Roof deck with access via spiral staircase

I am looking to add a roof deck with access via a spiral staircase. There is no roof access at the moment, and the roof it flat.

I am looking for some examples of access to the roof. I was thinking about building a sort of room around the access point. Also looking for ideas on the deck construction & material as well as redoing the roof. It might be difficult to fix the roof after the deck is on.

Has anyone used Bison deck supports (http://www.bisondecksupports.com/)?

Is the roof flat?

Yes, the roof is flat.