There are essentially two ways to put on a roof top deck. The 'lets do what we want and hope it all works out'' way and the legitimate, compliant way.
Of course you see lots of roof top decks around. Whether all of them are compliant or not is another matter. Unless your neighbors call on you, the City probably won't notice. Nonetheless there are serious considerations.
If this deck is for you and a friend or two to sit around drink coffee and yak about life, no big deal and nothing will probably ever happen. Do what you want. However, reality usually doesn't work like that. More and more friends show up, you have a party, put kegs and large plants on the roof, next thing you know there is some real weight up there. Since this is a NC building you'll need to establish what type of trusses were used for the roof and how much top load they can handle. The type of existing roofing material also needs to be figured out. Typically people need to either install additional roofing or suspend the deck over the roof, not on it.
Going the legitimate route will require plans and permits by the City. Plans will need to be done by an architect. Depending on existing building conditions its sometimes easier and cheaper to build a new structure over the existing roofing (I beams run into the parapets).
Typical requirements include egress path, guardrails and load rating. Depending on proximity to adjacent building fire separation may also come into play. Without a Spec sheet or Plans for contractors to bid off of you will continue to get varied answers.
A note of caution, the cheaper bids usually includes the contractor nailing everything down. You should NOT consider this acceptable. Everything should be screwed or bolted. Any perceived upfront savings will disappear if the roof needs to be serviced and nailed components need to come up.