My 2 x 4 rafters are 24" on center. They support 3 layers of shingles, a layer of cedar and a layer of pine sheathing - that's a lot of weight. So much so that it has cracked several
of the rafters.
After consulting buddies of mine that each have twenty plus years of roofing experience I am going to follow these directions;
replace floor joists from end to end
(the entire span in all structural framing is key) Use nail gun and circular for this
lay a 2x6 plate across and above the entire span of my attic floor / downstairs ceiling joists
climb step ladder, use speed square to measure pitch of roof, set chop saw and cut base plate side of 2x6 sister rafter
Have a friend hold base plate flush to floor (I don't need a seat cut - you might)
Grab other end of sixteen foot 2x6 climb step ladder use square to draw a line down from my 1x8 pine ridge plate, cut 2 x6 - lay sister rafter on the ground then
use a four foot level against cracked rafter and good rafter to inspect the difference in pitch (get a mental picture of how the good rafter looks)
jam a kneewall sized 2x6 between new base plate (step 2) and cracked rafter to take the sagg out of the roof (check pitch with level)
from end to end
nail up the sister rafter that I just cut both sides of with the nail gun my buddy lent me.
remove kneewall from new 2x6 rafters and put them under good 2x4's
Peace out y'all