Our organization is planning to construct a greenroof on the University of Michigan's Outdoor Activities Center (OAC). The greenroof will use a modular system of four square feet planter boxes with a soil depth of four inches to allow native Michigan plant species to grow successfully.

To date, there are no greenroofs on the University of Michigan campus, despite a history of environmental activism at the college. By placing this pilot project on our campus we hope to not only benefit the environment, but also to promote the idea of a greener community at the university.

In addition, we want to use plant species that are native to Michigan, including various types of grasses and perennials. Currently, most greenroofs use sedums, which are foreign, drought-resistant plants. By using native plants, the greenroof would contribute to current attempts to preserve Michigan's natural ecology.

Greenroofs also provide a way to reduce the waste produced by typical roofs. A normal roof, which is covered with shingles or tiles, must be replaced about every 20 years. Each time this is done, the old covering becomes garbage. A greenroof lasts for 60 or more years without replacement. Therefore, a building with a greenroof causes much less pollution in the long run. We plan to use recycled materials in the layers that make up the greenroof.

Implementation of a greenroof addresses several important environmental issues, namely storm water retention, energy conservation, reduction of air and water pollution, and native wildlife preservation. In addition, they are aesthetically pleasing. Each of these issues is discussed in depth below.

Storm Water Retention
A greenroof system is composed of several layers as demonstrated in Figure 2.1 below. The depicted layers are designed to be light and porous to serve as a highly absorbent sponge. This capability of the layers provides a storm water retention system in the case of heavy precipitation. A typical roof produces polluted runoff that needs to be contained in an external system, but the greenroof's absorbency reduces the volume of runoff that otherwise would have to be controlled elsewhere. This reduces the amount of pressure on storm sewer systems and contributes to the lessening of sewer flooding. During severe storms, the roofs delay and reduce the initial massive amount of runoff created by standard roof designs. The amount of water retained would depend on the amount of soil and vegetation -- for example, a 4 inch layer of grass and vegetation would retain about 71% of oncoming water.

Energy Conservation
Compared to a standard dark roof, a greenroof has the ability to conserve energy for the building on which it is installed. The vegetation protects the roof from extreme temperatures, unlike a dark roof, during the warmer months of the year. Figure 2.2 below shows a comparative analysis of roof surface temperatures over a two-day period for dark roofs and greenroofs. By reducing the surface temperature of the roof, the transfer of heat into the building is also reduced Ð diminishing the need for air conditioning. In winter months, greenroofs provide insulation and decrease the amount of heating needed. This conservation of heat and energy means less pollution would be created.

Reduction of Water and Air Pollution
In a city, the air and rainwater carry pollutants from vehicle and industrial emissions. A greenroof has the capability to filter these pollutants. One of the most concerning problems caused by rain is the heavy metals contained within, such as cadmium, copper, and zinc. However, studies have indicated that greenroofs can reduce the amount of such metals. These pollutants bind to the soil rather than being discharged with the runoff into rivers, streams, and other bodies of water. Greenroofs also improve air quality by filtering pollutants from the air and producing oxygen.

Native Wildlife Preservation
A greenroof also promotes preservation of biodiversity. For example, greenroofs can provide a stopping-zone for birds, bees, butterflies, and other insects. Greenroofs can also sustain a habitat for species of plants easily damaged on ground level by walking.

Our proposed greenroof is situated in a small city that experiences hot summers and cold winters, in addition to heavy rain in the spring and fall. Here the major concerns addressed by the greenroof are energy conservation and storm water retention. It is still very important to filter the pollutants and preserve the natural habitat, but these concerns are less pressing in this location than in a more urbanized area.

Aesthetics
Greenroofs are visually pleasing to the surrounding community and studies have shown that seeing vegetation outside one's window improves mental health. In general, greenroofs are much more appealing than a black roof.