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I am converting a barn which has 500mm+ thick wall and an exisiting concrete floor. The floor has to be taken up and re-laid with insulation & DPM. I understand that the DPM will "keep" water from rising through the slab but how do I fit this into the exisitng stone walls?.

The outside ground layer is 200-300mm lower than the top of the existing slab, so do I just chase out a grove and fit the DPM into it?. At what height above the slab do I do this?.
Doesn't sound right but I can't think what else I can do.

Thanks

I dont think you can,as if the wall is solid, unless you get the DPM right the way through you will not achieve anything.
What materials are the walls built of,and how old is the barn?
On a brick built solid wall you could damp roof render up to meter high,as damp will only rise about 2'3" above its entry point.
I have a similar situation, on a stone built barn, not in this country, and have used a lime based render/plaster, as this lets the walls breath, and haven't noticed any major problems.

Chemical DPC injected by specialist contractor. If you want to have a bash yourself there's this product http://www.dampness-info.co.uk

You can't insert a physical DPM into an existing wall.

Cheers