Expert systems are fast becoming the leading branch of artificial intelligence technology. An expert system tool, ‘The Deciding Factor’, was investigated for use in building a Lightning protection decision system, an expert system for structures. The system described in this paper is a diagnostic expert system, developed for IBM PCs. There are a number of factors affecting the risk of the structure being struck by lightning and consequential effects of a stroke, which are capable of assessment. National Building Code of India has specified some judgemental and heuristic rules to combine these factors. The system was developed to model this expertise and to provide consultation to the user. This paper addresses the issue of knowledge engineering and demonstrates the ease with which the ‘knowledge engineer’ himself can build the systems with the use of an expert system shell.