Steps to Repair
Remove any damaged shingles or felt paper. Many times when a porch roof is damaged, you will need to remove the damaged portions.
Mark the damaged area and cut it out. For example, if you have a 10-inch-by-10-inch area of damage on the roof, go just to the outside of that damage and make a square using your framing square. Try to hit on the center of the roof rafters. Rafters are set on 24-inch centers, so, if your damaged area is directly in between two rafters, go left and right to the centers of the rafters and cut the area out. There is no need to go 24 inches long, you can just cut 2 inches bigger on the top and bottom of the damaged area.
Cut a piece of plywood that is the same as the area that you cut out. Place it in the hole and renail using eight penny nails.
Cover the patch with felt paper. Allow the felt paper to overlap the old paper by about 3 inches all the way around. Secure using felt nails.
Replace the shingles. Realign the shingles by following the rows to the right and left of the new patched area. Secure the shingles using roofing nails.