A rooftop greening has many useful functions. In these functions, a flood peak reduced functions influenced by the rainfall patterns. Therefore, to implement the rooftop greenings promising control rainfall outflow, it is available to analyze rainfall patterns, propose “Rooftop greening suitable site map” , and discuss about these relationships. In this study, the aim was to clarify the properties of suitable site for a rooftop greening. Collecting the meteorological data and the flood disaster data, we grasped the urban structure and rainfall pattern of urban inundation disaster factors. Based on the evaluation data, the building ratio, the average of floor area, the road density, and land use, we considered the rooftop greening area. By the results from the simulation of the quantity of rainfall outflow, we could verify the reduction effect of rainfall outflow by rooftop greening in early stage of localized torrential rain.