Demystifying Green Roofs
[I am on vacation this week in Phoenix and then Kansas City so I bring you GUEST POSTS! I met each of the guest authors somewhere along the way and asked them to contribute. Hope you enjoy.]
By Linda McIntyre
Does the notion of plants on a roof seem bizarre, even dangerous, to you? If so, that’s understandable--there aren’t a lot of green roofs in the US yet. But their numbers are growing, because green roofs--relatively simple installations usually featuring low-growing herbaceous plants, not park-like roof gardens with furniture and trees--can be useful functional tools when properly designed, installed and maintained.
A green roof has a waterproofing membrane like any relatively flat roof. Above the membrane, plants grow in a layer, usually four to six inches deep, of mostly mineral-based medium that looks like gravel, not soil. A drainage layer comprising coarse aggregate or synthetic sheets ensures the system will drain properly after it reaches field capacity, and fabrics are available to protect the membrane from roots if the material isn’t naturally repellent. These layers make up the green roof assembly. Think of a ballasted roof with plants growing among the stones.
Why put a green roof on a building? It can help control stormwater runoff (a big and growing problem in cities), reduce energy costs (especially for air conditioning), and provide amenity space for employees or residents (if a building’s loading capacity allows for such access, a green roof assembly can provide a cost-effective garden). The green roof assembly, when properly designed and installed, protects the membrane, extending the lifespan of the roof. It can earn LEED points and make a building more marketable. If built on a mass scale, green roofs can mitigate the urban heat island effect that makes densely-built cities so uncomfortable in hot weather. A green roof aggregates benefits in a way that other approaches, such as reflective white roofs, which also save energy, can’t. And it will almost certainly look better than a black tar roof.
Maybe you’ve seen a couple of green roofs and been unimpressed. Not every green roof is flamboyantly beautiful, and not every project is successful. There’s no “Consumer Reports” for green roof components, and trial and error is unavoidable with any new technology. Green roofs are common in Europe, but still relatively new here, and we have a more variable climate and a different building culture. Research, project data, and the accumulation of experience are helping to fill knowledge gaps and making it easier to design and install successful green roofs.
A green roof isn’t the right choice for every building. It’s always going to cost more, and installation will always be more complicated. Especially for the first year or two of its life, it will require more maintenance than a conventional roof. But, more and more, it’s a choice worth considering.
Linda McIntyre is a writer specializing in environmental and urban issues. She is working with nurseryman and consultant Ed Snodgrass on a book about designing, installing, and maintaining green roofs, to be published by Timber Press in 2010.
This is a great article. Green roofs have so many benefits, like the ones listed above, but they also help minimize fine particulate matter air pollution.
There are several good sites to learn about green roofs, such as:
http://www.cleanerairforcities.blogspot.com
http://www.greenroofs.org
Greek Treasury Green Roof saves 5,630 euros in energy bills in one year.
The "green roof" created on the roof the economy and finance ministry building in Syntagma Square has resulted in savings of 5,630 euros a year, the ministry announced on Tuesday. The savings arise from a reduction in power used for cooling, amounting to 3,600 euros a year, and a reduction in fuel used for heating by 2,030 euros a year.
The green roof was created in July 2008 and covers about 650 square metres, or roughly half the surface at the top of the building.
Measurements carried out by the applied thermodynamics laboratory at the National Technical University of Athens (NTUA) mechanical engineering department have shown that the difference in temperature between the areas of the roof with plants and those without are 18C, with the former reaching 37C and the latter 55C, respectively.
The measurements also confirmed that the energy savings from the planting amount to 9.6 percent for cooling and 4.4 percent for heating. For the top floor of the building, especially, the energy costs for cooling can be more than halved.
The finance ministry noted that the project was initiated to help improve the environment in central Athens, save energy and reduce the buildings operating costs, as well as acting as a model that might encourage initiatives by other private and public organisations.
http://www.oikosteges.gr
http://media.causes.com/ribbon/568189
For owners who are uncertain about installing a green roof, or who cannot finance the entire thing at the opening of a project, there are some membrane systems that work totally fine on their own and can have plants added on top at a later date. Of course, the cost of the increased structural support needs to be installed at the time the building is constructed.
In addition to the increased insulation benefits and increased roof lifespan as mentioned above, garden roofs can help improve storm water quality by filtering out pollutants before the discharged rainwater is sent to an on-site collection cistern or to the municipal sewer. Roof gardens can provide excellent habitats for migratory animals and can be also use for small scale vegetable cultivation and as additional outdoor space for recreation. It is really a way to turn the unused fifth side of a building into a useful amenity.