Why Important
Unless there is a specific heritage requirement NOT to introduce a damp proof course, a damp proof course provides a permanent barrier to prevent the building structure taking up ground water, and thus reduces subsequent problems with walls, etc.
Where damp proof courses are NOT allowed, then other techniques can be used to reduce the problems with ‘breathing’ wall plaster (such as slaked lime plaster). Concrete based plaster does not ‘breath’ and will add to further problems/
What to look for
If provided, there should be a visual horizontal line near the base of structural brickwork of stone. The type of barrier could be slate, or a black mastic, or a plastic film. There is often a difference is visual colour between the brickwork below and above the damp proof course.
When to look
When the initial detailed survey is undertaken. Collect photographs of the existing situation.
Undertake annual inspections to identify areas of failure of a damp proof course.
How to fix
The construction — two skins of stonework with a rubble core — makes it impracticable, if not impossible, to insert a traditional felt or lead dpc into external walls.
There are proprietary methods that claim to alleviate rising damp either by;
- Chemical injection
- Electro-Osmosis – neutralising or reversing the positive potential of the wall relative to the earth using an electric charge
- High capillary ceramic tubes, but this is unsuitable for drying out basement walls. While techniques are continually being developed and studied, it is wise to obtain impartial advice before specifying any one proprietary system of damp-proofing.
Avoiding creating problems
Neither Chemical Injection or Electro-Osmosis methods can be recommended without reservation; it may not be possible to ensure full penetration by chemical injection into rubble walls of composite construction containing voids, and the claims for electro-osmotic systems have yet to be proven.
Historical background
Damp proof courses were required in new building from 1920’s, and became standard in 1930’s. Bulding before 1919 rarely had a permanent damp proof barrier, relying on ‘breathable’ plaster to allow dampness to evaporate naturally. With damp-proof courses came concrete-based plaster, which was quicker to apply and cured much faster.