Do Green Roofs Help the Environment?

Runoff Control

• As cities grow, more and more land area is covered with impermeable material. When it rains, water runs off roofs onto asphalt and concrete, picking up pollutants along the way. Storm water is delivered to sewer systems without a chance to seep into the ground, as occurs in natural areas. This large amount of runoff inundates storm-water systems with large volumes of polluted water. Living roofs mitigate storm-water runoff by slowing the flow of water off roofs, reducing peak runoff volume that sewer systems must handle.

Water and Air Pollution

• Living roofs are like giant water and air filters for urban environments. Plants absorb some pollution, such as heavy metals and nutrients from rainwater, and soil provides further biological and mechanical filtering of runoff.

  Plants are natural air purifiers that absorb pollutants such as nitrogen, sulfur and ozone from the air, and living roofs provide natural air purification right in the middle of urban environments, where pollution is greatest. Living roofs also battle climate change by sequestering carbon in their biomass.

Energy Conservation

• Not only do living roofs reduce pollution directly by acting as natural air and water filters, they also reduce pollution indirectly by reducing the cooling and heating load of buildings and reducing buildings' energy use. Living roofs act as thick insulating blankets for buildings. During summer, they insulate buildings from the hot summer sun, and in winter they reduce heat loss through the roof. Year round, they help building interiors remain at relatively constant temperatures, reducing the need for heating and air conditioning. With less heating and cooling requirements, buildings use less energy, resulting in less air pollution.

Urban Heat Island

• One of the major problems of urbanization is the "urban heat island effect." Pavement, rooftops and masonry building materials absorb solar energy and radiate it back to the surrounding air, making urban areas much warmer than adjacent vegetated areas. Heat islands further increase the need for air conditioning, increase energy consumption and pollution, and pose heat-related health risks to humans. Living roofs combat the urban heat island effect by providing shade and evapotranspiration from plant leaves, which cools air.

Habitat

• Urban development displaces natural habitat; but living roofs can return a level of wildlife habitat to cities, improving the urban environment for both wildlife and humans. Over time, successful living roofs become aesthetically pleasing, functioning ecologic systems. Instead of a lifeless expanse of hardscape, living roofs create sanctuaries for wildlife and people.