Rising damp is a symptom of groundwater in contact with a building.
What is rising damp?
Rising damp is a form of dampness that occurs, particularly in older buildings, when groundwater rises up through walls, floors and masonry via capillary action, which is the ability of a liquid to flow in narrow spaces in opposition to gravity.Safeguard; the main supplier to the damp proofing industry and chaired alongside the Property Care Association (“PCA”) by Hudson Lambert.
For rising damp to exist there most be two ingredients:
- Groundwater: Groundwater is water below the water table and normally found well below the soil zone. It is under pressure, causes pores to become and has different characteristics to undrained rainwater in soil.
- Absorption: Capillaries are used as an analogy to explain the effect of small pores sizes. The rise depends on both pore size and or porosity (which can be thought of as a product of how much water can be held in a particular substrate, e.g. brick, or the proportion of pores that are inter-connected). Walls are composed of various building materials, behaving independently and in combination.
But doesn’t water also rises up brick when in contact with wet soil?
Yes, any cause of dampness will result in a rise; groundwater, rainwater, mains water and condensation form excess vapour.
Water is water after all, wherever it came from.
For example if you introduce a leak, rainwater or condensation in the centre of a wall at normal relative humidity, then water will be absorbed upwards on normal, absorbent plaster. The water introduced is suspended. It will redistribute through absorption and evaporation, unless it is constantly added to.
- Water rises by about 10cm, but water will also fall.
- The fall of water in the plaster is in the order of 10 – 100 times greater than the rise.
A brick, or plaster in contact with suspended water will rise up to about 10cm, if the plaster is saturated, and less than this if unsaturated, and more if very high relative humidity.
Water in the soil zone comes from precipitation. It is suspended, as the pores below the soil zone are unsaturated. Gravity draws water down until it reaches groundwater.
So why is rising damp only dampness caused by groundwater?
Unlike water in the soil zone, which is suspended water, with unsaturated pores beneath it, groundwater is saturated. Water cannot be drawn down anymore, therefore a brick wall in contact with groundwater will rise to the maximise height above the water table, depending on the pore size, porosity and evaporation.