A layer of soil is placed over a waterproof roofing material, with raised sides to prevent soil washing into drains when it rains. The soil is planted with vegetation which can withstand extremes of hot, cold, dampness and drying out. Cooling happens not just because of shading the roof surface, but also because up to 60% of all rain is retained on the roof in the soil and as it evaporates, it acts as an air conditioner, cooling the entire surface. Plants themselves also lose water by transpiration, cooling their leaves down.