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The greening of the Postal Service

Case Studies, Landscapes | May 1, 2010 | By:

New York’s Morgan Station is one of the nation’s largest green roofs

It’s hard to get public agencies to fund poetics,” observes New York landscape architect, Elizabeth Kennedy, ASLA of the new green roof atop the Morgan Station Mail Processing Facility. Yet, at 1 hectare and seven stories off the ground, this green roof offers a poetry of its own with sweeping views of Midtown Manhattan and New Jersey. It was really cost-effectiveness and lower life-cycle costs that made this rooftop oasis possible.

As the largest green roof in New York, Morgan Station is a model project both for the city and for the US Postal Service nationwide. Built in 1933, the 198,000m2 facility became a historic landmark in 1986. Even though immense, the original historic building was designed for future upward expansion. In fact, the roof’s historic grid of 176 copper-capped columns, now oxidized green with the decades, became a key design element in roof design. Kennedy, who worked in collaboration with URS Corporation, explains that, “the roof is actually a floor with 976.4kg/m2 structural capacity that’s been waterproofed”— a real luxury for any green roof and one reason that Morgan made an ideal demonstration project for USPS.

Even with the benefit of a high-load roof, “green roof planting medium is very expensive,” Kennedy explains. To save budget the team changed the planting design to minimize intensive plantings and the volume of engineered soil required overall. “Very large, open roofs are harsh microclimates, and plants are subject to ferocious transpiration rates,” she adds. The designers used absorptive industrial felt to provide continuous and adequate moisture for the low-water sedum root systems covering most of the “extensive” roof (see “Terms to know when working with green roofs” for more information).

Rather than irrigating the entire roof, the designers doubled the thickness of the industrial felt lining the elongated planter, and added low-volume water emitters at each tree root ball of the nine Amelanchier trees along the central walkway. Visitors can rest on Ipe Brazilian wood benches made from lumber certified sustainable by the Forest Stewardship Council. Besides the Amelanchier trees, plantings include sedum varieties on the ground plane and vertical stands of native grasses and drought-tolerant perennials.

Even though it’s almost 60m above ground and the tree canopy layer, small birds already visit the roof. “We expect to find drowsy bees among the sedums after the second growing season, when the plants and trees are more mature,” Kennedy says. The roof is also attracting employees (although it is secured from public access). As the nine flowering trees grow taller and the sedum plantings spread, the roof’s glare will diminish as will summer ambient temperatures. The USPS estimates that the roof’s new waterproofing, protected by the green roof “overburden” from infra-red and UV rays, will last up to 50 years, twice as long as the roof it replaced. The green roof will also reduce the amount of contaminants in stormwater runoff flowing into New York’s municipal water system by as much as 75% in the summer and up to 35% in winter. Sam Pulcrano, USPS vice president for Sustainability, notes that, “the green roof will help us meet our goal to reduce energy usage 30% by 2015.”

Kennedy speculates that, “within three to five years, extensive green roof technology could involve even greater use of high-tech meshes, felts and other fabrics that can be directly hydro-seeded, or hydro-mulched with growing medium to support plant growth.” She adds, “Fabric tensile properties offer exciting opportunities to economically create lightweight forms and perhaps mimic topography.” Such new technology will make possible further demonstrations of urban green roofs that help to cleanse and cool city climates while saving costs over time.

Frank Edgerton Martin is a contributing editor for Fabric Architecture specializing in landscape design and planning. His profile on landscape architect Jeffrey Bruce, FASLA, appeared in the Nov/Dec 2009 issue.

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