There was a thread here a while back on if you have rafters every few feet you can push it a bit and have you boards overhang horizonatly, but 2 ft is a bit much w/out returns
I used purlins and I have 3 foot of overhang for my straw bales. What I did is put pockets in both sides of the rafter (8x10) with a 1.5"deep dado running between the pockets. I then notched the purlins (6x7)so that they ran in the dado but shouldered out in the pockets.
It is at this point so far. I plan to hang a 2x10 "barge rafter" Some might call it a a fascia board to connect all the purlin ends toogether running up the rake. I hope that all of the purlins acting iin concert will handle the load. I may need to run a return back to the bale walls, which will obvously be easier if you have a more conventional wall.
I cant vouch for the longevity of this design but it appears sound to this point. My computer has been down for a bit and my photo edit software is AWOL, but I will try to get some pics here soon if you want.