The roof features intensive and extensive areas, both of which use a lightweight expanded-shale aggregate growing medium. The expanded shale medium was selected because it is extremely light even when fully saturated. Nowhere on the green roof does the saturated weight of the soil, plants, and stone features approach the 62 lbs/s.f. live-load capacity of the roof's steel support system.
The extensive areas have a 4" soil layer and are planted with nine species of sedums. Sedums were chosen for these areas because they do not require a highly-organic growing medium, and their shallow root systems allow them to thrive in the thin soil. The intensive areas, which have between 4" and 9" of soil with higher organic matter content, are planted with a variety of native perennials and shrubs with deeper roots and greater demands for organic matter from the soil. In both the intensive and extensive areas, plant species were carefully selected to ensure they could withstand the conditions on the roof, which are hotter, drier, and windier than on the ground.
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