A green roof is a layered system consisting of a waterproof membrane, a drainage mat, a root barrier, growing medium, and plants. Each of these layers should be selected with long-range performance in mind. The media and plant layers protect those below from wind, steep temperature fluctuation, and UV light degradation. Because of the exposure of the plants and media to wind and sun, evaporation of water occurs. Likewise the plants transpire moisture into the air. Both of these actions help cool the roof. The downside to evapotranspiration is that extreme moisture loss will harm or retard plant growth. Plants are an important part of the green roof layer because they hold media in place. Plants, as living things, require light, water, macro and micro-nutrients to survive. Plantings on green roofs should be chosen for the environment in which they are to grow and survive. A key concept is to promote slow to moderate growth, so that during the dormant winter period plants are in balance with their root systems.

Green roofs will never look like a putting green or highly maintained lawn, nor should they. Extensive green roofs will have shallow soil layers (4-6 inches) and are highly porous. They rapidly drain internally so that the support system for the roof will not be overloaded. The problem with such a scenario is that plants must be able to take such dry conditions. It is recommended that temporary irrigation be employed so that the plants have thoroughly filled the media and maximize their ability to take up water. This may take 2-3 years. Media mixes should not have clay fractions. The fine particles in clay will migrate through the media and plug drainage mats and root barriers. This is problematic too, because the surface of the small clay particle hold quite a bit of water and clay will supply some nutrients as it breaks down over time.

Most of the work in supplying moisture and nutrients is in a silt-sized and larger organic fraction, usually compost. Unfortunately compost will break down in a short time and in 4-5 years could be nearly eliminated. Dead tops and roots of the plantings are supposed to replace the organic matter when compost disappears. Right now we have no idea of the turnover rates of organic material in green roofs. Also, without the addition of nitrogen-fixing plants such as legumes into the planting palette, a lack of nitrogen (a critical macronutrient) may limit growth. Green roofs should not have to be fertilized because they are billed as low-maintenance. In the long run, a lack of critical micronutrients may also become a limit to healthy growth. We simply have not had a long enough experience with maintenance of green roofs to know what will happen in the long run.

Remember that green roof plantings and media represent a human created assemblage and ecosystem. We must think like an ecosystem to ensure low maintenance, yet functional green roof plantings.

Hawthorne Hostel, Portland
Cedar River Watershed Education Center, Cedar Falls
Pioneers Park Nature Center(includes green roof video), Omaha

New Harbor, Malmo
Sidwell Friends Middle School, Washington, D.C.
Washington Mutual Roof Garden, Seattle
ASLA Green Roof, Washington, D.C.
California Academy of Sciences, San Francisco, California

ADDITIONAL RESOURCES
http://greenroofs.wordpress.com/great green roof blog. from Australia, but examples from around the globe

A firm by the name ofCTGcan help analyze the mitigating impacts green roofs will have on local temperatures. From the CTG website, the following figure illustrates the potential impact green roofs can have for one sample site. For years, urban observers have documented an “urban heat island,” which is the increased ambient temperature caused by extensive paving, hot roof surfaces and other urban development. Typical temperature rises are on the order of 8-10 degrees. This contributes to increased air conditioning use, increased peak electricity demand (+2% for every 1°F rise in temperature) and increased smog (+3% for every 1°F rise in temperature).

Green Roofs for Healthy Cities, the Green Roof Industry's Trade Asccociation, just released a new tool called theGreenSave Calculatorthat will let professionals evaluate the future. It looks like a pretty great tool.

Portland Bureau of Environmental Services Ecoroof Page

ASLA has recently releaseda book on green roofs as part of its LATIS series.
http://greenroofs.org/
http://greenroofs.com/
http://www.greenroofplants.com/
http://roofmeadow.com/
ECOROOFS (GREENROOFS) – A MORE SUSTAINABLE INFRASTRUCTURE
The Pomegranate Center'sHomeowner Green Roof Guide