This paper describes an approach to evaluate the safety of existing buildings, which is different from current reliability methods. The approach combines subjective judgement and empirical or objective data to give an approximate estimate of the possibility of failure of the building. Fuzzy sets are used to model linguistic and qualitative factors which are important to structural safety but are ignored in the current practices. Advantages and difficulties of the possibility approach are discussed. A three-members frame is used to illustrate the methodology.