This paper describes the life cycle environmental cost characteristics of intensive and extensive green roofs versus conventional roofs. A life cycle inventory and environmental impact assessment is used to document and analyze the similarities and differences in the environmental impacts of the fabrication, transportation, installation, operation, maintenance, and disposal of all three roof systems. This is important because there are additional resources committed to green roofs from which environmentally relevant benefits, such as reduced electrical energy use for building cooling, are derived. The extensive green roof design for the case study presented here is from an actual 1115 m~2 (12,000 ft~2) green roof project on a retail store in Pittsburgh, PA, USA. The case study includes a conventional ballasted roof, an extensive, or shallow growing medium green roof, and an intensive, or deep growing medium green roof. For the life cycle inventory and the material use, both the types of material used and the transportation distances to the site are with respect to this project. The study found that, for the Pittsburgh, PA climate, the energy use reduction that is realized because of the lower thermal conductivity of the roof due to the green roof growing medium is the critical factor in determining the relative magnitude of the environmental impact of the alternatives compared here. Although the energy use reduction is not very large in relation to the overall building energy use, it is significant for environmental impact over the life cycle of the building.
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