I want to build a small (16x24) rustic cabin, and the birdsmouth cut seems to require more experience than I have, but I wonder about drainage, if I install a flat roof. Is there a way to build a shed roof with fasteners, or is there a way to put a slight pitch to a flat roof?
You can easily go to a lean-to type roof, just build one wall higher.
On small projects like yours I manufacture my own trusses and put the crossties flat on the topcap and secure with hurricane tiedowns. If you beef up the crossties, you can floor over parts of them for storage.
I suggest that you pick up a DIY book on garage or home building.