Roof deck in Manhattan

A couple of questions about roof decks: - Anyone know what it takes to get one built ($ est, permit, etc.)? - Do you need a permit if you install one of those floating (non-permanent) systems like Bison? Do you need a permit to install a fence? We have a small space, about 100 sq. ft, that is next to a common space. - Any good recs for companies that do this? - What are the most common, practical flooring options? - How do people maintain the roof underneath after a deck is installed in the city? Do people typically install removable, non-permanent decking or do they tear it out and fix the roof? I see pictures of really permanent-looking decks and am just curious.