I would use some sort of a mechanical fastener, rather than relying on adhesive to hold the ledger in place. I'd also prefer it to have a bit of a standoff above the roof so that it's not going to sit in water for long periods when it rains. (Pressure treated wood would be a bit longer lived in such a situation, but modern pressure treatment chemicals are quite corrosive to aluminum and will cause holes in the roof sooner or later--quite possibly sooner, as in a year or two or three.)
One way to do both of these is to screw an aluminum angle to the roof (sealing the screws etc. with some appropriate caulking material, Dicor or ProFlex or OSI Quad for instance), and then attach the 2x4 to the vertical part of the angle with lag bolts or similar.
