Install a layer of roofing insulation as recommended by the roofing manufacturer.
Roof change plywood.
Loads will vary with the pitch of the roof.
Start from the top of the roof and remove the old shingles along with the flashing underlayment and vents.
Most roofs will be sheathed in plywood at least 3 8 inch thick.
Step 6 unroll new roofing felt over the new plywood and extend it at least 3 inches over each edge of the new plywood.
That will suffice on a roof with rafters set 16 inches apart with minimal roof loads.
Hammer down any large nails that are exposed by the removal.
Secure the plywood by inserting six penny nails along the rafters spacing them every 6 inches.
A half inch cdx plywood is often used with rafter with 24 inch spacing.
Moisture causes wood to rot and rotten wood absorbs water even more readily than fresh wood.
Rotted plywood can cause a number of issues.
Place a new plywood board in the same location ensuring the ends align over roof rafters.
Rotted plywood cannot support the weight of a roof the way healthy wood can nor can it safely grip the nails put into the roof and keep them held firm.