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Rising damp in walls - diagnosis and treatment (DG 245)

BRE (Building Research Establishment) is an independent, research-based consultancy, testing and training organisation, operating in the built environment and associated industries.

On 12 October 2007, BRE published Rising damp in walls - diagnosis and treatment (DG 245 revised edition), written by Peter Trotman.

Dg 245 rising damp.jpg

Excess moisture is a common problem in buildings, and may be apparent from; damp patches, mould growth, mildew, salts, staining, ‘tide marks', blistering paint, bulging plaster and so on.

Rising damp is caused by capillary action drawing moisture up through porous elements of a building’s fabric. DG 245 considers the causes of rising damp in walls and how to remedy it if found.

Porous walls which stand in water or saturated soil without a damp proof course (dpc) can have rising damp to a height of more than 1m, with the height depending on:

  • The rate of evaporation from the wall.
  • The porosity of the wall.
  • The salt content of the wall and soil.
  • The groundwater height and saturation.
  • Heating in the building.

Some accumulation of salts can occur in the walls of old buildings even when they do not have a damp problem, so high readings from a moisture meter alone are not conclusive. Appendix A of the 12 page digest describes how to drill samples from a wall in to test for moisture content and hygroscopicity, and the interpretation of moisture gradient profiles.

If a building has a physical damp-proof course (dpc), it is unlikely that it has failed, as most dpc materials have a long life, however, defects such as mortar droppings in a cavity wall, may make a dpc ineffective by bridging it and allowing moisture to pass up through the wall. This and other mechanisms by which an existing dpc might be bridged are discussed.

If it is necessary to provide a new moisture barrier, the digest explains methods such as; inserting a membrane or using chemical injection. The repair of plaster damaged by damp is also discussed.

The content of the digest includes:

  • Mechanism of rising damp.
  • Diagnosis.
  • Determining the source of the dampness.
  • Treatment.
  • Replastering.
  • Dry lining.
  • Further reading.
  • Appendix: The sampling method.

--BRE Group

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