Damp-proofing the Underpinning

One of my readers has asked a question about how the design ensures the damp-proofing of the underpinning of the existing house. I will reproduce my answer here, in case it is of interest to the wider audience. Hopefully the extract from the drawings reproduced below will help to explain this :

This is a cross section, from the back of the extension, with the new basement room on the left, and the existing house on the right. The rough cross-hatching represents the ground (mud, earth, rocks - that sort of thing !).

The concrete that was cast last Thursday (19 Jan) forms the base slab for the building. On top of this will sit a sheet DPM (shown in red above) that is carried up the underpinning to link in with the existing damp course above the old foundations. Inside of this DPM will be another blockwork wall that goes just as high as those foundations. At it's base it sits on the horizontal portion of the DPM. This wall is therefore on the "dry side". The cavity between the DPM on the underpinning and this wall is filled with insulation and mortar. A screed goes on top of the horizontal DPM across the whole floor. On the other side of the build the same applies - the DPM is carried upwards to the top of the retaining walls. These retaining walls are therefore on the "wet" side while the inner house wall is on the "dry" side of the DPM.

Interestingly, one of my other readers, Ian, who has just been through a project to rennovate a barn in France, wrote to tell me that over there they did not use a sheet DPM. Instead, thick black tar was painted on the outside of the walls below ground level. This means that there is no masonry work on the "wet" side. I wonder why there is a difference. Do we need a European standard for water proofing basements !!!!!!!