Home and Industrial
A damp-proof course (often abbreviated to DPC) is a horizontal barrier in a wall designed to resist moisture rising through the structure by capillary action – a phenomenon known as rising damp. A damp-proof membrane (DPM) performs a similar function for a solid floor. Moisture resistance is not necessarily absolute: it is usually defined by a specific test method, limits, and engineering tolerances.
In theory, due to capillary movement of water, water rises from earth to the building. Passing through the foundation, it rises higher to reach the walls. Contact between water and walls may damage them by creating cracks, breaking cement-paint bonds and creating dark-spots on the wall, etc. So, to avoid water from reaching walls, a DPC layer at plinth level (the joint level of walls and foundations) is laid.
DPC layer is usually laid below all the walls, regardless of the issue that the respective wall is a load bearing wall or a partition wall. Usually, a DPC membrane is 4″ to 9″ wide.
Rising damp in theory can occur for various reasons – the failure of an existing damp proof course, bridging due to the raising of external ground or internal floor levels, or in older buildings, the complete absence of a damp proof course.
Brick, stone and mortar are porous allowing damp from the ground to rise by capillary action, carrying with it ground salts including chlorides and nitrates. These salts from the ground can absorb moisture from the atmosphere leading to wall dampness in conditions of high relative humidity. Also they can ruin decorations and break down internal plaster. Modern damp proofing courses are generally of three types:
- Chemical damp proof treatment – Recommended for most types of walling, a chemical is injected into holes drilled at intervals in the wall.
- Injection mortar/creams – Ideal where a good sized bed joint is visible.
- Electro osmotic treatment – An active system that introduces a very small electric current into the wall that pushes moisture downwards
A DPC is usually a thick plastic strip bedded into the mortar between two courses of bricks or blocks. It can often be seen as a thin plastic line in the mortar near ground level.
A DPM is usually a thick polythene sheet laid under the floor slab, to allow the slab to dry out and keep out groundwater. It is often laid on a bed of sand, to prevent the sharp edges of the hardcore damaging it.
To create a continuous barrier, pieces of DPC or DPM are welded together. In addition, the DPC is welded to the DPM around the outside edges of the ground floor, completely sealing the inside of the building from the damp ground under it.
In a cavity wall, there is usually a DPC in both the outer and inner wall. In the outer wall it is normally 150-200mm above ground level (the height of 2-3 brick courses). This allows rain to form puddles and splash up off the ground, without saturating the wall above DPC level. The wall below the DPC may become saturated in rainy weather. The DPC in the inner wall is usually below floor level, (under a suspended timber floor structure), or, with a solid concrete floor, it is usually found immediately above the floor slab so that it can be linked to the DPM under the floor slab. This enables installation of skirting boards above floor level without fear of puncturing it. Alternatively, instead of fitting separate inner and outer DPCs, it is common in commercial housebuilding to use a one-piece length of rigid plastic, (albeit an angled section), which fits neatly across the cavity and slots into both walls (a cavity tray). This method requires the need for weep vents to enable rainwater ingress to drain from the cavities otherwise rising dampness could occur from above the DPC.
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