I found this interesting guide to building a green roof. I have not tried it (I do not have a flat roof) but it seems do-able.
Green roofs can make your home cooler in the summer and warmer in the winter and can control water runoff. 
Here is what a green roof might look like in cross section.
Via: Instructables LINK
Brooklyn may not be the first place you thing of when ROOFTOP gardens come to mind ... BUT Roof Top Farms grows pumpkins, peas and other produce - organically.
" ...The garden, 15 m above the Green
point sidewalk, is like a peaceful oasis in an otherwise throbbing industrial district. ..."
" ... Rooftop Farms project started in 2008 ... About 90 tonnes of specially mixed soil from Philadelphia was spread on the roof in March 2009 ... The garden users receive organic waste for their patches from the restaurants who by some of the vegetables, enabling them to avoid the use of artificial fertilisers."
Other benefits include controlling water runoff and less water treatment needed as well as possible reduced heating / cooling loads on the buildings.
Via: Sustainable Cities LINK
PSE&G, a New Jersey based utility, will double its current solar capacity with a program called "Solar 4 All."
Components of the program are (1) utility pole mounted solar cells and (2) roof top solar installations.
"... invest $515 million in 80 megawatts of solar projects ... creating green jobs ... expects to complete the installations by the end of 2013."
" ... installing a solar unit ... on 200,000 utility poles in PSE&G’s service territory ... the largest pole-attached solar installation in the world ... power ... sold into the PJM wholesale grid."
" ... effectively double the size of New Jersey’s installed solar capacity ... more solar capacity than ... any state other than California ..."
PECO is installing a green roof at it's Phila HQ location at 23rd and Market Sts. The planned 45000 sq. foot roof will " ... reduce the summertime temperature of a dark roof by 60 to 80 degrees, saves on heating and cooling costs, and grows without conventional soil..."
" ... green roof development ... is part of the company’s recently-announced comprehensive environmental initiative with a range of programs over the next several years.
Via: PECO PECO
Continue reading "PECO Green Roof in Center Citry Phila, Pa." »
The California Academy of Sciences is an example of GREN design - from solar cells, to green roof, to skylights to reused literature ...
See the Video for more ...
Via: ZDNet LINK
Newly opened condominiums in the Western Suburbs of Philadelphia combine energy efficiency, LEED design standards, a green roof and is within easy walking distance of 2 t
rain stations.
" ... highly efficient energy and mechanical systems meet stringent environmental certification requirements ... making Pembroke North the first multi-family residential buildings to be registered for LEED-certification in the Philadelphia area ...".
" ... buildings have heating and cooling systems that employ heat pumps connected to extensive underground thermal pipe systems for tremendous energy efficiency. ..."
"LEED FEATURES ... High-Efficiency Window Glass ... Well-Insulated Roof and Exterior Walls ... Natural Light and Views ... Geothermal Heating and Air-Conditioning ... Tankless Hot Water Heaters ... "
Nearby Radnor Middle School also has a green roof ... that's 2 green roofs (at least0 in Radnor, Pa!
Via: Pembroke North Link