fredii

In order to get the height, I am thinking about adding a ledger on top of a shingled roof, above the top plate. Can I just remove the shingles underneath and nail a ledger to the plywood sheathing? Are there angled joist hangers that
I can then use to connect my joists for a patio that I am adding?

nealtw

Where will the water be going if you do that. Without nowing all the details I would think about cutting off the roof and rafters just outside the wall and adding a ledger there, low enough so the water could continue down over your new roof. More detail and photo may be helpful.

fredii

The ledger would lay flat on the 5:12 roof, and there would be flashing for
the water from the asphalt roof to run down over the flashing and onto the less sloped polycarbonate roof.

I have not thought of your suggestion, but I have thought of something similar. Not doing a ledger but have the patio roof joists just sit on top of the top plate/house band and tying it in there.

I appreciate your thoughts.

oldognewtrick

If you put a ledger board on the sloped roof be sure you nail into the rafters and NOT just the plywood. You will probably have a water issue if you transition to a low slope with poly panels. You should be able to find foam "closures" that fit the ridges and valleys of the panels and are installed under your transition flashing at the shingle roof line. Closures help with wind driven water being forced back up under the flashing.