Hello, I hope this is the right place to post this, and I hope y'all can help.
I have a 1900 house that was renovated and expanded in the 80s (there's your problem, I know, and luckily I knew this when I bought the house, but I digress), and they tacked on (mildly exaggerating, but just) a second floor with two bedrooms on it. I've included a picture of the basic structure, but there is a wall on the sloped half, possibly structurally supporting the roof, but if the rest of the house is an indication, probably not. the left half was built in 1900, so it's built meh, but I'm not worried, it's lasted this long. the 1980s half is flat, which is asinine, since we get 2 feet of snow in a single event sometimes. before this time, the right half mirrored the left.
the members run as pictured, with no additional bracing. the left half and right side rafters attach to a single 2x6 running the length of the upstairs (26 feet or so) and the right side rafters then sit on a 2x10 that is slightly overlapping the 2x6. this flat roof does not work structurally (it could be holding 3 feet of snow in a bad winter, and it can't run off), it doesn't hold enough insulation (it's only insulated to r20, and to insulate it properly would be expensive), and the sheathing is rotten.
I'm liking the idea a buddy of mine has, but I'm just trying to figure out how to form this in reality so it works and is safe. the second picture illustrates this idea. the overall span is only about 20 feet, and the right side is about 12. I was going to leave the exising 2x6s to hold the drywall, and the new structure is going to be 2x8s (unless 2x6s would be enough? it's what's already there, and the 1900s half is 2x4s).
I was thinking of using the existing "ridge beam" and birdmouthing the left side member vertically so it matches to the existing roof, and then tying them at the top with a truss tie plate. this alone would appear to me to be at least as structurally sound as what's there already, but should I add a knee wall at the peak like my buddy suggests (I would think this would complicate construction on the roof), would a simple horizontal tie near the peak be sufficient, or would the 2x8s be sufficient as is? I appreciate your time and if there if this is the wrong place to post this, I would appreciate knowing where I should.