There are many forms of damp in a building, and the term damp is often used without understanding what the real cause or effect is, and how it can be remedied.
The best approach is to understand the causes, and identify the solution, but always remember that every home, or building will have moisture either in the structure or in the air.
Even in a normal ‘dry’ building, there is always a surprising amount of water present in porous materials, most of which does no harm whatsoever.
Although the amount varies widely, depending on the nature of the material and on the humidity of the surrounding air, the following figures indicate the range that may be expected in some common materials:
- plaster 2 – 1.0% wet weight;
- lightweight concrete >5%;
- timber 10 – 20%.
These amounts of moisture do little harm to the materials and such moisture is not usually regarded as dampness. That term is commonly reserved for conditions under which moisture is present in sufficient quantity either to become directly perceptible to sight or touch, or to cause deterioration in the decorations and eventually in the fabric of the building.
A building is considered to be damp only if the moisture becomes visible through discolouration and staining of finishes, or causes mould growth on surfaces, sulfate attack or frost damage, or even drips or puddles. All of these signs confirm that other damage may be occurring.
A high proportion of dampness problems turn out to be one of the big three:
- rain penetration;
As these are so common, it is easy to overlook other causes but, before forming an opinion, these other causes of dampness should be investigated:
- construction moisture;
- pipe leakage;
- leakage at roofing features and abutments;
- ground and surface water;
- contaminating salts in solution
The causes of condensation are quite complex. When making an initial dampness diagnosis, there are several distinctive features to look for:
- Condensation normally occurs only in the coldest months of the year
- Trouble starts on the coldest internal surfaces: external walls, particularly corners, single-glazed windows, cold-water pipes, wall-to-floor junctions, lintels and window
- Damp patches sometimes have definite edges on cold spots such as lintels; patches of damp or mould in exposed corners are crescent-shaped
- Condensation occurs most often in rooms where large amounts of moisture are produced, such as kitchens and bathrooms, and in unheated rooms into which moisture has migrated to outside walls
- It is common in rooms where flueless paraffin or butane heaters or unvented tumble driers are in use, or clothes are frequently dried
- It often concentrates in areas where air movement is restricted, such as behind furniture or inside cupboards on outside walls
Rain penetration occurs most often through walls exposed to the prevailing wet winds, usually south-westerly or southerly.
Even if rain penetration is certain to be the cause of the dampness, pinpointing the exact route that the rain is taking can be quite difficult. A damp patch on a ceiling could be due to a missing roof tile or to a faulty flashing some distance from the patch. Materials in parapets and chimneys can collect rainwater and deliver it to other parts of the building below roof level, unless they have adequate Damp Proof Courses (DPC) and flashings. Blocked or defective rainwater goods can lead to damp patches on walls that appear to be straightforward rain penetration.
The results of rising damp in walls leave characteristic signs. There is usually a fairly regular, horizontal tide mark, up to a couple of metres above the floor. Below it, the wall is discoloured with general darkening and patchiness; there may be mould growth and loose wallpaper. Hygroscopic salts brought up from the ground tend to concentrate in the tide-mark. In severe cases, it may cause rot in skirtings or dados.
If there is a physical DPC, it is unlikely to have failed. But it could be bridged by pointing or rendering, or by soil, paving or rubbish heaped against the wall outside, or by plaster inside. In a cavity wall it could be bridged by a build-up of mortar droppings.
If there is no DPC, the presence of rising damp must be confirmed by correct diagnosis before deciding to put in a remedial DPC.
Rising damp can affect solid and suspended floors. In solid floors, it is occasionally due to a faulty or missing DPM. Dampness in suspended timber floors often becomes evident by the discovery of wet or dry rot, starting in the joist ends.
In a wholly or partly new building, the fabric contains water used in concrete, mortar and plaster. In a typical brick-and-block, semi-detached house, about 8000 litres of water can be used for mixing. This can take a long time to dry out; for example, a 150 mm-thick floor slab may take about a year. In addition, bad weather during construction may have saturated the building before it was closed in. Water can also be trapped in the fabric of older buildings which have been open to the weather during repair.
Over time, even a small leak in a water supply, central heating, drainage pipe or rainwater goods can cause extensive dampness, often some distance from the leak. The dampness can easily be mistaken for rising damp, rain penetration or condensation.
Leaks at roofing features and abutments
Blocked valley gutters and downpipes can cause rainwater to pond and overspill the flashings. Parapets and chimneys can become extremely wet and, in the absence of effective damp-proofing, water will drain downwards to other parts of the building, showing as damp patches in rooms below.
Persistent or recurring spillages can occur from tanks, cisterns, washing machines and dishwashers. Frequent floor washing can also cause problems, for example in kitchens in institutional buildings. Water running through cracks or joints in an impervious floor covering can spread underneath and may reach areas where drying out is either impossible or which may take a considerable time to complete.
Ground and surface water
Water can seep into ground floors or basements from ground or surface water, or from repeated flooding.
Walls and floors can become contaminated by hygroscopic salts causing damp patches to form.
There are several proprietary solutions that will remove growths of these organisms but use only those which have been given a number by HSE. You can use wet brushing or, in the worst cases, hosing with a power jet, provided care is taken not to damage the surface or to drive water between the laps of sheeted or tiled surfaces. Future growth may be inhibited by fitting small diameter copper pipe or wire above the area to be protected but the disfiguring stains that will result may not be visually acceptable.
Copper chimney flashings inhibit organic growth only in the areas immediately under the flashing.
Health & Safety Executive Guidance Note EH 36, Work with asbestos cement, explains how to clean old asbestos cement roofs. Asbestos cement must not be brushed when it is dry. It must be thoroughly wetted to minimise the release of fibres from the material, and dust masks of the appropriate rating should be worn. Mould growth can be killed by toxic treatment but this provides only a temporary solution; the mould will return unless the source of moisture is removed. An essential first step is to identify the cause of the dampness, and then to remove it.
Do not remove mould growths by dry brushing or rubbing, as heavy growths release large amounts of spores into the air which occasionally induce allergic reaction. Use a vacuum cleaner then dampen the infected area with a 1:4 solution of domestic bleach in water containing a small amount of washing-up liquid. Wipe down the surfaces with a damp cloth rinsed out regularly. Wooden window frames may need several applications. Keep windows open to promote dispersion of spores and moisture and wear appropriate protection.
It is not easy to decide if mould is just growing upon the surface or has penetrated further. Where decorations can be stripped off (distemper, wallpaper, polystyrene tiles, flaking paint) it is often best to do so, but where the growth is slight it may be sufficient to clean down without stripping.
Before re-decoration, sterilise the stripped or cleaned surface with an approved wash and keep it under observation. At least a week is necessary but longer is advisable. If mould reappears, wash it down again with the toxic wash to ensure sterilisation is thorough.
Clean fabrics and soft furnishings by sponging affected areas with a solution of an approved wash, not bleach. Apply the solution sparingly, first testing a small, insignificant area for any adverse effects; dry thoroughly afterwards.
There are many products for use as toxic washes though not all are widely available. Suitable toxic washes and chemicals which are safe to use include quaternary ammonium compounds and sodium hypochloride. Some yellowing of paints may occur with some treatments.
Use only proprietary products with labels which state that they have been cleared as safe for this use; there is some risk in the use of all these materials. The supplier’s instructions and recommended precautions should be carefully observed.
If the toxic wash treatment appears to have been successful, redecoration can be undertaken. Fungicides incorporated into the decorative finish protect only the finish itself and do not obviate the need for the preliminary toxic wash treatment. Some manufacturers supply paints and wallpaper adhesives incorporating fungicides and it is better to use these than to add them on site.
Rising damp can bring salts up from the ground, which are then deposited into the fabric of the wall and remain there even after the original dampness has been cured. When the surrounding air is humid, the salts absorb moisture, and damp patches appear.
Salts can enter the building in various ways: from sea-sand or gravel which has not been adequately washed, or from additives used for frost protection or for rapid setting of mortar or concrete. Other possible sources are magnesite in composition floors, or previous use of the building, such as in a conversion of stables which have been contaminated by faeces or urine.