Nowadays, the whole country is overrun by the water crisis. Therefore, rainwater harvesting is desired as an efficient alternative for water supplying. Before the execution of any rainwater harvesting project in rural or urban buildings, we need to prepare a comprehensive manual for designing and optimizing the water reservoirs. In this work, an inclusive plan was designed to study all parts of a rooftop catchment system, i.e. rooftop area, transferring equipments, reservoir and also water quality problem in Golestan province. Finally, a guideline was prepared for designing and optimizing the reservoirs with consideration of social and economic scopes in the study area. The methodology was based on the field surveying and questionnaire filling in 12 selected sites in Golestan province. Then, each site was hydrological simulated and reservoirs’ volumes were optimized based on the mass diagram approach. Water quality monitoring was carried out, as well. Finally, technical report was prepared and split into 7 parts including “introduction”, “literature review”, “study area”, “hydrological simulation and optimization”, “water quality”, “guideline” and “conclusion”.