This sounds like a really exciting project! Even more than the shade, the rooftop location is a challenge for gardening because of the wind exposure. Choose plants that are hardy in one or two zones colder than your USDA zone, depending upon the amount of protection you can offer the plants. There are some tough plants that are also beautiful and very suitable for rooftop gardens. Pinus mugo (Mugo pine), Pinus thunbergii (black pine), and Prunus x cistena (sand cherry) are all great plants that can manage with some shade. Spring - (Spiraea prunifolia, x vanhouttei, thunbergii) and summer- blooming spireas (Spiraea japonica) are also good choices. Prepare the soil carefully to suit the plants you grow to reduce the care during the growing season. Happy gardening!

Courtesy of NYBG Plant Information Service