I want to use it as a real rooftop deck.
Besides the issues of increased load, what other issues do I need to address?
The house does have a fireplace chimney near by.
A widow's walk is a railed rooftop platform, typically on a coastal house, originally designed to observe vessels at sea. The name comes from the wives of mariners, who would watch for their spouses' return.
In some instances, the ocean took the lives of the mariners, leaving the women as widows. The women would often thereafter gaze out to sea wishing beyond hope that their loved ones would return home and hence the name widow's walk was born.
Besides the issues of increased load, you should be sure that the roof top decking is durable enough for foot traffic.
I have never built such a deck, but I did find a company whose product, DecTec® Walkable PVC looks like a possible solution.
DecTec® WALKABLE PVC
As for the chimney, any chimney terminations must extend a minimum of 3ft. above the roof line.
If the chimney will be within a horizontal distance of 10 ft. of any wall or obstacle, the chimney must extend at least 2 ft. above that wall or obstacle.
Carl Heldmann, byoh.com