I have four sagging rafters in garage in my 1952 home. They are 2x6 and all side by side. All four have knots in about the same location, resulting in sagging. I got 4 2x6s to sister these, but that's a lot of weight to add side by side. So, Im thinking of using angle iron, as least for two of them.
4/12 pitch. 12.5' rafter length. Single layer of shingles, which will be removed and replaced soon. House has no trusses or rafter ties. I will be adding a few rafter and collar ties.