Rising damp

Rising damp is a condition where groundwater (water located beneath the water table) ascends through the pores of a wall by a process known as capillary action or absorption. This phenomenon is distinct from the movement of rainwater, which typically descends through unsaturated pores in soil.

Background:

  • Groundwater and the Water Table: Groundwater is the water found under the water table. The water table represents the boundary between the soil’s unsaturated zone, where pores are not completely filled with water, and the saturated zone, where they are.
  • Capillary Zone: Above the water table lies the capillary zone, where pores are partially saturated with water. This zone typically extends up to about 1 meter above the water table.
  • Natural Rise of Water: The natural rise of water in materials with no unsaturated pores is approximately 10 – 20 cm, depending on the material. In cases where pores are saturated, or above a solid surface such as a solid floor or a damp proof course, the rise can be between 1 – 1.3 meters. This increased rise can occur in situations like leaks or condensation above the solid surface.
  • Rarity of True Rising Damp: True rising damp is relatively rare, as most buildings are not constructed in areas prone to groundwater flooding.
  • Standard Treatment: The conventional method for addressing rising damp involves damp proofing. This can include the installation of a damp proof course or the retroactive injection of siloxane into brick walls to reduce water absorption. Often, this is coupled with the replacement of internal plaster with an impermeable slurry to prevent hygroscopic salts from appearing as damp on the surface.
  • Challenges and Misdiagnosis: A significant issue arises when the actual source of moisture is not rising damp. In such cases, the real cause persists, potentially leading to hidden damage like rot and mould. Additionally, most instances of hygroscopic salts in residential buildings are attributed to historical factors such as the burning of coal, equine urine, or the use of de-icing chemicals on roads during winter.
  • Moisture Issues in Waterfront Buildings. While often mistaken for rising damp, the moisture issues in buildings situated near or on water bodies, such as in Venice, are distinct. In these cases, the primary cause of moisture at the base of walls is not groundwater rising through capillary action but rather vapour from the surrounding water. Vapour can exacerbate rising damp or be distinct, either way it is better to deal with the vapour first, using ventilation, heat, insulation, airflow and dehumidification.

Assessment and Diagnosis:

  • Online Resources: Online resources can be used to assess the risk of groundwater-bearing rocks under a property and the potential for groundwater flooding.
  • Physical Assessment: Drilling down around all four sides of a building to a depth of about 1 meter can help assess the presence of rising damp. If the water is not near the building or if the water levels are inconsistent, it suggests that rising damp is not the issue.
  • Capillary Zone Risk: Within the capillary zone, the risk of rising damp is proportional to the height within the zone. For example, if a building is 50 cm above the water table, the potential rise due to capillary action would be about half the rise at the water table from plus the natural rise above unsaturated pores. This means that the rise should not be more than about 1.3M above the water table. If the height of water inside is more than 1.3M above the water table then you almost certainly don’t have rising damp.

Importance of Accurate Diagnosis: Accurately identifying the source of moisture in a building is crucial. Misdiagnosing the problem as rising damp when it is not can lead to ineffective treatments and ongoing damage to the property. Accurate diagnosis ensures appropriate and effective remedial action.

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