|Fig. 1: Green Roof Construction Sample (Source: Wikipedia Commons.|
Green roofs, also known as eco-roofs or livings roofs, implement plants in order to improve roof functions. There are two basic types of green roofs: intensive and extensive. Intensive roofs tend to have deeper, natural soil and can support a wider variety of plants, including shrubs and even small trees.  Extensive green roofs on the other hand, have 6 inches or less of a mineral-based growing medium instead of soil and generally support tough, drought-resistant, low-growing plants.  Although intensive roofs often have a greater aesthetic reward, extensive roofs generally provide the greater economic and ecological return on investment. Most of the green roof research in existence today has been conducted in Europe. Green roof research began in the 1960s in Germany, and as a result there has been a viable green roof market since the 1970s.  Green roofs have just begun to become popular in the United States. However, they have been popular for a significant amount of time in Europe, where most of the green roofs are extensive. 
Many of the benefits to green roofs are ecological, including stormwater management. When it rains, water runs off of normal roofs, hits the ground and runs along paved areas picking up pollutants and depositing them in rivers, lakes, and oceans. Common stormwater contaminants are fertilizer, herbicides, oil, grease, salt, acid, and bacteria from animal waste.  Before the creation of houses and cities, the leaves, soil, and plants would be able to reduce and slow the water runoff. However, due to paving, and general land development, natures mechanisms are unable to work. Green roofs help solve this problem by minimizing roof runoff, and even when the roof water retention is at capacity it slows the water down, helping to control intense peak flows.  Water that is used by the plants gets evaporated back into the atmosphere and never even touches the ground.  In temperate regions of North America, green roofs even as thin as 4 inches are able to capture approximately half of the annual rainfall.  Green roofs can even help neutralize runoff from acid rain due to the filtering effect of the plants and medium.
Another environmental benefit is the reduction of the urban heat island effect in cities. It has been shown that cities are significantly hotter in the summer than their suburban and rural counterparts in the same region because clustered buildings and paved surfaces hold and then slowly release solar radiation. The urban heat effect can magnify heat waves, resulting in extreme temperatures in cities that have even resulted in deaths. For example, hundreds of individuals died in the July 1995 heat wave in Chicago.  Green roofs will increase the amount of vegetation in the city, which will help reduce this effect by decreasing the amount of exposed paved surfaces and by cooling the air through evapotranspiration.
Green roofs can provide a habitat for wildlife in urban areas. If properly designed, green roofs can help to preserve urban biodiversity by supporting birds, insects, butterflies, and other small animals. 
Green roofs also have economic benefits, including a longer life span. Due to increased heat on rooftops and exposure to the sun's ultraviolet rays, conventional roof membranes generally have to be replaced every 15 to 20 years. Green roof membranes generally have a conservative estimate of at least 30 to 40 years, and this longevity has been green roof membranes in Germany that have lasted decades intact.  Reduced frequency of roof replacement is not just beneficial monetarily for the homeowner, it is also beneficial to the environment due to reduced discarded roofing materials accumulating in landfills. 
Another financial benefit (and environmental benefit) to green roofs is lower energy costs. Although the degree of this effect will vary based on the design of the green roof (due to varying air and moisture content, biomass, species, etc.) the green roof will help maintain a stable temperature. In some instances in will act like an insulator while in others the thermal mass effect will result in reducing the temperature gradient above and beneath the roof.  The main mechanism behind green roof passive cooling is through providing protection from solar radiation. Green roofs reduce thermal fluctuation on the outer surface of the roof to cool in summer, and increase their thermal heat capacity in winter, thus reducing heat loss during winter.  This means that due to these thermal properties of green roofs, green roofs are able to save energy consumption costs by reducing cooling costs in summer, and heating costs in winter.
One of the largest impediments to green roofs in the United States is that our climate is significantly more heterogeneous than western Europe. As a result (and because green roofs are relatively new) there are no or few design and material standards. Instead, green roof design, installation process, and maintenance program have to be tailor designed for the regional climate and even the roof's local microclimate.  Another is the large initial costs of green roofs. Green roofs are significantly more expensive to build, and although the green roof may last longer than a conventional roof, it also will have greater maintenance costs over its lifespan. However, some of the other benefits, such as stormwater managements and reduced heating and cooling may be able to offset the increased cost.  Finally, green roofs face preconceived notions. Many Americans have difficulties accepting green roofs because they associate gardens/greenery with the need to be watered, and roofs with the function of keeping water out.  As a result, many are concerned about green roofs keeping water out of the house.
Green roofs are a great alternative to the traditional roof with both environmental and economical benefits, however there are still major obstacles to overcome before they become widely implemented in the United States. In order for green roofs to become more widespread, green roof advocates will need to join together and create region specific guidelines on design and plant species for the temperate regions (which will have potentially the greatest benefit) in order to help reduce design costs for new green roofs. It would also help if they are able to get federal incentives and funding, like solar energy has been able to. Finally, they need to inform the public about green roofs and their benefits, so that people choose to either install green roofs, or choose to live in apartment and condo buildings with green roofs and thus drive demand for green roofs.
© Michelle Nii. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 E. C. Snodgrass and L. McIntyre, The Green Roof Manual: A Professional Guide to Design, Installation, and Maintenance (Timber Press, Inc., 2010).
 S. Cantor, Green Roofs in Sustainable Landscape Design (Norton, 2008).
 C. Wertherman, Green Roof: A Case Study (Princeton Architectural Press, 2007).
 A Niachou et al., "Analysis of the Green Roof Thermal Properties and Investigation of Its Energy Performance," Energy and Buildings 33, 719 (2001).