Water in the ground rises into brickwork and concrete by capillary action, the way water climbs a paper towel. Two barriers stop it: the damp-proof course (DPC), a strip of membrane built into the walls, and the damp-proof membrane (DPM), the sheet under the floor slab. If either is missing, torn or bypassed, damp walls, peeling paint and salt stains follow. This guide covers where they go and what makes them fail.
The damp-proof course in the walls
SANS 10400-K 4.5.3.3 asks for a damp-proof course in a masonry wall at the level of the top of a concrete floor slab resting on the ground. And 4.5.3.7 says no horizontal DPC may be installed less than 150 mm above the adjacent finished ground. Put together, the floor level and the DPC both sit at least 150 mm above the outside ground.
AfriSam’s detail for a double-brick external wall uses a 375 micron polyolefin DPC, laid on the bricks and projecting about 10 mm beyond the face of the plaster so it can’t be bridged. Laps in the DPC go onto bricks, not over a gap.
The damp-proof membrane under the floor
Under a floor slab on the ground, SANS 10400-J asks for a polyolefin membrane at least 250 µm thick, turned up around the edges by at least the slab thickness and overlapped 200 mm at joints, with any punctures taped. AfriSam’s guidance has the membrane running halfway over the top of the foundation brickwork so it meets the DPC and leaves no gap for damp to find. See floor slabs, DPM and mesh.
Changes in level
Where the ground outside is higher than the floor inside, or where floors step down, a horizontal DPC on its own isn’t enough. A vertical damp-proof membrane is built into the foundation brickwork, often with a protective skin of bricks against the backfill. AfriSam notes these heavier membranes come in thicknesses up to 1,000 µm.
How damp gets past
AfriSam names the most common cause of rising damp in modern buildings: plaster carried down over the DPC. The plaster forms a bridge, and water climbs through it past the membrane. SANS 10021 8.4.6 says plaster should not be applied across a damp-proof membrane in a wall; where a wall must be plastered top to bottom, a V-joint 10 to 15 mm wide is cut at DPC level. A shallow decorative groove does little.
Other common causes:
- Garden beds, paving or fill built up against the wall above the DPC.
- A DPC recessed into the brickwork, or damaged during building.
- A torn floor membrane, or one whose overlaps were never sealed. Damp then comes up through the slab, damaging floor coverings and the walls built on the slab.
- Mortar droppings in a cavity, bridging from the outer skin to the inner.
Treating rising damp
Where the DPC in a wall has been bridged or damaged, AfriSam describes the usual repair: remove the plaster, inject a chemical damp-proof course into a mortar joint, apply a waterproof slurry to the outside from ground level to two brick courses above the DPC, and re-plaster, cutting a proper V-joint at the DPC and using a salt retardant in the new plaster inside. Damp through the slab is harder: floor coverings come up, the floor gets a waterproof coating lapped onto the walls, and walls are treated as above. Prevention is much cheaper than the cure.
On site
- Set the DPC at least 150 mm above the finished ground, and keep finished ground and paving below it.
- Let the DPC project past the plaster face, and stop plaster above it with a V-joint.
- Lap the floor membrane onto the foundation brickwork to meet the DPC.
- Protect membranes from tearing until they are covered.
- Use a vertical membrane wherever ground outside is higher than the floor inside.
The strip foundation calculator includes the DPC length for a house, and the floor slab calculator includes the membrane.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.