In Scandinavia and Finland, nearly all constructions, including monumental and sacral architecture, used only timber until the 11th century. In Finland, timber remained the dominant construction material until the 19th century. Due to many fires, wars, and other catastrophic events, the number of still-standing wooden buildings and constructions erected before industrialization is very limited. Medieval wooden buildings can only be found in archaeological excavations, and the last remaining medieval load-bearing wooden structures are preserved in the medieval stone churches. While medieval roof constructions have been widely researched elsewhere in Europe, a comprehensive understanding of medieval roof constructions in Finland is largely lacking. This article presents an ongoing study of roof structures in Finnish medieval stone churches, focusing on two churches, Pernaja and Hammarland, which provide evidence of the transition from tie beam roof trusses to roof trusses without a tie beam. We examine the typology of the trusses, and present new results based on a comparison of the churches and dendrochronological analyses. Finally, we discuss possible explanations for the structural innovations and the emergence of the roof truss without a tie beam in a wider cultural context.