The Taiwanese government gave an impetus to the 'Green Building Promotion' movement in the year 2000, and set the 'Water Resource Norm' to increase water conservation in building facilities. Furthermore, in 2003, the norm was changed to the 'Water-Resource-Substitution Design' act to reflect the conception of utilizing rain as a secondary water supply in order to reduce the water-clarified needs and enhance increase demand of water supply. In "Water Resource Indication" of green buildings, the condition of water replacement is different in different geographic area. While the water usage replacement condition at Taipei is based on the rainfall data collected in the Taipei Weather Station of the Central Weather Bureau, Taipei Basin's geographical distinction reflects the rainfall differences among all districts. This paper investigates use rainwater to replace the huge water usage of 137 elementary schools at different regions. According to data school' factor are extracted by the factor-analysis statistic method for the balance influence between rainwater supply and demand at Taipei elementary schools. Results of this study can provide a valuable reference to calculate water collection and to estimate how much rainwater a school needs for washroom use.