The definition of a green roof according to Nian She of the Journal of Hydrologic Engineering, "is a multi-layer green roof covering composed of an anti-root barrier, a waterproof membrane, a drainage system beneath and a layer of growing medium covered with plants” (Theodosiou 2009). Green roofs, also called living roofs, have been around for centuries. world used only natural materials for construction Today they are considered as the sustainable building design of the future They are used for reasons such as energy saving for heating and cooling, reduction. of the urban heat island effect, absorption of air pollutants and dust, management of stormwater runoff and social and psychological benefits (Theodosiou 2009). Many experiments and computer models have been carried out to study them in the context of integrated building design. Typical living roofs contain a lightweight soil mix and a drainage layer. Additionally, to keep the layers separated, there is a fabric filter and then a special layer under the drainage that protects the roof from root penetration. High-quality waterproofing is also essential. The cultivated vegetation determines the height necessary for each layer (Theodosiou 2009). There are two main categories of green roofs: extensive and intensive. The most extensive is considered the simplest. Extensive residential roofs require minimal construction, maintenance costs, soil layer weight and depth. They are generally not irrigated and the vegetation is mostly self-sufficient and indigenous to the site. They are versatile, meaning they can be applied to flat paper or in the middle of the durable world. Np, and Web. December 8, 2010. .Passive cooling for homes. (nd). House-Energy. Retrieved December 1, 2010 from http://www.house-energy.com/Landscape/Passive-Cooling.htm Peck, Steven W. and Chris Callaghan. “THE GREEN FILES OF GREEN ROOFS: FORGING A NEW INDUSTRY IN CANADA. » Peck & Associates I (1999): 18-39. Print.Theodosiou, T. (2009). Green roofs in buildings: thermal and environmental behavior. Advances in Building Energy Research, 3.1, 271+. Retrieved December 1, 2010, from Academic OneFile database. Wilmeth, M. (October 1, 2008). Green roof study reveals wide variation in stormwater retention - EBN:17:10. BuildingGreen.com. Retrieved December 1, 2010 from http://www.buildinggreen.com/auth/article.cfm/2008/9/25/Green-Roof-Study-Finds-Big-Variation-in-Stormwater-Retention/