Examine the plywood, planks, fascia, soffits, and other areas of sheathing that may need to be repaired, then follow the steps to get it fixed.
Once the old roof has been torn off, repair any damaged sections of sheathing. Inspect the plywood and planks; repairs are usually relatively easy and quick.
A roof does not have to be as firm as a floor, but it should be strong enough to hold the roofing without sagging. If the roof has dips that can be spotted from the ground, be sure to make repairs in those areas.
If the edge along an eave is rotted or otherwise damaged, be sure to inspect the fascia board and rafter ends as well. This is a common location for rot. Replace any damaged fascia boards. If rafter ends are rotted 2 inches deep or less, you may be able to solve the problem by applying liquid wood hardener. Or cut away the damaged area and install a "sister" piece alongside the rafter. Then cover the rafters with fascia and soffit boards. Repair sheathing with plywood that is the same thickness as the existing sheathing if possible. Pressure-treated plywood is affordable and will prevent rot.
Once you have the materials, less than an hour for most sheathing repairs
Tape measure, circular saw, framing square, layout square, chalkline, drill, hammer, screwdriver
Basic measuring and cutting
Remove shingles, flashings, and nails
1/2-inch or 3/4-inch plywood or OSB (oriented strand board) to match the existing sheathing, 2X6 boards, 16d nails or 3-inch deck screws, 8d nails or 2-inch deck screws, underlayment
Check for damage. Look for places where the sheathing is rotted or discolored from water damage. Poke with a screwdriver; if it goes in easily, the wood is rotted. You can also inspect from inside the attic. Shine a flashlight at the sheathing and rafters and poke with a screwdriver.
Remove the damaged area. Use a chalkline or a pencil and framing square to mark a rectangle around the damaged area that spans from the inside of a joist to the inside of another joist. (Do not attempt to cut along the center of the joists.) Use a circular saw to cut out the rectangle.
Pull out the damaged plywood and inspect the rafters below. If they are slightly rotted, install long sister rafters, made of boards the same width as the rafters, alongside them instead of the nailers shown in the next step. Attach the sisters from inside the attic. If the damage is extensive, call a carpenter for advice.