What Is a Damp Proof Course?
A damp proof course (DPC) is a horizontal barrier built into the walls of a building near ground level to prevent moisture from rising upwards through the wall fabric. In modern construction, DPCs are typically a layer of polyethylene membrane or bituminous felt built into the mortar course at a set height above ground level. In older buildings, the DPC may be slate, lead, engineering brick, or an early bituminous product.
A DPC works in conjunction with the damp proof membrane (DPM) in the floor slab to create a continuous moisture barrier at the base of the building. Where these two elements are correctly installed and connected, moisture cannot rise from the ground into the building fabric.
What Is Rising Damp?
Rising damp is the upward movement of moisture through a wall from the ground by capillary action. It typically manifests as a tidemark at low level on internal walls (usually no higher than around one metre above floor level), salt deposits (efflorescence), peeling wallpaper and paint at low level, and plaster deterioration at the base of affected walls. The tidemark height is characteristic — genuine rising damp typically does not rise significantly above one metre because the weight of moisture counteracts the capillary force.
However, it is important to note that rising damp is often misdiagnosed. Many properties that appear to have rising damp symptoms are actually suffering from bridged DPCs (where external ground, paving, or render has been raised above the DPC level, bypassing it), condensation issues, penetrating damp from external sources, or leaking plumbing. A qualified survey is needed to distinguish between these causes.
Do You Need to Replace or Inject a DPC?
The damp-proofing industry has historically over-recommended chemical injection DPC treatments — drilling a row of holes into the wall and injecting a silicone-based cream or fluid that creates a hydrophobic barrier. These systems are legitimate where the original DPC is genuinely absent or has failed, but they are often recommended and sold where the underlying cause of damp is something else entirely.
Before commissioning any damp treatment, have the property assessed by an independent, non-remediation surveyor — a building surveyor or structural engineer with no financial interest in selling you a treatment. Identify the actual cause of the damp. If the DPC is bridged by raised external ground levels, the solution is to lower the external ground. If the issue is penetrating damp through external walls, the solution is external waterproofing or repair, not a DPC treatment.
Other Causes of Damp in Period Properties
In Victorian and Edwardian properties, the original DPC is often slate, which is highly durable and rarely fails. Where damp symptoms appear, the cause is more commonly a bridged DPC, impermeable surface treatments on external walls (such as cement render or masonry paint trapping moisture), blocked cavity trays, or condensation arising from the mismatch between an old breathable wall and modern airtight internal finishes.
Condensation is particularly commonly misdiagnosed as rising damp. Condensation occurs when warm, moist indoor air contacts cold surfaces, causing moisture to deposit on walls. It is more common in poorly ventilated rooms, in walls with cold bridges, and in properties where heating is intermittent. The solution is improved ventilation and insulation — not a DPC injection.
DPCs and Extensions
For new extensions, a continuous DPC at the correct height and a DPM in the floor slab are standard building regulations requirements. The junction between the existing house DPC and the extension DPC must be properly detailed to prevent moisture infiltration at the connection point. This is a common source of damp problems in extensions if not correctly specified and executed.
At RCB Design and Build, damp proofing specification and detailing is part of our standard pre-construction review for all projects involving ground floor works. We ensure the DPC and DPM are correctly specified and that the connection to the existing building is properly detailed and inspected by building control. Contact us to discuss your project.