HI Lucky I've not dug out my books on structural framing in responding to your concern, so I'll do this from experience and intuitive logic. But for a definitive answer, you'll need to check with a structural engineer or your local county bureau of buildings.
A wood framed 12' X 24' (say 4 pitch) roof will dramatically change your 'deck' to a sort of garage or shed. The reason is the lateral structural support that will be required. Putting that much framing weight that high will require cross braced and rigid plywood walls, unless you go to timber style framing. You may be able to get by with 4X6 posts if your roof is a fairly shallow pitched metal roof, requiring considerably less wood framing. To me, a #77 live load for a deck is a bit high, unless you are going to have it packed with partying people all the time :-) .......although overkill hurts nothing but your pocketbook.