Building survey on house purchase says "front step appears to have breached damp proof course and be blocking subfloor ventilation". Some of the internal joists near the front door have therefore rotted. We are trying to work out how to rectify this with the least disruption, could we just put a channel/gap between the paving slabs and the front of the house in the area that is protected by the porch? If so approximately how wide would this have to be ?