A green roof is a layer of soil and low plants laid over a building's roof surface. Because the soil holds rainwater and releases it slowly, buildings with green roofs send far less water into the drains during a storm, which matters in cities where old drainage systems flood easily. The plants also keep the roof cooler in summer, so less energy is spent on air conditioning. Installing one costs more than an ordinary roof, although the covering underneath tends to last longer because it is shielded from sunlight.