Environmental Design + Construction - October 2008 - (Page 26) Cool Roofing Figure 1. This crosssection of an extensive green roof system shows the waterproofing membrane, protection course, root barrier, drainage layer, insulation, aeration layer, moisture-retention layer, reservoir layer, filter fabric and growth medium. (Detail courtesy of the NRCA.) are the terms used in the International Building Code [IBC].) Specifically, fire- and wind-resistance requirements are being discussed. THREE TYPES OF GREEN ROOFS A green roof combines roofing, waterproofing and landscaping concepts. NRCA defines three types of green roofs: extensive, semi-intensive and intensive. The principal difference between these three types is the depth of the growth medium. Growth medium for extensive systems is 2 to 6 inches in depth; for semi-intensive systems, 6 to 10 inches in depth; and for intensive systems, 10 inches or more in depth. Extensive systems generally are not irrigated and the reservoir layer is optional. This is primarily based on the common plant varietals used with extensive systems. Semi-intensive systems may include aggregate in the reservoir layer. Intensive systems often require irrigation and need large reservoir capacity. The basic components of the three types of green roof systems are similar. Figure 1 shows a cross-section of an extensive green roof system. NRCA recommends all green roof systems include a waterproofing membrane adhered to a stable substrate, protection course, root barrier, drainage layer, insulation, aeration layer, moisture-retention layer, reservoir layer, filter fabric and some type of growth medium or engineered soil. It’s impor tant to point out there is an aeration layer above the insulation and a drainage layer beneath the insulation. When insulation gets wet or saturated, its R-value is significantly reduced. Having an aeration layer above and a drainage layer beneath the insulation helps maintain the R-value by giving water a place to evaporate and drain. If these components are not included, Figure 2. A low parapet flashing detail for a thermoplastic waterproofing membrane is designed for use with an extensive green roof system. (Detail courtesy of the NRCA.) 26 Cool Roofing > Fall 2008
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