Dreaded rising damp

Rising damp is widely misdiagnosed in existing buildings, based on the incorrect interpretation of visual evidence.

Rising damp is the upward movement of ground water through a permeable masonry wall. The water rises through the pores in the masonry via a process called capillarity. Rising damp is also a problem if it affects vulnerable materials or finishes that are in contact with the wall. For example it affects timbers such as skirting boards or the bottom of door frames. It causes wall plaster and wall coverings to deteriorate and become stained. Whilst injected chemical damp-proof courses may provide useful short to medium term protection for certain types of structures, this application is rarely the most cost-effective way of controlling damp problems in buildings, and may be wrongly specified and ineffective.

Cause and effect
When there has been a long-term problem with moisture penetration, evaporation at the edge of the damp area leads to a distinctive ‘tidemark’ as a result of salt deposition. Where this occurs at the base of a wall, the tidemark is often taken as a typical diagnostic feature of ‘rising damp’. However, these salt accumulations may remain even when the water penetration that originally caused them has long gone. Similarly, water penetration may have occurred from causes other than ‘rising damp’. The most common source of moisture in the base of the walls of buildings is from defective ground and surface drainage. This is present to some degree in almost every building in the country, due to a combination of ground levels, ineffective ground drainage systems, and the increased use of concrete or paving around buildings without consideration of drainage slopes. Damp masonry at the base of walls may lead to a number of problems:

  • The moisture in the structure may rise to a level at which decay organisms may grow, or the materials themselves may be adversely affected. For example, timber skirting boards along the base of walls may become infected and decayed by dry rot, wet rot, weevils or woodworm.
  • In very damp conditions, the building materials themselves may lose their structural strength.
  • Damp conditions on the surface of walls, particularly in conjunction with condensation, allow the growth of moulds both on the surface and within porous or fibrous materials, such as wallpapers or carpets fitted against the base of the wall. Not only is this aesthetically unacceptable and damaging to finishes, but it can be a significant health hazard to occupants.

Treatment options
As described above, ‘rising damp’ in the base of walls, is rarely the primary source of moisture. The correct defect of the moisture source must be identified before the most cost-effective solution to the problem can be determined. Damp and its effects may then be controlled by one or more of the following measures:

  • The provision of suitable moisture sinks or drains to dissipate the moisture at its source without causing problems to the structure and the repair of any defects acting as moisture sources, such as broken pipes.
  • The introduction of damp-proof membranes or materials to form a ‘damp-proof course’ or ‘chemical damp-proof courses’.
  • The isolation of vulnerable materials such as timber and interior finishes from damp fabric.

Moisture barriers
The control of moisture movement using damp-proof materials to create a less permeable ‘moisture barrier’ is not necessarily a cost-effective option in controlling damp problems. It may even be counter-productive. This is because impermeable materials will restrict moisture and prevent drying. As a result, moisture may be ‘locked’ into damp walls for many years causing chronic problems. Moisture may also be prevented from dissipating from permeable materials, resulting in the build up of moisture or even damper walls. This may result in moisture moving into previously dry structures or evaporating from previously unaffected surfaces, causing further problems. One reason why those injecting ‘chemical damp-proof courses’ generally insist on re-plastering treated walls with a salt-proof and waterproof mixture, is to cover up these potential problems.

If it is decided that a moisture-barrier at the base of the wall is essential, the most reliable method is to use a physical barrier rather than a chemical one. This involves ‘cutting in’ a layer of damp-proof material to form a barrier. As the wall above this barrier will remain damp for some time, it is then necessary to isolate all vulnerable materials above the barrier, such as skirting boards, from the base of the wall with a damp-proof membrane.

‘Chemical damp-proofing’ may provide a useful barrier to damp in the short to medium term where the walls are of uniform construction such as sound brickwork laid with strong cement mortar, especially if they are combined with building detail which allows moisture to dissipate. However, any gaps which are left, or which appear over time as the chemical barrier deteriorates, may lead to an accelerated decay. This ‘damp proofing’ is unreliable where walls are of natural stone, because the injected material will not fill all cavities where needed, especially when the wall is made up of different types of materials.

Surface water drainage

The most cost-effective way of preventing damp problems in buildings, is to minimise moisture ‘build-up’ and provide adequate run-off to dissipate any water. One should start with adequate ground drainage and sloping around the building to minimise water at the foot of the walls. Buildings with no gutters can also be a problem as rain water falls to the ground and splashes into the walls. This water can penetrate through very porous paint and thus takes longer to dry out, especially in wetter conditions and if there is less sun on that side of the building. Consider either installing gutters to lead the water away, or using a less porous paint.

Wall construction

If the exterior walls are plastered, make sure they are plastered below ground level and are not cracked. Cracked plaster allows water to penetrate into the foot of the wall and prevents drying, thus forcing the damp through to the interior. Also consider using clay bricks for the foot of the wall to a height of roughly one metre, as they are less porous than cement bricks. Constructing a wall with a cavity may provide a way of dissipating moisture and preventing moisture penetrating into the interior. The cavity must be ventilated with air bricks or ducts. Make sure the cavity is not bridged with debris or other types of insulation. These defects can also bridge existing damp-proof courses, allowing water to penetrate to the interior. In some cases, the most cost effective solution would be to put ventilation ducts in. Failures in existing damp-proof could be the result of bridging by repairs and alterations, by raised ground levels such as filling and flower boxes, or by damage due to structural movement or poor building work. If a damp-proof course is an original design to control moisture in the structure, it may be necessary to repair the said fault. This is best done by ‘cutting in’ a new layer of damp proof material rather than by the injection of solvents and chemical damp-proofing.

As described earlier, walls should be kept ventilated and not sealed. This can be achieved by using through-ventilation. In all cases, the cavity behind should be ventilated at the top and at the bottom to allow through-ventilation to dissipate moisture, as moisture will accumulate to cause damp and decay problems. This happens when insulation material or debris blocks the cavity.

Even with the loss of skills and complexities in building with new materials and new styles, the conditions resulting in damp to the base of walls can easily be avoided or solved with a little thought and understanding. Indeed, new materials and techniques can often be used to advantage if their properties are analysed. In contrast, the misdiagnosis of rising damp and the general application of the incorrect products and techniques, leads to the unnecessary waste of limited budgets available for  maintenance and refurbishment. ‘Treat the cause not the symptom.’