Watch this video from This Old House to learn how to reshingle a roof.
1. Pry off old roof shingles with pitch fork.
2. Nail down loose roof sheathing to rafters.
3. Cover roof deck with 30-pound felt underlayment; staple felt every 6 to 8 inches.
4. Nail drip-edge flashing to lower edge of roof.
5. Snap vertical chalk line down center of roof.
6. Trim tabs off roof shingles to create starter course.
7. Center starter course on the roof's centerline; allow the shingles to overhang drip-edge by ½ inch, then nail them in place.
8. Snap horizontal chalk line across roof so first course overhangs starter course by ½ inch.
9. Set top edge of first-course shingles on the chalk line and nail them down.
10. Snap horizontal layout lines every 5 inches all the way up the roof.
11. Snap vertical layout lines every 6 inches across the roof.
12. Continue laying roof shingles, aligning the top edges with the horizontal chalk lines, and the ends with the vertical chalk lines to create the proper 6-inch offset.
13. Secure each shingle with four nails.
14. Trim the shingles at the ridge and ends of roof.
15. Cut 12x12-inch ridge-cap shingles from standard roof shingles.
16. Center each ridge-cap shingle over the ridge and attach with two nails.