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The paper was performed to reduce cooling load on the top floor and to use for toilet washing using rainwater. The rainwater on the roof top acts on as thermal buffer zone and thermal storage. To use rainwater for toilet washing and thermal buffer zone, it is decided on the proper capacity of rainwater and the proper height from the surface of roof top to the improved roof drain. According to the surface temperature is calculated by weather data in Pusan, the cooling load between the existing top floor and top floor equipped with improved roof drain could be calculated. The heat transfer of roof top with improved roof drain is occurred by conduction, convection, radiation and evaporation but that of the existing roof drain is occurred by conduction, convection and radiation except evaporation. As the cooling load of top floor is decided on the surface temperature difference between roof top(outside) and top floor(inside), the rainwater could be used as thermal buffer zone between roof top and top floor. According to this paper, the surface temperature difference between the existing top floor and improved top roof is showd 20℃ difference at 14:00. The heat transfer in the roof top equipped with improved roof drain occurred from indoor to outdoor.