Evaluate how steep or complex your roof is. Shingling a simple gable roof on a single storey home is relatively straightforward. However, if the roof is steep, has multiple planes, needs valley work and complicated flashings, consider hiring a professional. As with any roof work, make sure you use the proper safety equipment to guard against falls. Then it's time to thoroughly inspect the following:
Structure. Go into the attic and check sheathing for voids, separating plywood, and broken boards. Also inspect the rafters for signs of damp or rot. All rotten or damaged boards under the old roof must be replaced. Look for rot along the edges of eaves. Make sure fascia boards are sound and solidly connected to the rafter tails. If necessary, hire a professional to do this work.
Flashing. Examine the flashing around chimneys, vent stacks, and skylights. It is always a good idea to check the condition of the metal valley flashing if applicable. Replace if damaged.
Shingles. Check the condition of the old shingles. Bumps or dips in the old layer of shingles cannot be hidden by new roofing. If the old surface is not uniformly flat it should be removed before adding new shingles. Also check how many layers of shingles are already on the roof. You can see this along the rake (sloped) edge of the roof. Most codes allow a maximum of the original layer plus two reroofing layers. Check your local building code to be sure