Start from the top of the roof and remove the old shingles along with the flashing underlayment and vents.
Roof change plywood.
Loads will vary with the pitch of the roof.
Secure the plywood by inserting six penny nails along the rafters spacing them every 6 inches.
Rotted plywood can cause a number of issues.
Install a layer of roofing insulation as recommended by the roofing manufacturer.
That will suffice on a roof with rafters set 16 inches apart with minimal roof loads.
A half inch cdx plywood is often used with rafter with 24 inch spacing.
Inspect the underlying roof sheathing for damage and make any needed repairs.
Place a new plywood board in the same location ensuring the ends align over roof rafters.
Moisture causes wood to rot and rotten wood absorbs water even more readily than fresh wood.
If your roof carries a heavier load like if there s less pitch and or it snows in your area a roofer may recommend the use of the inch variety.
Hammer down any large nails that are exposed by the removal.
Step 6 unroll new roofing felt over the new plywood and extend it at least 3 inches over each edge of the new plywood.