Green Roof Technology

Center For Urban Ecology, USDI, NPS, NCR, 4598 MacArthur Blvd., NW, Washington, DC 20007

I had meetings this afternoon at three locations with National Park Service employees that I used to work with.  While at the Center for Urban Ecology, I had a good look at their functioning green roof system.  A set of steps takes you to the roof, at two sections of the natural resource science complex.


The green roof system has been in place for two full growing seasons now, going into the third.  They have tried dozens of species of plants and have swapped a few out of the roof due to mortality.  The dominant plants are sedums, which are xeric species, tolerant of dry, arid conditions.  The roof currently has 22 species.


Once the structural engineer determined that the existing roof could handle the weight load, the National Park Service went hog wild.  They first laid down a clear colored PVC tarp and installed a center isle of recycled rubber 2-foot by 2-foot interlocking blocks for the purpose of a quiet walkway.


Each thick plastic tray is also 2-foot by 2-foot and 4-inches deep.  I lifted up this tray so that you could see the sides.  Each tray weighs about 10-pounds (wet).  The soil medium is two-thirds shale aggregate and one-third topsoil.  Eight quarter-inch weep holes are in the bottom of each ‘ridged’ tray.  The outer bounds of the roof has a three-inch aluminum lip, drilled into the roof, also with weep holes, which holds all of the trays in place.  The trays have never blown out of place in high winds.  They osmocote fertilize the roof once per annum with a hand-crank spreader.  They never water the roof, except to establish new plantings.  Most all of the plants are capable of overwintering without freezing and/or desiccating.


This system cost the NPS $80,000 for 7,000 square feet of roof coverage, costing approximately $11 per square foot.  They purchased everything from Emory Knoll Farms in Street, Maryland, a specialty nursery for green roof plants and supplies.  Check the web link for the research and development section to see the recognized benefits derived from a green roof system.  This flat roof design is the most functional and practical to date.  The NPS states that the most obvious benefit is thermal cooling in summer and heat insolation in the winter.  Even on the roof top on a hot day, the vegetated trays maintain ambient air temperature.


I took dozens more pictures and lots of notes, but don’t want to bore you.  I was told that these flat roof systems have come down in price considerably, and can be moved at will to inspect the roof, a much more efficient design than vegetated roofs with soil mediums and irrigation systems spread over large areas.  The key to success, keep it simple, affordable, and low maintenance, which then constitutes “state of the science,” over other designs.

Emory Knoll Farms, 3410 Ady Road, Street, MD  21154, 410-452-5880