Green Roofs

What is a green roof? Green roofs consist of plants being grown on roofs, thus replacing the vegetated footprint that was destroyed when the building was constructed. The highest density of green roofs occurs in Germany, widely considered a leader in green roof research, technology and usage, where it is estimated that 10% of all flat roofs are green. Modern green roofs can be categorized as ‘intensive’ or ‘extensive’ systems depending on the plant material and planned usage for the roof area. Intensive green roofs use a wide variety of plant species that may include trees and shrubs, require deeper substrate layers, are generally limited to flat roofs, require ‘intense’ maintenance, and are often park-like areas accessible to the general public. In contrast, extensive roofs are limited to herbs, grasses, mosses, and drought tolerant succulents such as Sedum, can be sustained in a substrate layer as shallow as 2.0 cm (1.5 in), require minimal maintenance, and are generally not accessible to the public.

Benefits of green roofs. Numerous benefits can result from the adoption of green roof technologies. Besides the obvious aesthetic and psychological benefits of being surrounded by garden-like settings, common ecological and economic benefits include the recovery of green space, moderation of the urban heat island effect, improved stormwater management, water and air purification, and a reduction in energy consumption. The mitigation of stormwater runoff is considered by many to be the primary benefit because of the prevalence of impervious surfaces in urban areas. The rapid runoff from roof surfaces can result in flooding, increased erosion, and may result in raw sewage that is discharged directly into our rivers. The larger amount of runoff also results in a greater quantity of water that must be treated before it is potable. A major benefit of green roofs is their ability to absorb stormwater and release it slowly over a period of several hours. Green roof systems have been shown to retain 60-100% of the stormwater they receive. In addition, green roofs have a longer life-span than standard roofs because they are protected from ultraviolet radiation and the extreme fluctuations in temperature that cause roof membranes to deteriorate.

The green roof research program at Michigan State University. The green roof research program at MSU was initiated in collaboration with Ford Motor Company in an effort to advise them on the installation of a 450,000 square feet green roof on a new assembly plant in Dearborn. The objectives of our ongoing research are to evaluate plant species, propagation and establishment methods, substrates, water and nutrient requirements, and water quality and quantity of runoff. Numerous experiments are currently being conducted on 27 simulated roof platforms at the Horticulture Teaching and Research Center at MSU. The site is equipped with a weather station, thermocouples measuring temperatures at various depths in the growing substrates, and electronic tipping buckets that record the volume and rate of stormwater runoff from the individual platforms. Measurements are taken every ten minutes, 24 hours a day, and are recorded on a datalogger that can be downloaded onto a laptop computer. Combinations of 18 herbaceous species native to Michigan and nine species of Sedum are currently being evaluated. Criteria for evaluating species include rate of establishment, capability to exclude invasive weeds, heat and cold tolerance, drought tolerance, survival and persistence, and fire resistance.

The future of green roofs in the United States. Will green roofs ever catch on in the United States like they have in Europe? Several barriers to widespread acceptance exist such as a lack of awareness regarding green roofs, potentially higher installation costs, limited quantifiable data pertaining to the benefits they provide, no technical information on how to build them, and a lack of government incentives or tax breaks. However, all of these problems are currently being addressed. Furthermore, the same barriers were overcome in Germany. In the U.S. the concept of green roofs is just now being introduced and will likely become more common in the future. They represent an entirely new market for nursery stock and landscape contractors, and the potential market includes all existing and future roofs in the country. A market that it too large to ignore.