Planting grass or trees like in Manhattan (cf. picture) on top of buildings have many advantages as it decreases the air pollution, the local temperature of cities and greenhouse gases emissions and so on.
In this article I will review the various specificities of green roofs and walls and what they can bring to large and medium sized cities.
There are numerous advantages to such roofs as they :
- Improve the insulation of the building in an important way, indeed to livingroofs.org : “Green roofs have a positive effect in terms of thermal insulation through their ability to cool buildings and insulate them during the winter.”
- Absorb the carbon dioxide present in the air and generate oxygen thanks to photosynthesis, . Various air pollutants are also absorbed. To livingroofs.org : “Green roofs reduce the heat island effect, which is the main cause of ozone production ; Plant roofs remove heavy metals, airborne particles and volatile organic compounds.”
- Decrease the temperature within cities.“Roof areas are a significant part of urban hard surfaces. Plants on green surfaces absorb heat and then use it through evapotranspiration. Green roofs therefore would play an important role in reducing urban temperatures, and subsequent improvements in air pollution/smog, as associated with the albedo effect.”
- Absorb the excess water as “Green roofs store rainwater in the plants and growing mediums and evaporate water into the atmosphere. The amount of water that is stored on a green roof and evaporated back is dependent on the growing medium, its depth and the type of plants used. In summer green roofs can retain 70-80% of rainfall and in winter they retain between 25-40%.”
On top of all these advantages, green roofs can be combined with solar thermal or photovoltaic panels so roofs can still be used to generate hot water or electricity.
Green walls also exists and their advantages are similar to those of green roofs.
In France such a concept is very seldom seen. However, the train station of Perrache in Lyon installed last month 200 square meters of green walls to decrease the air pollution from its parking lot.
To infer this article, I would like to state that albeit the sources I found gave many advantages on such roofs, I found no reliable data on the costs. The article on Wikipedia mentions a price of 15 to 35 USD per square foot but no source is given.
Further reading :
- For more data on this subject, you can consult the website www.livingroofs.org which is an “Independent UK Resource For Green Roof Information”.
- For data regarding green roofs in the United States and Canada, one can have a look at www.greenroofs.com. According to this website, there are currently 639 projects due to create up to 444,623,650 ft² of green roofs ( 445 km²).
- The time for green roofs, an IHT article by Elisabeth Rosenthal. (see also the article in the Business of green blog)
- Time to Mow the Roof ; from the green piece blog.