Green roofs present several advantages at multiple scales. However, the applications are still not spread enough, which could be mainly due to the uncertainty about some factors such as water consumption in the dry season, the usage of the roof for other purposes like solar panels, and construction issues such as initial and maintenance costs. Thus, this study aims to develop an optimization method for an independent green roof system disconnected from the urban water network. It means the optimized size of a green roof, in which the precipitation, runoff, atmospheric water harvesting (AWH), and greywater could be enough for the green roof's water requirements without further irrigation. In this regard, a mathematical model is developed that can be verified based on experimental data of a green roof and determine the optimized area according to determined scenarios. The developed model is used in a Mediterranean climate, considering different scenarios. The results showed that for a house with a roof area of 100m2 and three inhabitants, an independent green roof, unconventional-based water resources, is possible in 75% of the roof area.