Roof decks are usually built in situations where there is limited space for a conventional deck. Roof decks can be built on penthouse apartment buildings or on top of garages. You will probably need to hire a structural engineer or an architect to verify that the building is capable of supporting the added loads. Roof decks are not usually built with conventional deck framing. A rubber membrane is often installed over the roof and sleepers or 2x6's are laid flat on top of the roof. The decking material is attached to the sleepers. The deck is actually a thin platform that rests on top of the roof.
Garage roof decks will typically require some additional structural support. Most garage roofs are not designed for the kinds of loads a deck will require.
Many roof decks feature built in benches, planters, pergolas, kitchen and bar areas. Lighting can also add to the ambiance of a roof deck. You can also try to use elements of the building's design by adding brick, stone, or glass. Many roof decks in New York make use of modern materials like stainless steel and hardwood flooring.