Green Roof Practice

Initially, WSSI wanted to cover the entire building (approximately 25,000 square feet) in an intensive green roof able to support trees and a multitude of other flora. Unfortunately, the cost of such a green roof was prohibitive, forcing WSSI to alter its design. The main roof was instead covered in a high-albedo membrane to mitigate the heat island effect, and the green roof was built on a one-story, 3,626 square foot extension of the building above employee work areas. The green roof is accessible from the second-story training room, and has become an employee amenity with sitting areas and walkways.

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.

A unique feature of WSSI's green roof is the two wetland pods. Each pod was created from a loop of 6" flexible HDPE pipe, a pond liner, and thick, impermeable soil. The pods are kept wet by individual moisture sensors and drip irrigation systems. A variety of native rushes and sedges grow inside the wetland pods. During the first growing season in 2006, the bull rush grew nearly 6’ tall.

The green roof is supported almost entirely by direct rainfall, and water from the main roof is not recycled for green roof irrigation. In times of drought, rainwater is supplemented by a drip irrigation hose that feeds from the building's water supply.

More Information on WSSI’s Green Roof