Q. Why do I have to remove the wall plaster if I am having a new damp course put in?

Submitted by: PH - Isle of Man
 

A. If you are having a new damp proof course installed it is assumed that rising damp is present.

Rising damp in a load bearing wall at ground floor level will almost certainly be water originating from the ground. Dissolved in ground water are naturally occurring salts that are transported in solution up and into a wall affected by rising damp. As salt is only able to exist as a liquid or solid it is left behind when the water in which it was dissolved evaporates.

In a rising damp situation evaporation takes place at or near to the surface of a wall which is usually the plaster. Over a period of time, with on going evaporation, the concentration of  salts in the plaster increases with the highest deposits being near to the maximum height of the rise of moisture. This is referred to as the salt band.

The salts that cause problems in a rising damp situation are chlorides and nitrates both of which are what is termed hygroscopic. This means they have the ability to attract moisture from the atmosphere. Therefore even though a new damp proof course has been installed successfully, unless the chloride and nitrate contaminated plaster is removed, the decorative surface of the wall will remain damp due to the hygroscopic action of these salts.

The amount of dampness attracted by hygroscopic salts will vary subject to the amount of water vapour in the atmosphere and the level of salt contamination present. It is possible therefore for a damp patch to be evident on a wall one day and for it to have disappeared the next day purely as a result of hygroscopic salt contamination and climatic conditions.

Therefore the reason why it is necessary to remove wall plaster from a wall known to be affected by rising damp is to remove the chloride and nitrate salt contamination that will have accumulated in the wall plaster. Always make certain that the re-plastering is carried out in strict accordance with the manufacturer’s specification of the damp proof course material being used otherwise there is a risk of further contamination occurring and on going problems.