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Rising damp, Salt damp, Efflorescent salts and Penetrating damp

Rising damp can often provide visual evidence of its presence via;

• Eruption of finishes or disintegrating plaster
• Staining of finishes i.e. tide marks
• Dampness to finishes

Rising damp is a physical phenomenon which can be defined as ‘the passage of groundwater up through masonry. The water rises through a series of interconnecting pores by a process loosely called capillarity; in other words, porous masonry can act like a wick. If the suction of the wall is greater than the suction of the ground then water will rise’. Graham Coleman, Remedial Technical Services

The height of which rising damp will rise is mainly dependant on the pour diameter in which the water resides, the connectivity of the pore structure and the evaporation rate of the moisture rising through the wall.

As the source of moisture which rises through the capillary network of the building fabric is generally from the ground, it brings with it ground salts, most commonly chloride and nitrate. These hygroscopic ground salts are visible as they rise to a point until the moisture evaporates leaving behind a visible damp band, usually located 1 – 1.5 meters up the wall. However, as described above, this height can be greatly increased depending on the building material and rate of evaporation.

Common causes of rising damp can be:

– a bridge in the DPC by plaster touching a damp slab;

– mortar beds bridging the DPC line;

– mortar droppings in a cavity wall connecting the external and internal leaf 

Often, a different type of salt contamination can be seen in buildings; a fluffy white material can develop on the internal face of a wall. This type of salt contamination is called efflorescent salts, which in French means ‘’to flower out’’. This type of salt is usually a sulphate and is very common around chimney breasts, where waste material from burning coal has contaminated the surrounding area.

Efflorescent salts can occur in render where the sand is contaminated with sulphates – they aren’t hygroscopic but they can cause render to blow when they form.

Degradation in the construction can also be a cause for dampness to enter the structure. This can be defined as penetrating damp and can cause moisture to appear on the internal side of the structure particularly in solid wall construction.

Cracks that appear in the structure or render which are exposed to prolonged moisture either by wind driven rain, faulty rainwater goods or inhibition of evaporation can be the main cause for penetrating damp.