Historic assets, as a part of cultural heritage of a country, reflect traditions, knowledge and beliefs of different communities. It adds distinguished meaning to places where we live and provide source of identity. Accordingly, Historic environments need to be conserved and sustained while enabling people to enjoy those assets and make beneficial use from it. However, continuous guidance and support need to be available to assist in sustaining such environments. This paper presents an expert system, named ES_HXWall, that can be used as a decision support tool for conserving and maintaining external walls of heritage buildings. The proposed ES_HXWall takes the needed criteria in a form of rules coded using CLIPS shell. ES_HXWall knowledge is extracted from two main resources; Historic Scotland Inform Guide and domain knowledge of experts who are specialist in historical sites restoration.