The damp-proof course (DPC) is an important element in construction that prevents the penetration and ingress of water into a building.
Where serious dampness occurs, health problems and damage to internal finishes and fixtures and fittings are consequences, although only in exceptional cases will it affect the structural integrity of the building.
When DPCs are incorrectly installed, or are not there at all, not only is it difficult to rectify the problems without major reconstruction, tremendous inconvenience and cost, it can also be visually unsightly and “patchy”. “It is completely unnecessary since it can easily be avoided by building in the DPCs correctly the first time,” says Paul Koning, honorary member of the Waterproofing Association of Gauteng (WAG) and chairman of the Roofing and Waterproofing Institute (RAWI).
Where does it go wrong?
Koning points out that it is easy to put the blame on the contractor or bricklayer, since DPCs are rarely correctly installed on site or the wrong types of membranes are used, but he notes that there is seldom a drawing or instructions to guide the work and enforce best practice.
To prevent potential damp problems relating to DPCs, he suggests that architects and designers should provide better details and drawings showing all the positions and specifying the type of DPCs required around the building as a whole.
There should be detailed drawings indicating how the DPCs are to be terminated, how they must be continued around columns and other intrusions into the cavity, or how they are to go around corners of the buildings. These will also provide a means to hold contractors accountable and avoid disputes.
Where DPCs are normally installed
The general applications of DPCs in clay or cement brick walls are; at ground floor concrete slab levels; around window and door frames; over arches and openings; in concrete framed multi-storey buildings, where lower roofs butt up against the upper levels of a building; and under copings on top of exposed walls or parapet walls.
“Frequently, there is also an interface between DPCs in walls and flat-roof waterproofing systems, for example on balconies, terraces and parking decks,” says Koning.
“In a 230mm solid brick wall, the DPC prevents rising damp at ground level. A stepped damp-proof course in a 280mm cavity brick wall is not only to prevent rising damp at ground level, it is also to catch and shed, to the outside, any water that might penetrate the outer brick skin and run down inside the cavity. The higher a building is and the more floors it has, the more exposed the walls are to the weather, and the more important the installations of the stepped DPCs are in the construction of the walls.
“In coastal regions such as the Western and Eastern Cape especially and KwaZulu-Natal, where the rainfall is usually high and accompanied with high winds, it is strongly recommended to build cavity brick walls with DPCs. In areas with high humidity, such as KwaZulu-Natal, a bituminous coating should be applied to the inside face of the inner brick skin as a vapour barrier to prevent fungus growth inside the building,” he explains.
What it should look like
“Where the stepped DPCs in cavity brick walls bridge the cavity between the two brick skins, there must be a sand or cement fillet in the cavity between the different brick course levels to prevent the DPC membrane from sagging into the cavity. It is important that the DPC membrane protrudes on the external brick skin and does not stop short,” Koning states.
“Plastic-type DPC membranes (polyolefins) must be sandwiched in the middle of the mortar bed and the DPC level in the outside brick skin should not be lower than two bricks, or 150mm from the ground level.
“When lengths of DPCs have to be joined, a substantial overlap of the membranes must be made and joined with self-adhesive tape. At the end of a concrete column face, the DPC membrane must be turned up and bonded to the side of the column with a self-adhesive tape.
“In instances where cavity walls butt up against concrete columns or slabs, a drip mould must be formed in the concrete to prevent water bridging the cavity along the concrete surfaces. PVC (plastic) angles are typically used to form the drip due to ease of installation and low cost. The drip moulds must be fixed at suitable centres to prevent PVC bowing and they must be bedded in a suitable sealant or mastic to prevent water by-passing underneath the moulding.
“If the outside brick skin also continues across the face of a concrete column, the DPC must be continued in the outer brick skin and be bonded to the face of the concrete column with a self-adhesive tape. The complete face of the concrete column between the brickwork must also be coated with a bituminous coating,” he explains.
When pipes bridge the cavity
“It is important that the cavities between the two skins of brickwork are kept clean and free of rubble and broken pieces of brick and mortar. No pipes or conduits should be built inside the cavity that can cause a bridging effect from one brick skin to the other,” Koning adds.
“However, if there is no other option and pipes or conduits have to bridge the cavity, then they must be installed in such a way that they slope upwards to the inside to prevent the chance of water running along the conduits or pipes from the outside brick skin to the inside.
“Any chasing of pipes or conduits should be done on the inside brick skin and should not penetrate the DPC membrane. If there is no alternative, and pipes or conduits have to penetrate the DPC, then the membrane must be sealed to the individual pipes or conduits using a self-adhesive tape.”
How to make proper weep holes
Weep holes must also be made in the brick joints at 600mm centres to let water drain unrestricted to the outside. Koning explains that the weep holes must be at the DPC level and be completely open and with no mortar blocking the slot or on top of the DPC membrane.
“It is recommended that weep hole inserts are built in while constructing the cavity walls, protruding slightly so that the plasterer can plaster neatly around them, creating neat and even weep holes in the plastered walls,” he says.
“The bricks left out for cleaning the cavity walls during construction should then be between the weep holes. After the cavities have been cleaned correctly, the openings should be closed with bricks and all the perps and joints closed. This prevents having to try and form weep holes when the bricks are built, resulting in weep holes that vary in size and are often half or totally closed.”
Specify appropriate materials
Covered in SANS 10021 (SABS 021) and SANS 10952 (SABS 952), standard materials are the polyethylene plastic membrane and the bituminous felt membranes. In coastal regions where wind-driven rains are prevalent, polyethylene-backed rubberised asphalt self-adhesive membranes, and APP modified bitumen membranes are used as the DPC membrane.
Damp problems related to DPCs are a pervasive problem in South Africa and one that can easily be avoided. “It therefore makes good sense that DPC installation is correctly designed and detailed by designers, given special attention by all supervisors on site and be a specific inspection item on all sites,” Koning concludes.
Full thanks and acknowledgment are given to Paul Koning for the information given to write this article, which contains extracts taken from his technical paper published in the Journal of Construction in 2007.
Captions Main Image:
Example of incorrectly installed or damaged DPCs.