Damp (structural)

Structural dampness is the presence of unwanted moisture in the structure of a building, either the result of intrusion from outside or condensation from within the structure. A high proportion of damp problems in buildings are caused by ambient climate dependent factors of condensation and rain penetration. Capillary penetration of fluid from the ground up through concrete or masonry is known as "rising damp" and is governed by the shape and porosity of the construction materials through which this evaporation-limited capillary penetration takes place. Structural damp, regardless of the mechanisms through which it takes place, is exacerbated by higher levels of humidity.

Dampness control is fundamental to the proper functioning of any building. Controlling moisture is important to protect occupants from adverse health effects and to protect the building, its mechanical systems and its contents from physical or chemical damage.

Dampness tends to cause secondary damage to a building. The unwanted moisture enables the growth of various fungi in wood, causing rot or mould health issues and may eventually lead to sick building syndrome. Plaster and paint deteriorate and wallpaper loosens. Stains, from the water, salts and from mould, mar surfaces. The highest airborne mould concentrations are found in buildings where significant mould infestation has occurred, usually as a result of severe water intrusion or flood damage. Moulds can grow on almost any surface and occur where there is moisture from structural problems such as leaky roofs or high humidity levels. Airborne mold concentrations have the potential to be inhaled and can have health effects.

Externally, mortar may crumble, and salt stains may appear on the walls. Steel and iron fasteners rust. It may also cause a poor indoor air quality and respiratory illness in occupants. In extreme cases, mortar or plaster may fall away from the affected wall.

Health concerns around mould include infections, allergenic or immunological illness, and nonallergic illness. Asthma is also triggered by the sensitization of dust mites accruing humid, wet regions of a structure. Another health effect associated with structural dampness is the presence of bacteria in an indoor environment. Bacteria require water to grow and multiply and certain species can cause disease in humans, thus the intrusion of water into an indoor environment can place the health of occupants at risk from bacterial infections. Water removal and drying of wet building materials within two days will likely prevent mould and bacteria growth, therefore reducing occupants' vulnerability to disease.

A Visual Guide to Damp, Mold and Indoor Pollution states that:

Excess moisture leads – on almost all indoor materials – to growth of microbes such as moulds, fungi and bacteria, which subsequently emit spores, cells, fragments and volatile organic compounds into the indoor air. Moreover, dampness initiates chemical and/or biological degradation of materials, which also causes pollution of the indoor air. Exposure to microbial contaminants is clinically associated with respiratory symptoms, allergies, asthma and immunological reactions. Dampness has therefore been suggested to be a strong and consistent indicator of risk for asthma and respiratory symptoms such as cough and wheeze.

Section 5.2 of The Building Regulations 2010 Approved Document C, "Site preparation and resistance to contaminants and moisture" requires that buildings should be constructed to resist rising damp, penetrating damp, and condensation.