Basement Waterproofing, Damp Proofing, Expert Answers

Our kitchen wall has been damp since we moved in, but we are reluctant to have a damp proof course because we have fitted kitchen units against the damp wall. Part of the wall is also 60cm underground on the outside. Friends of mine have suggested that we externally damp proof this wall by digging a trench, sealing the wall with bitumen or such like, filling the trench with gravel so the water can flow freely. I have never heard of this system. Is it a recognised system for damp proofing a wall and does it work? Many thanks.

Answer: A damp proof course in isolation is designed to protect against moisture rising up through a wall via capillary action, where surface tension in the water wicks water up through the pores in building materials.  Therefore, they should only be used above ground, because where a wall retains soil, inclusion of a DPC below ground level would be pointless because moisture can simply bridge above it via the retained ground.

So, a DPC may be appropriate where the wall does not retain ground however a degree of 'waterproofing' (as opposed to damp proofing) is what would be necessary where ground retaining.

In respect of the proposal, this may be effective to a degree in stopping water within the soil from coming into contact with the wall, since the loose stone would encourage free drainage and the bitumen may provide a physical barrier, however it is assuming that water draining down will continue to drain down/away when it reaches the base of the gravel, when there is a chance that what you might actually create is a moat at the base of the excavation/gravel, concentrating water on the wall at that position.

Furthermore, even if such a waterproofing detail is effective, water from the ground is not pure, typically containing salts held in solution (nitrates and chlorides).  Where water moves through walls (and internal finishes) then evaporating, the salts which cannot turn to vapour, are deposited in the walls and can collect in time to contaminate.  Such salts are 'hygroscopic' which means that they are capable of easily absorbing moisture, including from the air.  The implication of this in a kitchen is that when water vapour is produced, as it would be in a kitchen, this could result in salt contaminated walls becoming readily damp.

This is why the installation of a damp proof course to address true rising damp, should ALWAYS include replastering up to 1m or so, because this removes contaminated plaster, which in then replaced with new finishes containing salt retardents. 

So, some external work may be of benefit (depending on drainage) particularly if any internal work to address potential salt contamination can also be undertaken.  With this being in our home town, we would certainly be able to inspect and give you further verbal advice if that would be of assistance.