A damp-proof course (DPC) is a barrier built into masonry walls to prevent moisture rising from the ground by capillary action—a phenomenon known as rising damp. A damp-proof membrane (DPM), in contrast, is a membrane material applied to stop moisture transmission, most commonly polyethylene sheeting laid under concrete slabs. Both systems serve a single goal: preventing moisture from passing into interior spaces and causing damage to finishes, plaster and structural timber.
According to the American Society for Testing and Materials (ASTM), damp proofing is defined as a material that resists the passage of water with no hydrostatic pressure, distinguishing it from waterproofing, which must resist water under pressure. In masonry construction, DPC layers are usually laid below all walls, whether load-bearing or partition walls, and may be horizontal or vertical depending on the structure.
Material options for damp-proof courses
Materials used for damp proofing fall into three categories. Flexible materials include butyl rubber, hot bitumen, plastic sheets, bituminous felts and sheets of lead or copper. Semi-rigid materials such as mastic asphalt offer a middle ground in durability and application method. Rigid materials include impervious brick, stone, slate, cement mortar or cement concrete painted with bitumen.
A DPC typically consists of a durable, impermeable material—slate, felt paper, metal, plastic or special engineered bricks—bedded into the mortar between two courses of bricks or blocks. It often appears as a thin line in the mortar near ground level. To create a continuous barrier, individual pieces of DPC or DPM may be sealed together, and the DPC may be sealed to the DPM around the outside edges of the ground floor, completely isolating the interior from damp ground.
Placement and installation in cavity walls
In a masonry cavity wall, a DPC is typically installed in both the outer and inner wall. In the outer wall, the DPC is normally positioned 150 millimetres to 200 mm above ground level—the height of two to three brick courses. This elevation allows rainwater to puddle and splash upward from the ground without breaching the moisture barrier. Physical DPCs are installed during construction of the structural walls, typically around 150 mm above ground level, and are now standard in modern construction to combat rising damp.
By blocking rising damp, a DPC serves as a protective barrier that restricts the infiltration of moisture upwards through walls, preventing damage to internal wall coverings and finishes. Unwanted moisture in buildings can cause deteriorating plaster, damaged decoration, black spot mould and even wet or dry rot in timber components.
Alternative methods and remedial applications
Beyond physical DPCs installed during construction, other damp-proofing methods include integral damp proofing—adding materials to the concrete mix itself to make it impermeable—and surface suppressant coatings with thin waterproof materials such as epoxy resin for resistance to non-pressurised moisture. Cavity wall construction, such as rainscreen assemblies, separates interior walls from exterior walls by a cavity, providing an additional line of defence.
Chemical damp proofing, also known as an injectable damp proof course, involves injecting chemical agents or treatments at regular intervals into the mortar bed between courses. This method is often employed as a remedial measure in older buildings where physical DPCs were not originally installed or have failed over time. DPCs can fail or be bridged by high external ground levels such as pavement or driveway works, making it important to verify that the DPC has not been compromised when internal damp issues arise.
Causes of damp walls beyond DPC failure
Damp issues on internal walls are not solely caused by water rising from the ground. Other common causes include damaged roof coverings, malfunctioning rainwater gutters and downpipes, defective external render or masonry, condensation problems, inadequate window and door sealing, plumbing leaks, defective drains in older buildings and external flood events. In all such cases, a DPC would not provide effective protection against damp, underscoring the importance of correctly diagnosing the source of moisture before implementing remedial measures.
Pressure grouting of cracks and joints in masonry materials, coarse sand layers under floors and continuous plastic sheets under floors are additional techniques used to control moisture in construction. Wood and stones also feature in traditional damp-proofing systems, though modern polymers have largely superseded these materials in new construction due to their durability and ease of installation.
Takeaway for specifiers and contractors
Selecting the appropriate DPC or DPM material and ensuring correct placement—typically 150 to 200 mm above ground level—are critical steps in preventing rising damp and prolonging the service life of masonry structures. For remedial work, chemical injection offers a viable solution where physical barriers are absent or compromised, but accurate diagnosis of the moisture source remains the first priority in any damp-proofing strategy.
