Silicone or 3M 5200 caulk for Wood to Aluminum Roof?

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.

Sticking stuff together is getting so complicated with all the advances in that industry. I use this web site to help me. Just plug in what your wanting to stick together in the drop down boxes.

Usually with good results. I have also became a user of either alcohol swaps or denatured alcohol to clean surfaces.

http://www.thistothat.com/cgi-bin/glue.cgi?lang=en&this=Wood&that=Metal

Silicon is the worst thing to use.
As for 3M 5200 it's great stuff just remember it's listed as permenit
as in once you lock parts together don't plan on ever taking them apart.
I'm told there is a 3M 4200 which is not as aggressive as 5200.