Good luck on building your deck. In addition to consulting an architect, you may also want to investigate the zoning of your property to make sure it is even allowed.
Here is the link:
If you don't want to wade through all this, a simple call to your Alderman's office may help you get started.
I would consult an architect as there are differences between a single-family home and a mult-unit building when it comes to a roof deck. They can help you with permits and also let you know if the structure that will support the deck is adequate.
Good Luck, Ken.