You are seeing green roofs in the news more and more these days. Most recently France passed a law that all new commercial buildings across the country need to be at least partially covered in plants or solar panels. The green roof or what is sometimes called a living roof are becoming a common buildings practice throughout the world. There are many benefits to these types of roofs from reducing energy costs to the absorption of storm water. One of the primary reasons the adoption of these roofs has increased is their effectiveness at combating urban heat island effect.
In urban areas the traditional roof covering consists of asphalt, tar, and gravel. This traps a lot of heat that makes metropolitan areas much warmer than their surrounding rural areas. This is called urban heating island effect. The trapping of heat causes a lot of usage of energy and water. The use of green roofs can quickly reduce this effect.
There are two main types of green roofs:
Extensive Green Roof
This type of green roof’s only purpose is for environmental reasons and not used as elevated gardens. They require a relatively thin amount of substrate ( only about 100 pounds per square foot) that supports native vegetation. They have all the benefits of cooling and storm water absorption that comes with a green roof but very little maintenance. An over simplified way to look at a Extensive Green Roof is if you took your yard and placed it on your roof.
Intensive Green Roof
This is the type of green roof what comes to mind for the general public. Ultimately an elevated park with shrubs, trees, gardens, solar panels, amenities spaces, etc… Of course these have the benefits of cooling and storm water absorption of the Extensive Green Roof but they also have the added benefits of food production, energy production, and ultimately being a nice place for people to go and enjoy.
This type of green roof does have a much higher cost of maintenance. Some one has to take the time and resources to care for it all. It is also tougher on the structural engineers because the weight of the substrate needed to support Intensive Green Roofs is almost double at 200 ponds per square foot.
One of the world’s most famous green roofs is Chicago’s City Hall that is a mixture of an Intensive and Extensive Green Roof. There is a great article about it here.