After the shed roof framing is complete finish the lean to along with any part of the existing roof from which you ve removed shingles with new shingles to blend the lean to into the old roof.
How to extend lean to roof.
The rafter nailers spanning between the posts on the eave side of your lean to must be strong enough to support the load of multiple rafters.
Remove sections of siding immediately adjacent to the top edge of the roof.
If the siding is vinyl use a siding removal.
From this existing post measure 3 feet distance in the same line dig a hole and drive the second post in.
Using a coil of aluminum flashing like this cut a length that will run the length of.
Drive the post in and fill the hole with concrete to set it in place.
Use a minimum size of 2 x 6 nominal southern yellow pine or other strong wood.
Use a razor blade to cut it and have it the size of the roof.
After is adding the metal panels you will want the metal to extend 1 2 past the framing will run from tall side to short.
Finish the shingles up to the top edge of the lean to as described in steps 1 5 above.