What is Rising Damp?

 

Rising damp, a worldwide phenomenon, is a major cause of decay to masonry materials such as stone, brick and mortar. Even when mild it can cause unsightly crumbling of exterior masonry and staining of internal finishes. It may also cause musty smells in poorly ventilated rooms.

 

Rising damp occurs as a result of capillary suction of moisture from the ground into porous masonry building materials such as stone, brick, earth and mortar. The moisture evaporates from either face of the wall (inside or outside), allowing more to be drawn from below. The height to which the moisture will rise is determined by the evaporation rate and the nature of the wall. The normal limit for rising damp ranges from 0.5 to 1.5 metres above ground level.

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Rising damp may show as a high-tide-like stain on wallpaper and other interior finishes, and, when more severe, as blistering of paint and loss of plaster. Damp walls encourage the growth of moulds, which, with the high humidity, can lead to health problems for occupants.

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How to prevent Rising Damp?

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To prevent rising damp it is normal practice to build-in an impermeable barrier at the base of the wall just above ground level. This is known as the damp-proof course (DPC) or sometimes as the damp course. Modern DPCs are generally 0.5mm thick black polyethylene sheeting. Early DPCs included overlapping roofing slates, lead sheets, glazed ceramic tiles (made for the purpose) and various bitumen-based materials, including tar–sand mixes which were laid hot.

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Control and Treatment

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Having diagnosed the cause of the damp problem, the obvious response is to prevent it recurring by fixing leaks, by removing bridges, or by inserting a new DPC. Good housekeeping measures should be undertaken as well. These will help prevent further damp problems and may reduce the severity of an existing problem to an extent that major works are not necessary. These measures include regular maintenance of plumbing and roof and guttering systems, and attention to site drainage and to underfloor ventilation.

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The Importance of Subfloor and Underfloor Ventilation

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Maintaining underfloor ventilation is an important part of controlling damp, as it allows soil moisture to evaporate beneath the floor and to pass out through the vents in the lower walls. Without this ventilation the moisture ‘stress’ on the walls would be much greater. One of the worst mistakes of renovators is to remove a ventilated timber floor and replace it with a concrete slab poured on sand or fill. The concrete prevents evaporation and all the soil moisture rising beneath the building is now focused on the walls. Rising damp problems are almost guaranteed, whereas before there may have been no significant damp, even though the walls may have lacked effective DPCs.

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