Green roof systems of this type are drained by a base layer of lightweight, inorganic, granular media. To maximize water storage and minimize weight, the granular media should contain a significant percentage of porous material such as heat-expanded rock. To minimize separation, a well-graded blend of particles with sizes ranging from 1/8” to 1/2” is preferred. A slotted plastic drainage conduit with a triangular cross-section is embedded within the media to transport excess water.