"INTRODUCTION Green roofs are an emerging sustainable technology that is becoming more popular in North America (Miller et al.@ 2005). As a definition@ green roofs are ""specialized roofing systems that support plant growth on rooftops"" (Liu et al.@ 2004). From top to bottom@ a typical green roof consists of several layers: (1) vegetation@ (2) substrate@ (3) filter membrane@ and (4) drainage layer. Plants used for extensive green roofs are typically drought tolerant@ and selected from the group of native or Sedum plants. Substrate is the soil-like layer where plants grow@ and it has to be porous@ retain moisture and nutrients@ and support plant growth (Snodgrass et al.@ 2006). The filter membrane prevents drainage clogging by containing the substrate and roots. The drainage layer transports the rainfall water runoff to the roof drainage (Peck@ 2002; Snodgrass et al.@ 2006). There are basically two types of green roofs: extensive and intensive green roofs. Extensive green roofs have lower weight@ lower capital cost@ minimal maintenance@ and a substrate depth between 2 and 6 inches (5 and 15 cm). Intensive green roofs have greater weight@ higher capital costs@ wider planting selection@ higher maintenance requirements@ and a substrate depth between 8 and 24 inches (20 and 60 cm). However@ intensive green roofs are less cost-effective than extensive and required more structural support (Peck et al.@ 1999; Tanner@ 2004). Moreover@ extensive green roofs represent about 2/3 of the total green roof square footage installed in North America (Johnston@ 2007). Therefore@ this research project focuses on summer thermal performance of extensive green roofs as a more economically viable solution to be adopted in the building industry. The popularity of green roofs is increasing due to their potential benefits. In general@ green roofs have a potential to (Liu et al.@ 2004): ? reduce energy demand on space conditioning ? reduce storm water runoff ? improve air quality@ and ? reduce the urban heat island effect in cities. Therefore@ green roofs can help address three of the four top problems facing the society in the next 50 years: energy@ water@ and environment (Smalley@ 2005)."