scope:

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).