Damp Information
Implications of rising damp
One of the problems caused by ingress of ground water in a brick wall from rising damp or low level penetration is the presence of local salts. These are present in the earth whether they be natural or sometimes in greater abundance due to contamination from fertilisers. The salts (Nitrates, Chlorides and Sulphates) are soluble in water and so become transported from the ground into the walls and plaster. The water then evaporates from the wall and leaves the salts usually randomly but sometime quite concentrated in the mortar joints of the bricks and in the plaster layer of the internal wall. Thus contaminating the wall. The downside of this is that these particular salts, are capable of attracting atmospheric moisture such as water vapour and humidity in the house back into the wall via the contaminating deposits. Which means that the new damp proof course installed to arrest further groundwater from rising does not mean that a wall will automatically dry out. Even if it did for a short while sooner or later humidity will activate the salts (Hygroscopic activity) and the existing plaster will begin to show damp patches again.
A certain way of curing this phenomenon is to remove the contaminated plaster and reinstate with a damp resistant plaster or a sand and cement barrier, (Render) which will allow old moisture to continue to evaporate so that given time excess moisture leaves the wall permanently. To ensure that new plaster does not become contaminated it is recommended that old plaster is left in place for at least two weeks after the new damp proof course has been installed. This will allow the semi solvent salts in the wall to migrate into the plaster layer as final evaporation takes place now that the wall isn’t being fed by new moisture. So that by the time the old plaster is removed it contains a significant amount of active salts purged from the brickwork which are then disposed of, thus allowing new plaster to remain free of migrant salts for many a year to come.
