Bethesda, Md., a suburb of Washington, D.C., is home to numerous healthcare facilities, educational institutions, housing districts, and entertainment, dining, and nightlife venues. It also is home to the 32-acre, mixed-used development North Bethesda Center. Once completed, the development will encompass 930,000 square feet of office space, 202,000 square feet of retail space, and 1,275 multifamily housing units. It also will link to the adjacent Metro station, which is expected to have an additional 6,500 daily commuters.

Berwyn, Pa.-based LCOR, a real estate investment and development company, is the project's master developer. Work began in 2006 and will be completed in phases. Phase one included a luxury apartment building and two-story retail structure at the its base, which houses a Harris Teeter grocery store. The grocery space is capped with a 17,500-square-foot green roof that features landscaping and gardens to reduce the building’s overall stormwater runoff.

The architect’s plans originally called for a standard inverted roof membrane assembly with gravel ballast. After examining several options, LCOR decided to install an extensive garden roof assembly from Chicago-based American Hydrotech Inc. Because LCOR wanted to create a showpiece, similar to a courtyard-type layout, as a visual amenity for the residents of the tower, they explored additional options for enhancing the plant media and roof’s design beyond standard vegetation or sedum carpet.

Landscape architect Parker Rodriquez of Alexandria, Va., suggested a combination of sedum and grasses in addition to plant species in varying sizes and colors. The final design features growing media up to 6 inches deep across 17,500 square feet, which is the maximum weight that could be allocated to the pre-existing structure without requiring extensive design modifications.

The various roof levels are composed of 55,000 square feet of American Hydrotech assemblies, including 32,000 square feet of stone ballasted protected membrane roof complemented by 17,500 square feet of garden roof and 6,000 square feet of ultimate assembly. The roofs on each of the levels were waterproofed with 215 mil. fabric reinforced hot rubberized asphalt system. There are protected membrane roof assemblies made up of protection sheet, filter fabric, and stone ballast on numerous levels. The garden roof assembly above the waterproofing membrane consists of a protection sheet, root barrier, insulation, a water retention/drainage medium, and about 6 inches of engineered lightweight soil. Pre-cast curbs were used as the perimeter and metal dividers separate the aggregates. The swirl feature received 10 inches of insulation to create a mounding effect. Three types of aggregate and 24,000 sedums are installed in patterns to provide visual interest.

Phase two of the center began in early 2009 and includes a second apartment tower, office space, and 125,000 square feet of retail space. The entire project is expected to be completed between 2013 and 2015.