Sources of moisture

Penetrating dampness

Penetrating dampness (sometimes referred to as 'lateral penetration') refers to any moisture sourced from outside the habitable space. This includes:

  • precipitation (rain, snow, sleet, hail and fog);
  • moisture from external plumbing leaks (soil, waste, rainwater and so on);
  • any moisture entering a wall above the ground, but below the dpc level (i.e. a dpc controls moisture sourced from below the ground and moisture sourced just above the ground - penetrating damp).

Lateral penetration of dampness usually refers to penetration through external walls above ground, but is sometimes also used to describe dampness below ground penetrating into basements.

The moisture literally penetrates the protective external envelope of the building, and is often first noticed when internal finishes become damp or stained.

Penetrating dampness occurs through roofs, chimneys, parapet walls and walls above ground level. Various physical mechanisms enable moisture to enter the fabric:

  • gravity pulls rain vertically in through weak points in the roof: through open chimney pots, poorly detailed or defective flashings, cracked or broken tiles, roof wall abutments or other horizontal surfaces;
  • lateral winds force moisture through cracks and gaps in walls; 
  • surface tension enables moisture to penetrate under horizontal soffits and cills where there is no effective drip; and
  • a higher external pressure can cause water to penetrate through gaps in the envelope.

Leaks in rainwater goods outside the building may enable rain to soak the building fabric. External leaks from soil and waste plumbing goods can also soak into the building fabric as penetrating dampness - even though the source of the moisture is inside the building. At the base of walls, rain may splash up from the ground, soaking masonry, or surface water may pool and penetrate the building because of unsatisfactory falls or blockages of gullies, and raised ground levels externally.

Many buildings suffer from dampness problems caused by the long-term dripping of cistern overflows. These can erode pointing if allowed to continue for long enough, and can cause unsightly staining to the building exterior and internal dampness problems, often to several floor levels in higher buildings.

Conflicts sometimes arise where measures to prevent one type of dampness problem actually result in a different problem. For example, an airbrick might improve the ventilation of a subfloor void, but at the same time it could offer an easy pathway for penetrating damp that could run down the face of a wall and track in via its holes. Raised external path levels may also enable water to run into the building via the airbricks. Modern plastic vents are designed to shed water away from the ventilation opening and older style airbricks can be provided with a lead hood bedded on the top to act as a drip. (The British Standard for airbricks and gratings did not take into account the risk that airbricks could present an easy route for water to enter.)

Internal damage and staining to finishes are usually easily linked to the area of external penetration. But where damp is penetrating a cavity wall or flat roof construction, the link between internal symptoms and external building faults may be less obvious (figures 1 and 2). Infrared thermography can be useful here, because this technique will identify surface temperatures that can be linked to symptoms of dampness. An optical illuminated endoscope can also be used to inspect cavity wall voids for potential blockages, especially where there is corresponding dampness internally to an external defect such as shown in figure 1 below.

Figure 1: Vivid stain patterns on a cavity wall soaked from blocked rainwater pipes above, but is the internal face of the wall affected?

Figure 2: Mould development of the type usually associated with a condensation problem, but in this case the problem was moisture penetrating via a defective roof

Wet outside, dry inside

We inspected a school building that had severe wetness on an external wall, which seemed to have been caused by faulty guttering. The wall had been wet for a long time and carried a variety of green biological growths and white fringes from salt deposition by evaporation. But after photographing the severely stained masonry, we were surprised to be told that there had never been an internal stain on the walls (see figure 2).

On inspection the wall proved to be of cavity construction. Unlike in a solid wall, it would be possible for the outer leaf to be very wet but not transfer its wetness internally as long as the cavity wall was well designed and built, and particularly at a position away from openings. Such wetness to an outer leaf would not measurably affect the thermal resistivity of the inner skin.

It is sometimes possible to single out the source of penetrating dampness because laboratory reports can provide information on the constituents of the moisture sample, clearly indicating waste, foul or tap water as the source. Penetrating moisture travelling through the building fabric may pick up soluble salts that are then deposited on the surface of plasters or papers, or the moisture itself may already contain salts (e.g. from tap water). In extreme cases, leaks from the roof can be traced by blocking up roof drainage outlets, flooding the roof with coloured water and watching where it emerges.

Note that, although repairing an external plumbing leak will enable the masonry to dry out over time, the close contact between masonry and timber in most constructions may lead to wetting of timbers that could result in rot or beetle attack if not remedied.