britannia Preservation's surveyors may wish to test for the presence of particular salts to assist in diagnosis and to assist in the design of appropriate remedial methods. In these circumstances, salts analysis is undertaken on samples of masonry, plaster, or decorative coverings removed from a wall.

This is a result of salts analysis carried out on a plaster sample. The test is colour-metric: the red colour of the liquid in the smaller container shows that the sample contains nitrates, and the yellow colour of the liquid in the larger container reveals chloride contents. The deeper these colours, the higher the concentration of salts.