Greenhouses, Sheds and Sun Rooms - Ramp for shed

The floor of my shed is 20" off the ground so I cant build a free standing ramp from 2 x 8's. I was planning on attaching a ledger and then using joist supports to attached the 2 x 8's to the shed. I will be driving a lawn tractor on the ramp regularly so let me know if this approach is wise. Does anyone know what angle I should cut the 2x8's before attaching to the shed ?

First, I don't understand what's stopping you from making the ramp freestanding. I build freestanding decks that are 8' or more above grade all the time. 20" is nothing.

The slope you use is going to have to be gradual enough that your lawn tractor can handle it. I would suggest making it about 10' long to rise the 20" you need it to. 2X8 joists at 12" to 16" O. C., with plywood, 2X decking, or better yet a composite decking should work just fine. The ledger, with joists hangers is a start, but I would probably add a beam about 4' from the shed as well. Then pour a concrete pad for the bottom of the joists to be attached to abd rest upon.

I went to the AARP site and used there design and it came out perfectly.

Thanks