Rising damp (or “salt-damp” as it is often called) occurs on the bases of walls. Water accumulating there has a tendency to ‘wick up’ through the capillaries that are present in the walls. Damp-proof courses are placed in the base of the walls below floor level to block this upward movement but sometimes they are ineffective. Aside from treating and curing the rising damp itself, you’ll find that other areas have been damaged by the rising damp and will need to be repaired.
Repairing Damaged Plaster
Plaster that has been wet because of rising damp will have reacted with the salts in the rising water. This process will have changed the composition of the plaster, making it more likely to attract and retain moisture. As such, you will need to replace the contaminated plaster or you won’t have a completely dry wall ever again!
It is best to remove that plaster to a height about 300 mm above the level to which water was observed to have risen, but it is also wise to wait several weeks - perhaps as long as three months between repairing the damp problem and replacing the plaster. This time period will allow the rising damp moisture to evaporate off the bricks, draw the undesirable salts into the plaster layer and thus enable that salt to be removed with the plaster.
Repairing with Waterproof Plaster
When rising damp has been caused by an inadequate damp-proof course, people sometimes apply waterproofed plaster to the damp areas. Sometimes rising damp is too smart to be cured by this and the result will be to simply cause the rising damp to move further up the wall and appear in what was previously an unaffected area.
To solve this problem, some contractors replace the entire surface with waterproof plasterboard. Others use a technique of attaching battens to affected walls and nailing on plasterboard, providing a 5mm to 10mm gap between the old surface and providing ventilation slots top and bottom. All timber should be decay-resistant and fastenings rust resistant.
All of these methods fall into the category of cover-ups rather than cures and consequently cannot really be considered permanent solutions.
Repairing Fretted Mortar
Where rising damp has caused mortar in external face brickwork to fret away and the cause of that dampness has been cured, the appearance of the brickwork can be restored by repointing the joints. If you do this, make sure you rake out the existing joints to a depth of 25 to 30 mm before repointing and don’t use too strong a mortar in that repointing. The maximum strength of the mixture should be equal parts of cement and lime plus six parts of sand (a C1 :L1 :S6 mortar).