The ledger typically a 2 by 6 is usually designed to hold one end of the patio roof rafters.
Shed roof attached to house.
You save one of the walls.
If the outside edge is 8 feet high and the area being covered is 15 feet attach the shed roof addition at least 9 feet 3 inches above the level of the porch or other area being roofed or 10 feet 6 inches for a 2 pitch.
More often than not attached shed use lean to roofs because they are easier to build and allow water snow and all that stuff to drop easily instead of trooping between the shed roof and your house wall.
A shed roof should slope at least 1 inch per foot but 2 or 3 inches will be better.
You will need to attach the porch rafters to the house roof rafters and replace the roofing and flashing.
Here are the main positives of building attached shed instead of a standalone one.
The roof is typically sloped down from the connecting wall if the shed is attached to the side of your house or garage.
Shed roof design combines advantages of gable roof and flat roof with beams spanning between two walls one of which is higher than the other one thus creating a slant usually ranging usually ranging from 7 to 35.
Rafters should be attached to the header beam using hurricane ties.
However if you choose to build a freestanding shed and use this style of roof you should plan the slope of your roof in such a manner as to slope down from the front to the back of the shed.
The good and the bad.
The ledger should be attached before the foundation is built for a deck patio roof posts and other structure.
Work from the height of the outside of the new shed roof.
If you are tying the shed roof into an existing roof on the house you will need to remove the shingles and sheathing to expose the roof framing.