A damp course (also known as a damp-proof course or DPC) works by creating a horizontal barrier within a wall to prevent rising damp, which is the process of groundwater being drawn upwards through the wall's material by capillary action.
Understanding Rising Damp
Rising damp occurs because porous building materials, such as brick and mortar, act like a sponge. They absorb water from the ground due to the tiny spaces within them (capillaries). Without a DPC, this water can travel upwards, causing damage to the wall and creating dampness inside the building.
The Damp Proof Course (DPC) as a Barrier
The DPC is a layer of impermeable material inserted into the wall, typically near ground level. It's designed to block the capillary action of the wall, preventing water from rising above that point.
Materials Used for Damp Proof Courses
Various materials can be used for a DPC, including:
- Plastic (Polyethylene): A common and effective modern material. It's durable and resistant to water penetration.
- Bitumen Felt: A traditional material that consists of a felt base impregnated with bitumen.
- Slate: Overlapping layers of slate can create an impermeable barrier.
- Engineering Bricks: Certain types of dense, non-porous bricks can be used as a DPC.
Installation
The DPC is usually installed during the construction of a building. It is placed horizontally in the wall, typically 150mm (6 inches) above ground level. It must be continuous to be effective; any breaks or gaps will allow moisture to bypass the barrier.
How the DPC Prevents Rising Damp
- Groundwater Access: Groundwater is present in the soil around the building's foundations.
- Capillary Action: Porous building materials naturally draw this water upwards.
- DPC Intervention: The DPC, being impermeable, interrupts this capillary action.
- Moisture Blockage: Water is unable to rise past the DPC level, preventing dampness further up the wall.
Effectiveness
A properly installed DPC is highly effective at preventing rising damp. However, its effectiveness can be compromised by:
- Bridging: External features such as soil piled against the wall above the DPC, or internal plaster touching the floor, can create a "bridge" that allows water to bypass the DPC.
- Damage: Cracks or punctures in the DPC can compromise its integrity.
- Incorrect Installation: If the DPC is not continuous or is installed at the wrong height, it will not be effective.
In essence, a damp course acts as an impenetrable layer in a wall, interrupting the capillary action that allows groundwater to rise and cause dampness.
