You can save a great deal of time and clean-up by firring out the rafters. Use a decent grade of wood also, not those crappy 1X3s you see. Probably could rip 1X8s into thirds and use that as firring strips. Then screw them to the rafters - be sure you find centers for anchoring. 2 1\2" screws should be good. And if you're tapping into some old lumber - say oak or GA Loblolly Pine, you'll drill pilot holes.
If you don't have time to do it right, when will you ever have time to do it over?