Maybe not the answer you want to hear...
I have torn down two garages, and, in both cases, I bought several
really cheap carbide blades for around $4 / each. I went through 3-4
on each project cutting the roof into reasonable pieces.
At this point do you KNOW it is the rafters?
Most often what I've seen with uneven roofs is plywood problems. It sags
between the rafters because it's thin, or it was placed/nailed too close
to the edge vs on center. The place I just finished roofing had no
spacing on some plywood ends which caused buckling. One of the planes had
no plywood spacers on the long side with tightly butted edges. Buckling
I'd recommend a reciprocating saw (Sawzall) with demolition blades. You
should be able to find a decent one for ~100. If this is for one time
use, consider renting.
Demolition blades do exist for circ. saws, but I don't know how common
they are in 7-1/4". Fire departments use these for opening up roofs --
most likely 10 or 12 inch blades in a gas powered saw.
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