Rising damp is the action of water moving vertically through porous building material, which is in contact with the ground. This is brought about by the drying action of air on the upper surfaces which causes the wall to act like a wick, drawing water upwards from the ground by capillary action, which then evaporates from the surface into the atmosphere. The moisture will continue to rise until it reaches a height where, unless evaporation is possible, gravity pulls it down again.
Rising damp occurs because the building’s damp proof course (DPC) has broken down or is being bridged by high ground. A DPC installed during construction will be a physical layer of impermeable material. This can consist of slate, bitumen impregnated felt or plastic, for example. Many older properties do not have a DPC.
Water coming from the soil will have various dissolved nitrates, chlorides and other salts (hygroscopic ground salts) which will be carried upwards into the brickwork and plaster. As the water evaporates, an increasing amount of these salts will be deposited in the wall. As ground salts are easily dissolved they also readily attract moisture from the atmosphere.
As a result of this the plaster will always be damp or contaminated, even after the rising damp is cured. For this reason the plaster is normally removed when treating rising damp. Wood rot in adjacent timbers may often be as a result of rising damp in a wall.
There may be a number of reasons which result in penetrating dampness into walls above external ground level. One of which could be wind-driven rain which, depending on the direction of the rain is largely out of the property owner’s control. Unfortunately, this can lead to a downward movement of moisture within the structure.
Whilst it may not be possible to control the direction and force of wind-driven rain, it is possible to prevent such rain entering the structure and causing internal damage. It is important to ensure the property is well-maintained to prevent water ingress; which could be via cracks in external render coatings, loose and missing pointing on walls and around window and door frames, defective flashings around roof structures, defective rainwater goods such as leaking or blocked gutters, down-pipes and hoppers.
If the property is not maintained, penetrating damp could result in significant damage occurring within the property including outbreaks of fungal decay (Dry Rot and Wet Rot attacks).
Penetrating damp can also result in the migration of hygroscopic salts into the internal plaster finishes which causes random and isolated patches of dampness to appear during spells of high humidity.
Damp proofing case studies
Following an initial inspection by one of our CSRT qualified Surveyors, on behalf of the Managing Agents and Landlord, Preservation Treatments were instructed to carry out Damp-Proofing works to this 3 bedroom Victorian semi-detached property in Camberley, Surrey.