The deck isn't cantilevered at all: its supported by the house on one side and the perimiter posts on the other. The plan doesn't acually show how large the upper level deck is, but it does look comfortable. The surface of a deck over an enclosed space is usually pitched slightly, enough that the water rolls off, not so much that a glass tips over. Typically a few inches in 12 feet. There are several ways of doing this. Sometimes you see a regular 2x6 deck built over a low pitched roof, but more often the roof and deck are the same material similar to the asphault used on flat roofed structures. A cool new option is LockDry, an aluminum surface that you walk on, but interlocks in a way to channel off water. A certain amount of care must be taken with flashing, siding etc. so that water can't inflitrate, but your builder won't have to invent the wheel, here: the procedures are well established. Just make sure your guy know and follows them: he'll probably tell you it won't be a problem, but ask how many such structures he's done. Still, a certain amount of all this will depend on your area [more care has to be taken in Seattle than Phoenix], and the exposure of the deck [A south or west exposure will dry more quickly, lowering chances of mold, etc]. But if proper practices are followed, you can have a large deck on the north side of a Seattle home. Here is a link that might be useful: LockDry

Your roof top deck has plenty of clearance,most of mine have the width of the stringers 1 1/2''. I would sugest using counstruction adhesive as well as SS screws be very sure the screws do not go thru the stringers be very sure as well during install you dont damage the roof...like hiting it with the edge of a board,droping a drill bit first on it,steping on a screw things like that. Your right if your going to use anything on it use the S American lumber not manmade material. With your pool deck provide cross ventalation. This can be done with screen vents in the framing if this is not completly possible install floor vents in the decking combined with the frame vents. Cross ventalation is a vastley over looked but much needed addtion to any decking project. The gap between the decking boards will have no effect one way or the other venting your project unless there is air moving under it. If possble a drain system under the pool deck might help if done corectley but do not put a ground cover like plastic sheet/tar paper under it 1'' ipe will react the same way 3/4'' ipe will under the same conditions. The life span of both is also the same. J.

Our home has decks off every room because of the views, and they are all plywood covered with tar paper then sealed with something that looks like concrete. The deck over the garage is the same. Whatever you use, be sure to make sure there is a drain and that the deck slopes toward it. Also. Gutters. Don't forget those. Suzi

"Is this something I should be concerned about from a safety perspective?" No. A structure gains strength from its whole. Two short rafters do not a building failure make. "Also, is it something that can be fixed?" Yes. You want a Simpson Strong Tie Adjustable rafter hanger. Those shorties aren't going anywhere after that hanger is installed. Use the recommended fasteners too please. For the record, toe nailing is second only in strength to rafter/joist hangers. It is end grain nailing, as your picture appears to show, that is weak and prone to pull out failure.