Drill holes into a piece of 1.5-inch by 1.5-inch steel L channel. The L channel should be a minimum of 2 feet long to fix the bowed rafter. Use a diamond-tipped drill bit and power drill to make four 3/8-inch holes on either side of the L channel. The holes on either side should be staggered from each other.
Use a chisel to remove any splinters on the bowed rafter. These splinters can get in the way of your repair.
Attach the four holes on the right end of the L channel using 3/8-inch by 2-inch lag bolts and your power drill. Note that you are first attaching one side of the L channel so that it fits straight across the sagging rafter.
Measure the gap between the bent rafter and the other end of the L channel that you have not secured to the rafter yet. Choose an appropriately sized 3/8-inch diameter bolt. The bolt needs to be 1.5 to 2 inches longer than the gap you measure.
Put the lag bolts in place and start tightening. As you tighten, the bowed rafter will slowly start to straighten out as the bolt gets pushed into it.
Continue installing lag bolts in the three other holes in the L channel. By the time you're finished, the bowed rafter will be straightened out and reinforced by the steel L channel.