I'm sure others from around here will agree - there are plenty of houses that fit your rafter description that are doing just fine. When you look at the structure of a home, you analyze both the manner in which it was built, and the degree to which it has performed. About a month ago, I did a 1904 house, original cedar shingles plus 3 layers of ashpalt shingles, 2x4 rafters, 24" on center. Not a steep pitch. Not a bit of sag, no cracks in the rafters, no bounce. It always brings a smile to my face, literally, kneeling in an attic like that and looking around. I report on the methods of construction. No recommendations necessary. I'd love to hear from others on this.