Rising damp is most likely to be found on the ground floor of very old houses or blocks of flats. It is rarely seen in flats above the ground floor.
Where external walls are constructed of permeable or absorbent materials such as brickwork, stone or plaster, and there is either no damp proof course or it has become defective, such constructions allow moisture from the ground under and around the building to rise up the walls until internal plaster finishes become saturated. This then leads to mould growth and the degradation of the finish. Eventually the fabric of the wall will begin to degrade too.
The usual height at which rising damp will be seen in plaster finishes is usually up to 1 meter above ground floor level, and it usually shows as tide marks because of the evaporation of water and salts from the ground. If these are not present then another common sight is that of brownish/yellow damp patches or staining above the skirting board and up to around the same height.
Loosening and curled up wallpaper or peeling paper at the skirting board is another clear pointer as to the problem.
The usual solution for this kind of damp is to drill holes at regular intervals around the perimeter of the affected area (just above external ground level). A damp proof course is then applied (DPC) with a pressurised chemical injection into the holes. The chemical is absorbed into the masonry, forming an effective barrier to moisture traveling upwards by expanding into a line across the brickwork. The wall is then re-rendered using a waterproof rendering system
Before the affected interior can be redecorated, the decorations and plaster-work must be removed to get rid of most of the hygroscopic/deliquescent salts which will have accumulated. If decorations and plaster are not removed, any salts that stay in the masonry will return and show through newly decorated surfaces. Walls take time to dry down and the lower part of the walls are always likely to remain damp because of the limitations of the chemical injection systems in terms of controlling the rising damp. So chemical injection damp-proofing is a 2-part process: the injection to ‘control’ the rising damp, and the replastering (inside) to prevent future show through- the processes are inseparable.