ABSTRACT THE PERFORMANCE OF MEDIUM AND LONG SPAN TIMBER ROOF STRUCTURES: A COMPARATIVE STUDY BETWEEN STRUCTURAL TIMBER AND STEEL ERTASTAN, Evren M.S, in Building Science, Department of Architecture Supervisor: Assoc. Prof. Dr. Erc&uuml / ment ERMAN December 2005, 174 pages This thesis analyzes the performance of structural timber and steel in medium and long span roof structures. A technical background about roof structures including structural elements and roof structure types, span definitions, and classification of roof structures are discussed. Roof structures are detailed with traditional and the contemporary forms. The thesis comprises the comparison between structural timber and steel by using structural, constructional and material properties. Structural forms and the performance of timber and steel are discussed. The research also includes the roof structures built with structural timber in Turkey, application, marketing and examples in Turkey are indicated. In the conclusion part the performance criteria of timber and steel are summarized, the researcher has prepared a table to compare the performance of timber and steel. Keywords: Timber, Steel, Roof, Structure, Span