What is it?
A damp proof course (DPC) is a continuous impermeable layer built into a wall to stop moisture rising up from the ground into the structure above. A damp proof membrane (DPM) does the same job across a horizontal area, typically under a slab, and the two are usually tied together to form one unbroken barrier around the base of the building.
In a cavity wall, both leaves need their own DPC. The DPC can also span across the cavity, if so it should be stepped so that any water reaching one leaf's DPC is directed back out through the wall rather than across to the other leaf.
- 1Brick (external leaf)
- 2DPC
- 3Cavity Insulation
- 4Brick (internal leaf)
- 5DPM / DPC
What does it do?
It stops groundwater and rising damp travelling up through masonry by capillary action, keeping the building fabric above it dry. It has to form a genuinely continuous barrier (DPC in the walls, DPM under the slab, tied together at every junction), since damp will simply find and exploit any gap or unbridged joint in the line.
Where is it used?
Built into masonry walls a minimum of 150 mm above external ground level, continuing under the floor slab as a DPM, and dressed around every penetration and junction, including door and window openings, where the continuous line would otherwise be broken.
Typical sizes
Wall DPC is typically supplied as a roll around 100–150 mm wide to suit standard wall thicknesses, in flexible polymer or bitumen-based sheet material.
How to specify
DPC/DPM material and detailing are specified together as one continuous system, not selected wall-by-wall. The critical part of the specification is how the junctions and laps are detailed (steps between leaves, laps at joints, dressing around openings) rather than the choice of material itself, since most modern DPC materials perform similarly well when properly lapped and continuous.
A fire-rated DPC may be required if used in a tall building where all materials are required to achieve a minimum standard of combustibility.
Other considerations
If the DPC spans the cavity, weep vents should be allowed for in the outer leaf of the wall to allow water to drain. If there is sufficient depth in the cavity beneath the DPC, then there is no need to span the cavity with the DPC.
Interacting elements
- Weep vents: metal or plastic elements that are inserted into the perpends (vertical joints) between bricks on the outer face of the wall. These allow moisture out (via drainage or ventilation) and prevent insects from crawling in.
- Concrete slab: the DPM beneath the slab ties into the wall DPC to form one continuous barrier
- Cill / EPDM: dress the DPC line around the door opening, where it can't run as a simple flat strip