Fixing rotten wood on a roof requires the removal of the roofing materials, which is why repairs are usually completed during the replacement of a roof. Rotten wood is the result of water damage, and if not repaired, will eventually result in rotten rafters and a damaged ceiling. The first major sign of rotten wood is usually water damage on the ceiling, but this is normally preceded by aged and damaged shingles. If you have rotten wood, have your roof inspected to see if you need a new roof or just a patch.
Remove the shingles and tar paper around the rotten wood. Be careful not to fall through the roof. For small repairs, use a flat bar to slip under the shingles and tear the shingles off with the claw of your hammer. Use a roofing shovel to remove larger areas.
Remove the nails in the upper row of shingles so that the new shingles can be slipped under the existing shingles.
Cut the existing tar paper with a utility knife and remove it.
Snap lines with a chalk box to mark the area to be cut out.
Remove all nails near the lines and in the sheathing to be removed with a cat's paw.
Set your circular saw to cut 1/16 to 1/8 inch deeper than the thickness of the roof boards, and cut out the rotten wood.
Remove the sheathing and nails.
Cut new sheathing with your circular saw and nail it to the rafters with 8d common nails.
Staple 30-pound felt paper to the sheathing and install new shingles.
When installing plywood, install edge clips between the rafters to support the joints. The clips are normally available where plywood is purchased.