A Damp-Proof Course (DPC) protects walls from 'rising damp', or moisture rising up from building foundations.
Moisture resistance is not total, but it is sufficient to defend against dampness, which can cause staining and cracks to appear. In the case of an inferior damp proof course, or the absence of one, dampness will be visible on both internal and external walls.
Rising damp occurs because moisture is able to rise up the wall of the building through what is defined as a ‘capillary action, ’ whereby a liquid is able to rise upwards, against gravity, in a narrow tube. This is the result of complex inter-molecular attractive forces between the liquid and its surrounding solid surface, much like when water rises upwards between the hairs of a paintbrush.
The DPC layer is usually installed horizontally where walls meet a building's foundations in order to prevent ground moisture penetrating the building. However, a DPC can also lie vertically to prevent moisture from rain that has been blown against a building's external walls from passing, through the masonry, to the building’s interior.
Many older buildings were not constructed with a DPC, causing significant dampness problems. Damp-Proof Coursing was made compulsory by the Public Health Act in 1875, however many DPC’s installed in the immediate period after this were laid shoddily and no longer fulfil their purpose.
Traditionally DPCs consisted of slate or lead barriers, fitted into the walls to protect rising damp. However in the last 30 years DPCs have been comprised of plastic. In addition, they are welded to a damp course membrane (DPM), which is a layer of polythene or bitumen-polymer, laid under the floor slab to keep ground water out.
There is a further option, best be described as a ‘remedial DPC’. This involves the injection of a chemical (often silicon) into holes drilled in a wall, at regular intervals, with the purpose of performing the same task as any other DPC.
It is strongly advised that you contact a Residential Surveyor for their advice and expertise when considering Damp-Proof Coursing.