When roofing a garage, does it matter which way the new boards go?
Our company did our roofing Liverpool and I would suggest using OSB (oriented strand board) instead of plywood. It's stronger, costs less per sheet, and there is no directional grain as in plywood to consider when spanning rafters. OSB has a slick side and a textured side. The textured side can go up to give you additional grip and footing. Use ply-clips around the top edges in-between each rafter to give you needed expansion. If you don't have a pneumatic decking stapler, use 8d box nails to fasten. Apply felt over the decking before you lay the new shingles. Start at the bottom, as you do with the decking, and overlap a few inches each time.