I have a 1929 craftsman house with dimensional 2x4 rafters on 24" centers. They're about 13.5 ft long, spanning 11.5 ft each (measured horizontally, wall to ridge beam) at a 7:12 pitch. About 25% of them have diagonal 2x4 bracing tying them to either the center interior wall or ceiling joists (also 2x4) over interior walls. The top of the rafters were angle cut to sit flat against the 1x6 ridge beam, then nailed through the ridge into the end of the rafters. (I know, definitely not up to current standards.) Over the years, 3 consecutive rafters split at the top end (maybe a tree branch fell on the roof?). I've sistered the top 6 feet on 2 of them with additional 2x4's, screws, and plates and plan to do the 3rd one soon. The roof decking is 1x6's and 1x8's nailed on top of the rafters. Besides a few knots and a couple split decking boards, the are no apparent soft spots or sags in the roof. It currently has 2 layers of 3-tab shingles.
I need to re-shingle the roof. I want to strip the existing shingles, add deck and ridge vents, and go back with an impact resistant dimensional shingle with a long warranty but I'm concerned about the weight. I've read that some can weigh over 400lb per square (100 square feet). A couple roofers I asked out for bids also said I would have to add 15/32" plywood or osb decking to ensure the nails have something to hold to, not empty knot holes or gaps between the current decking boards.
I'm in the central United States, Kansas City. We normally get 6-8" of snow on the ground, but occasionally up to 24-30" of heavy wet snow. We also routinely get 60-80 mph wind gusts with thunderstorms, sometimes higher.
Will the 2x4 rafters support the weight of the dimensional shingles, possible additional decking, and snow? Since not all the roofers recommended the additional decking, would that increase the roof strength, or is it an unnecessary added load?