The purpose of this study is to clarify the current state and the change of construction method of external insulation and asphalt waterproofing of flat roofing. The object of this study is the Kodan Housing, covering a period of 19 years from the year of 1984 to 2002. The reasons for the change of construction method were clarified as follows: (1) improvement of the durability of buildings and the waterproof ability (especially from 1984 to 1992); (2) improvement of the efficiency and rationalization of construction (especially in 1984); (3) the securing of quality of construction; (4) readjustment with new materials, new building methods, and new regulation; (5) readjustment with the problem of human health (especially in 1992, due to the problem of asbestos).