Consult a structural engineer who can conduct a thorough roof inspection and make recommendations on what you can and can't do.
The Indoors Outdoors
Plants, hardscape materials and furnishings can be an extension of the decorating theme already going on inside.
Create a user-friendly design.
Take the Scenic View
Hide unsightly scenes, like pipes, walls and industrial parks, and create privacy with a screen of plant material, a self-supporting trellis with climbing vines or an overhead structure like a pergola or shade sails.
Not only will these structures provide shade, they'll also create a privacy screen from neighbors.
Sounds of Silence
Mask noisy sounds with a water feature or grasses.
Light the Way
Light up your rooftop getaway.
Grilling and Chilling
Use lightweight furnishings, like teak furniture, polypropylene carpet, ipe decking and other materials.
Use plants that are hardy to one zone lower than yours. They'll be more likely to survive the rooftop winter conditions.