I can understand this for situations where boiling tar is the next
> substance to be applied, but does this work with fibreglass resin?
> Would the resin soak into the black bitumen surface as is needed to
> make what is essentially a composite roof?
>
Highly unlikely even with untreated wood. All wood will shrink and move a lot more than the fibreglass, so any 'bond' wouldn't last very long, but it's not important.
I assume that we really have to take the film off, unless that
> dissolves in resin. I can't seem to find any manufacturer's data
> sheet about this Metsa board, just brochures that say it's wonderful.
> I could only inspect the boards today in half-light as I didn't have
> the key to the shed, but the black looks like more a matt paint than
> any sort of bitumen I've ever seen.
>
> As is obvious, I haven't ever done a roof before.
>
> Because the roof is 4 feet wide and it's all we could find locally, we
> have bought the 8x4 sheets rather than the 8x2 t&g that seems to be
> suggested. I assume this means we have to allow for expansion at the
> edges, but I don't really understand how this works as we then bond
> the whole lot solidly together.
You don't need to allow for expansion, just nail or screw the boards on, apply the fibreglass and forget about it