damp proofing?

[jungle_jim01]

Here's a question which i'm sure has been discussed many times before but here goes anyway....

The front room of our Victorian extension has had the dreaded concrete floor treatment at some point in the past. and one side of the room is about 6inches below the outside ground level. Hence lots of damp on that wall. To make matters worse (i suspect) the exterior has been rendered so my guess is that moisture has nowhere to go. I've had the plaster off for about 6weeks now and most of the wall is drying fine, but at ground level there is clearly damp ingress. Now i'm looking for a solution to keep the damp out so we can replaster. My plasterer suggested 'bitumen tanking'...(i can hear the gasps of horror!) I should say that with the rest of the house we've been quite sympathetic to traditional methods but time & money means that we cant remove the floor and render and start again. We just need a damp free room. So any thoughts on tanking methods? or alternative ideas.
Thanks as always for the helpful advice.

 

[Zebra]

Is it possible to lower the external ground levels? Can you remove the cement render externally?
If you tank the inside, the damp will still have nowhere to go, and will be trapped in between your cement render and your bitumen tanking. This will mask the problem for a while, but won't cure it.
How about a French drain along the outside wall?

 

[jungle_jim01]

Thanks, yeah i'd wondered about a French drain, at the moment it's not an option but maybe a bit further down the line. although i was hoping to be able to avoid that. Would tanking suffice in the interim, followed possibly by a French drain at a later point?