Conventional green roofs, although having numerous advantages, could place water resources under pressure in dry periods due to irrigation requirements. Moreover, the thermal efficiency of green roofs could decrease without irrigation, and the plants could get damaged. Therefore, this study aims to improve the efficiency of conventional green roofs by proposing a new multipurpose green roof combined with fog and dew harvesting systems. The analysis determined that the average water use of green roofs in the summer (in humid regions) is about 3.7 L/m2/day, in the Mediterranean regions about 4.5 L/m2/day, and in arid regions about 2.7 L/m2/day. During the dry season, the average fog potential in humid regions is 1.2 to 15.6 L/m2/day, Mediterranean regions between 1.6 and 4.6 L/m2/day, and arid regions between 1.8 and 11.8 L/m2/day. The average dew potential during the dry season in humid regions is 0.1 to 0.3 L/m2/day, in the Mediterranean regions is 0.2 to 0.3 L/m2/day, and in the arid regions is 0.5 to 0.7 L/m2/day. The analysis of the suggested multipurpose green roof combined with fog/dew harvesting systems, in the summer, in three different climates, show that fog harvesting could provide the total water requirement of the green roofs, and that dew harvesting by PV (photo-voltaic) panels could provide 15 to 26% of the water requirements. Moreover, it could show a higher thermal impact on the building, higher efficiency in stormwater management, less dependence on the urban water network, and greater efficiency in decreasing urban air, water, and noise pollution. Finally, the novel green roof system could consume less water due to the shaded area by mesh and solar PVs and maximize the utilization of the roof area, as solar panels could be applied on the same green roof.