Bonding roof decking

I don't have the answer but, maybe you can find it on This to That.

There are a few options off the top of my head.

Epoxy works fine on styrofoam, but is expensive and messy to work with.

Some polyurethane construction adhesives (the sort that you squirt out of a caulking gun) are safe for styrofoam panels. Consult the labels on the various products—while some are safe, others are not. PL Pro I think is one that is safe for styrofoam (at least some variants are), and it is very effective.

Some contact cements are safe for styrofoam. There are water-based contact cements that are safe, but I strongly recommend avoiding them, as (at least in my experience) they utterly fail at holding things together.

I would probably go for construction adhesive if I were in your shoes.

A related question that may be more difficult to resolve is figuring out how to keep sufficient contact between the luan and the foam to get a decent bond. I'm afraid I don't have many great ideas there.

(By the way, I would guess that your rafters may be aluminum, not steel. I could be wrong.)

Thanks drew
A useful response. I was thinking a pl adhesive myself. I was going to read the label at the lumber yard tomorrow when I grab the Luan.
I'm not a fan of water based anything which you are hoping to get a long life from (except interior paint).
The stuff that was on there must have been quite thin as it looked rolled or sprayed on.
I have an idea to apply pressure. I will post a pic of it if I go through with it.
The rafters are steel as they have a bit of surface rust on them.

@ gbopp cool site. I cruised around it for a bit. Thx