This should be easy for someone. The exterior of my house is roughcast extending down to concrete paths on the northfacing wall. It's wet there a lot of the time. The roughcast sounds hollow in places when knocked, so I know it's loose. When I redo the roughcast, should I leave a couple of inches or so clear of the concrete path or can the join be successfully sealed? I don't know where the damp course is yet but I don't appear to have a problem with that. The house was built around 1922. Many thanks for any advice.
Yes leave a gap. Water resistent products may be available but nothing beats an air gap to prevent possible rising damp bridging existing DPC which in your case is probably slate positioned in wall just below base of floor joists. It's fairly likely that capillary action from concrete to roughcast has caused your problem in the first place.