In 2007, Columbia joined with nine other universities in the City in Mayor Bloomberg's PlaNYC Challenge, pledging to decrease carbon dioxide emissions 30 percent by 2017. One initiative that is in the process of being explored to aid with this effort is the expansion and development of green roof (or vegetated roof cover) architecture.

Photo 1: Green Roof Con Edison Training, Long Island City

According to the National Institute of Building Sciences Whole Building Design Guide there are two types of green roofs: extensive, which are the most common and lightweight consisting of 3-4" of soil and low growing vegetation, and intensive which consist of 4" or more of soil. Intensive green roofs can support a larger variety of plants, shrubs and trees.

All successful, well-designed green roofs employ several features that are not commonly found as part of your typical roof.

1. Drainage: must maintain optimum growing conditions and manage heavy rainfall conditions without damage due to erosion or build up of water.
2. Plant nourishment: the medium must meet specified requirements for grain-size distribution, moisture retention etc.
3. Provide protection of underlying waterproofing systems through drainage layers, which might consist of plastic sheets, fabric or synthetic mats, or granule mineral layers.

There have been several notable benefits to the development and facilitation of green roof technologies to date. These include:

1. Controlling storm water runoff- Rapid runoff of storm water from roofs contributes to flooding, erosion, and pollution. Green roofs minimize this through their imbedded retention and detention properties. According to the National Institute of Building Sciences The Whole Building Design Guide, green roofs can be "designed to achieve specified levels of storm water runoff control, including reductions in both total annual runoff volume (reductions of 50 to 60 percent are common) and peak runoff rates for storms". Successful trials have been measured in Germany where large zero-discharge developments that rely heavily on green roofs are operating.

As a result of decreased volume and rate of storm water runoff, green roofs promote additional benefits such as

2. Improving water quality- by reducing the frequency of combined sewer overflow (CSO) events. Cities often face problems when heavy storms cause systems to overflow resulting in the discharge of raw sewage material mixed with storm water runoff. This poses tremendous human health and ecological dangers. Green roofs control this runoff, and aid in the improvement of water quality.

3. Mitigation of Urban Heat-Island Effect and Conserving Energy- Replacing dark roofs with green roofs results in reduced temperatures above the roofs. The plants act like a reservoir for heat, absorbing energy from the sun and removing heat from the building through the process of evapotranspiration. Additionally, green roofs are highly effective insulators and they act as an envelop to prevent heat loss. Through temperature moderations green roofs conserve energy and reduce costs for building residents by eliminating or reducing heating and cooling systems.

While I have highlighted these benefits above, I should also note that green roofs have ancillary benefits in the form of prolonging the service life of roof materials, absorbing sound that otherwise bounces off hard roof surfaces, creating new wildlife habitats, and improving the aesthetics of city buildings.

Below is a brief video report by Wall Street Journal's Shelly Banjo, investigating the potential costs and benefits of green roof adoption in New York City.