section_path: Green infrastructure/Green roofs
section_name: Green roofs
paragraph_text: Green roofs
  
  A green roof is a rooftop that is partially or completely covered with growing vegetation planted over a membrane. It also includes additional layers, including a root barrier and drainage and irrigation systems. There are several categories of green roofs, including extensive (have a growing media depth ranging from two to six inches) and intensive (have a growing media with a depth greater than six inches). One benefit of green roofs is that they reduce stormwater runoff because of its ability to store water in its growing media, reducing the runoff entering the sewer system and waterways, which also decreases the risk of combined sewer overflows. They reduce energy usage since the growing media provides additional insulation, reduces the amount of solar radiation on the roof's surface, and provides evaporative cooling from water in the plants, which reduce the roof surface temperatures and heat influx. Green roofs also reduce atmospheric carbon dioxide since the vegetation sequesters carbon and, since they reduce energy usage and the urban heat island by reducing the roof temperature, they also lower carbon dioxide emissions from electricity generation.