Reduction in rainfall, population growth, environmental issues, and a need for development of green areas in residential buildings have become a major challenge for water resource management. The majority of urban surfaces are impermeable and as a result precipitation mostly flows on the surface as runoff. Hence, storage of rainfall in dry urban areas has an important role in water resource management. Tehran, with a population of more than 10 million people, has a dry semi-arid climate. Storage of rainfall can be effective in providing water for various purposes. The aim of this study was the evaluation of rainfall in Tehran and to assess the required volume of storage units to be built on roofs by designing and optimizing collection reservoirs for irrigation of green areas in residential units. To this end, precipitation data of two synoptic stations of Shemiran and Mehrabad, have been used for the northern and southern parts of Tehran, respectively. The evaluations were performed under two scenarios. In the first scenario, the highest monthly rainfall, and in the second scenario, the average annual precipitation was considered and storage volumes of tanks were calculated for roofs with areas ranging from 100 to 550 square meters. The results showed that in Tehran, using rainwater harvesting system in the months with the highest rainfall can meet the needs of green areas in drier month, as well as some other non-drinking water uses. Under the scenario which utilized the months with the highest rainfall, for every 100 m² of roof space, about 5000 liters of rainwater can be stored which lessens the pressure on the municipal water supply network and offers economic benefits .