A damp-proof course is impervious layer built into a wall ideally 150mm above ground level in order to prevent moisture rising up the wall by capillary attraction. Dpc's used over time have been slate, bitumen, lead, copper, bituminous felt or in modern times, polythene or polymer. These are generally known as a physical damp-proof course.

If water rises up a wall from the ground it is known as rising damp and the water will bring with it minerals and salts which can affect the internal surfaces of the structure, plaster, timber, etc. A Public Health Act was passed in 1875, after which it became obligatory to include a damp-proof course in the walls of buildings. If a building does not have an effective damp-proof course it may be possible to install an injected damp-proof course. Typically a chemical injection damp proof course is created by drilling holes at about 100mm centres in the brickwork or mortar joints just above ground level.

A damp proof course fluid is then pumped into these holes under pressure. The fluid reacts with carbon dioxide to form a water repellent silicone based resin within the capillaries of the bricks or mortar. This resin stops the further passage of water but is permeable to water vapour thereby allowing the materials to breathe and dry out.

In recent years a new generation of damp proofing products has emerged. Water soluble thrixotropic gels can be injected into any damp substrate using a simple cartridge gun or compression pump. This gel then reacts to form a silicone repellent which is completely water resistant. Silicone emulsion creams are injected into any form of masonry at low pressure through holes pre-drilled. This formulation then creates polysiloxanes once the liquid cream has migrated into all of the pores of the substrate.

Problems with damp-proof courses.

NB These problems can affect both physical and injected damp-proof courses

Bridging, i.e. the partial or total covering of the damp-proof course with a porous material allowing moisture from the ground to by-pass the damp-proof course.

1. By flower beds and paths. The damp- proof course is normally found approximately 150mm above external ground level. Flower beds and paths must not be taken over the damp-proof course level.
2. By flower beds and paths. Where the external levels are close to or at damp-proof course level "splash over" bridging by heavy rain splashing up from the ground and over the damp-proof course may occur. Current building regulations recommend that the damp-proof course should be at least 150mm above the external ground level.
3. By solid floors and or hearths introduced subsequent to damp course installation and above the level of the damp-proof course.
4. By render plinths and external renderings/stucco over the damp-proof course level.
5. By internal plaster over the damp-proof course level.
6. By mortar droppings built up over the damp-proof course inside a cavity wall.
7. By mortar joints/brickwork. E.g Abutting garden wall or adjacent building walls where no vertical damp-proof course has been included to isolate the buildings/walls.
8. By mortar joints. A blue/engineering brick course(s) can provide some degree of damp proofing, but this will not prevent some damp rising through the mortar joints.

Other sources of dampness

Causes of dampness to the walls of buildings, other than rising damp are:

A. Water penetration caused by:
i) defective or inadequate rain water goods;
ii) porous brick or stonework;
iii) cracked renderings
iv) defective pointing/sealing;
v) uncapped projections (trap driving rain, particulariy when situated on the weather side of the building) e.g. chimneys, abutting walls;
vi) cracked cills;
vii) cills and other projections that lack an effective throating or drip;
viii) defective external and internal plumbing/drainage.

B. Atmospheric Moisture:
i) Condensation
ii) Moisture attracting properties of materials contaminated with hygroscopic salts

C. Ground water dampness to the section of the wall below the damp-proof course level.

D. Dampness to solid floors which lack an effective damp-proof membrane in their construction. Note: many old solid floors lack a damp-proof membrane.

E. Failure to link the damp-proof membrane in solid floors with the damp-proof course in the wall. (see fig.3)

F. Lateral penetration of ground water where walls are earth retaining. Where the walls are partially earth retaining dampness is likely to be due to a combination of the lateral penetration of ground water and rising damp. The damp-proof course will only prevent dampness to that section of the wall above ground level and a form of vertical barrier will be necessary to deal with the dampness in the lower part of the wall and floor.

When a building has suffered from rising damp it is usually necessary to remove the lower 1m or so of plaster from the internal wall, since this is usually contaminated with hygroscopic salts. The walls can then be re-plastered with a 1:3 sand cement render and a salt retardant additive followed by a layer of gypsum multi finish plaster. Alternatively a suitable renovating plaster can be used, typically a cemetitious, lightweight plaster containing lime which acts to inhibit mould formation. Great care must be taken when replastering not to bridge the damp-proof course and to follow good "post damp-proofing" replastering specifications.