This project requires two parts.
Old roof new roof joined rafters fix.
The new part of the roof system can be erected both above and below the old elements of the roof.
I e the original roof was a light wood shingle roof and the new replacement roof is a concrete tile roof weighing 5 or 10 times more than the original roof.
Notice the rafter spreader bolt.
Save those old shingles and number them for replacing later.
If a roof has been re roofed with heavier roofing material than the original roof was engineered for.
Wood rotting from the inside out compromises the structural integrity of the rafter.
To avoid this there are additional pads on the connection phase with a base.
Because driving nails up into the sheathing material will make things worse glue a piece of 1 inch rigid styrofoam to your piece of plywood.
However the trick to this process is to do it very slowly so it may take a few weeks before you get the rafter into a position where it is straight again.
Repeat the process for the joist beneath your old roof rafter.
When repair is no longer an option the roof rafters must be replaced.
Attach to the rafters using metal clamps or serrated plates.
Starting two courses higher than where the new ridge meets the old roof unseal the shingle tabs a thin flat bar or a joint taping knife works well and pop the nails of the overlying course.
First remove the rotten wood and then replace it with new rafters.
Use your tape measure to record the distance of both rafter legs.
Then the beam or roof may sag.
High enough to give room to nail the mid span cross tie at the desired location.
With the roof rafter pair straightened clamp new mid span cross ties in place and then nail the tie ends to the rafters.
Remove two shingles to each side of the new valleys until you reach the bottom.
Cut a piece of 1 2 inch plywood that will cover the area.
The easiest way to straighten rafters is to jack them into position.
Measure the damaged area between your roof rafters or trusses.
Move the jack in increments of one inch to allow for the roof to be able to move with it without cracking.