Global warming and increased air-conditioning demand drive up energy use. A passive cooling system for the building's roofs is one solution to this problem. The current study intends to provide a unique evaporative cooling concept for cooling building roofs that use significantly less water than prior approaches. To reduce room temperature, saturated-activated alumina beds with 2, 4, and 6 cm thicknesses were put over the roof. Pure and 35 PPT saline water were utilised to evaluate system performance and sustainability. The device successfully reduced rooftop temperatures by 84 % and 70.9 % after 24 h of testing (6 h during daytime and 18 h at night) under heat loads of 800 and 1000 W/m2. The findings show that salt water is a feasible alternative to freshwater that does not sacrifice cooling efficiency. From an environmental standpoint, compared to a bare roof, a 6-cm layer of saturated activated alumina tables can reduce annual carbon dioxide emissions by 71.3 tCO2/m2 and 92 tCO2/m2 for roof areas subjected to daily solar radiation of 6000 W h/day and 4800 W h/day, respectively.