Given the span and height restrictions there is NO WAY 2x4 is anywhere NEAR enough, much less 16" o.c. it is easy to say there were no permits pulled on the above mentioned job as even a blind inspector would've caught that and threw the red tag on the job until it was remedied...what you just read above is the typical "well that's what i did and it's still holding up" and that's fine and good, but regardless span tables are built based on historical facts and loads for safety.
HERE is a span chart for you so you can see what i'm talking about, hopefully you can see what it all means, but if you need explaination, let me know. The 2x4 system listed above is "safe" for an 8'2" span with an unfinished underside. Your roof is going to be just like the deck cover we did since it is almost flat, and to retain your headroom and given the height under the soffit you have to work with, you can get by with 2x6's 12" o.c. or 2x8's 24" o.c. Personally if i were hired to do the job i would use 2x8 16" o.c. for the over kill factor sicne you never know how much snow you will get, and being how the roof is almost flat that is a huge amount of dead weight up there so it's better to be safe than sorry, esspecially when our name is on it and we'd be liable should it fall onto whatever is being covered under it.
You have 2 roofing options as well with the darn near flat roof. You cannot use asphalt/fiberglass shingles on anything under a 4/12 pitch since there is'nt a enough drainage to prevent water from backing up under the tabs. So you can either use steel or rubber roofing. Both are fine if installed correctly. We chose steel on the roof above for decreased labor installation and roughly the same material cost vs rubber membrane. It goes up extrememly fast, and like i said, we still installed roof sheathing over the rafters like a normal roof, but we installed ice guard over it to ensure if water did somehow get under the steel it would not degrade the roof structure.
If you decided to use rubber you will still have to install a plywood deck over the rafters and then install a layer of asphalt saturated board/horse hair board.....the light weight black fiberboards. They have large washers you will use to screw these panels of fiberboard to the wood deck, and then you will rolll out the adhesive and start installing the membrane. It sounds easier than it really is, membranes are a PITA to install correctly, but when done right they are very durable and carry a 50yr warranty.
So the choice is yours, have to let us know what you decided to do.
