As well as being energy efficient, green roofs also absorb storm water and curb runoff. Typically consists of the following components: an insulation layer, a waterproof membrane; a root barrier; a drainage layer, usually made of lightweight gravel, clay, or plastic; a geotextile or filter mat that allows water to soak through but prevents erosion of fine soil particles; a growing medium; plants; and, sometimes, a wind blanket. There are two basic types of green roofs: intensive and extensive. Intensive green roofs require a minimum of one foot of soil depth to create a more traditional rooftop garden, with large trees, shrubs and other manicured landscapes. They are multi-layer constructions with elaborate irrigation and drainage systems. Intensive green roofs add considerable load to a structure and require intensive maintenance. In contrast, extensive green roofs range from as little as 1 to 5 inches in soil depth, adding less load to a building. Extensive green roof systems also generally require less maintenance than intensive systems. Some green roof designs incorporate both intensive and extensive elements.