Sealing the bottom of a roughcast wall Damp can be a major issue in the home. Find answers to questions or post your own here.

[STILLUCAN]

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.