In traditional Korean wooden buildings, purlins(dori) are longitudinal structural members which deliver load of roof to beams and columns. Among them, mid-purlins(jung-dori) determine the slope of sectional curve of the roof and at the same time closely engage with interior layout of the building. This study aims to investigate various factors affecting the position of mid-purlins as well as historic changes of the position itself and their result in the structure and design of the building, focusing on the 5-purlin type roof structure, which is the most common in traditional Korean wooden architecture. In general, mid-purlins are horizontally located at the points ranging from a quarter to one-third of the whole transverse building length inwards both from the front and back outer columns. What takes attention is that there is a sharp distinction in the position of mid-purlins between buildings built before 16th century and those built after 17th century. Moreover theses change is closely related with several unique compositional techniques in the traditional wooden architecture of the late Joseon dynast.