Building and Environment 42 (2007) 2606–2613 Comparative environmental life cycle assessment of green roofs Lisa Kosareo, Robert Ries à Department of Civil and Environmental Engineering, University of Pittsburgh, 949 Benedum Hall, 3700 O’Hara Street, Pittsburgh, PA 15260, USA Received 5 April 2006; accepted 12 June 2006 Abstract 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. r 2006 Elsevier Ltd. All rights reserved. 1. Introduction A green roof, i.e., a roof with a vegetative cover, is one passive technique that can be used to address environ- mental issues in an urban setting. Research has shown that green roofs can be used to mitigate problems associated with storm water runoff, the urban heat island effect, wildlife habitat, and air and water quality [1,2]. Building owners are hesitant in considering the use of a green roof due to its increased initial costs and uncertainties in the construction and maintenance of such roofs. In one of the few studies that consider the life cycle of green roofs, it was found that the life cycle cost of extensive, i.e., shallow soil roofs, are less than conventional roofs but that intensive, or deep soil roof systems have a higher life cycle cost (LCC) than conventional practice [3]. However, these are not the only deciding factors in choosing a roof system for a building. Throughout the life of a building, many environ- mental costs will be incurred as well. A careful examination of all of the environmental aspects of building and maintaining either a green or conventional roof will be used to develop a life cycle assessment (LCA). LCA and LCC can be useful ways to determine the environmental and cost benefits of green roofs [14]. This LCA case study examines a conventional roof, an extensive green roof, and an intensive green roof to determine the option with the fewest environmental consequences. 2. Background Green roofs come in two forms, extensive and intensive. An extensive green roof has between 50 and 150 mm of growing medium to support plant life. This limits the size of plants that can be used on the roof, thus limiting the weight of the green roof on the building structure. Generally foot traffic is not allowed on extensive green roofs because of the shallow and fragile root system of the vegetation. Often this type of green roof is retro-fitted to ARTICLE IN PRESS www.elsevier.com/locate/buildenv 0360-1323/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2006.06.019 à Corresponding author. E-mail addresses: lmkst45@yahoo.com (L. Kosareo), robries@pitt.edu (R. Ries).