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.
If the building has a damp proof course then it should be visible in one of the horizontal mortar joints just above the external ground level. If it is in isolated places, this usually means a defective or bridged damp proof course with the latter occurring where abutting features (i.e. steps, ramps, walls), or simply high external ground levels, allow moisture to pass into the external wall of the building above the damp proof course.
If it is defective then it is necessary to cut out and replace the defective portion.
Injected damp proof courses must be installed by specialist contractors, who usually supply a guarantee but before installation, it is necessary to remove any internal plaster finishes from all affected walls up to one metre above the internal ground floor level. This will get rid of most of the hygroscopic/deliquescent salts which will have accumulated.
The usual solution for rising 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
Once the damp proof course has been injected, the plaster can be replaced with new, waterproof plaster and any other areas made good, such as skirting boards. If decorations and plaster are not removed from the affected areas, any salts that stay in the masonry will return and show through newly decorated surfaces. Hardwall or lime plaster should not be used as both of these will absorb moisture from the brickwork and transfer it to the plaster top coat.
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.
An alternative is to cover the interior of the wall with a damp proof membrane and fasten a thin partition onto the face. Although this doesn’t entirely solve the problem then a) if the dpm is not breached, or b) there was never on put in, then it will at least provides a barrier between the wall and the new inner surface. This technique is often called “dry-lining”.