Green roofs provide three main benefits: Stormwater-management, energy conservation and urban habitats for insects and birds. They also provide significant documented benefits such as noise reduction, improved air quality, aesthetic (human health), local food production (urban farming) and job benefits for urban residents.
1. Stormwater Management
Urban areas often have large areas of impervious surfaces, which contribute significantly to stormwater runoff. When there is rainfall, water is retained by the green roof substrate (i.e soil) which reduces the immediate runoff volume from up to 1″ of rainfall. Large runoff volumes are a problem when there is an excessive amount of surface water that impacts local streams, increasing erosion and sediment pollution. Green roofs reduce this immediate runoff, and green roof plants are able to absorb and transpire that water back to the atmosphere through the process of transpiration. Essentially, green roofs convert impervious roof areas to structures which store water and decelerate the rate of runoff. Several factors contribute to the efficiency of this process, including:
- Slope of the roof
- Rainfall patterns
- Type of plant community
- Substrate material
The type of plant community and substrate material are the two most important factors that determine the efficiency of the green roof is as a stormwater management tool. We’ll discuss this issue in more detail (see our research, below).
2. Energy Conservation
Green roofs reduce the heat load (radiation) from the sun, as plant leaves intercept that radiation and use it, together with carbon dioxide (CO2) and water in photosynthesis.
Leaves also transpire water to reduce the radiation heat load, thereby cooling the surface even more. In fact, an unvegetated roof can heat up to more than 70 degrees Celsius (°C) in the summer, while the surface temperature of the green roof typically will only reached 30-35°C. It is via these mechanisms that green roofs reduce the ambient heat load on a roof, saving in heating and cooling a building. This has a major impact on the heat load of air intake by air-conditioning units oftentimes placed on roofs.
Another study done by the General Services Administration (GSA) revealed that green roofs can reduce peak electricity demand and lead to an annual savings of $0.23 per square foot of the roof’s surface (REFERENCE). A study in Madrid showed that green roof reduced the cooling load on an eight-story residential building by 6% during the summer. In the winter, green roofs also minimize the heat loss through added insulation on the roof.
Urban Habitat Provision
As mentioned above, the cooling effect of green roofs is important because mechanical air-conditioning (HVAC) units must pre-cool the temperature air taken from the roof, requiring a much greater amount of energy. Since the surface air is cooler when a green roof is present, the thermal difference between outside and inside air is less, greatly decreasing the energy use of HVAC systems.
Additionally, many human-made changes to the environment such as buildings, pavements, and etc., are made of materials that have different thermal properties (heat capacity and thermal conductivity). This will oftentimes cause an effect known as the urban heat island effect which is when the metropolitan area is significantly warmer than its surrounding rural parts. Green roofs stabilize the temperature by reducing the dark pigmented or low heat capacitance material which would otherwise cause temperatures to fluctuate.
- Waste Diversion
- Improved Air Quality
- Fire Mitigation
- Urban Wildlife Habitat
- Urban Farming
- Noise Reduction
- Job Availability