Teardrops n Tiny Travel Trailers

I am new to the whole teardrop trailer build process, and need some advice. I have read quite a few post on putting the aluminum roof skin on about letting it float or glueing it down, but I have noticed just about everyone is using a plywood skin underneath the aluminum, is there a purpose for this? I would like to just use the aluminum if possible, and wondered how the trim works are you fastening the skin down with the trim, or is the skin sitting in between the trim fasteners? any help would be great thanks.

It can be hard to glue aluminum or anything that is smooth and does not have a porous surface. On our camper I have a layer of Harding board ( peg board) and then installed the aluminum sheet. I used aluminum gutter/siding caulking to seal the edges and them used 1/16 inch by 1 inch strips of aluminum held in place with screws to hold the center down. That way the metal can expand somewhat. I think, at least for me, the reason to back up the metal is just for support. It is a lot harder to dent it when it has something on the back.