[Cob] roof trusses

[Jilly]

In the event of using timber trusses, if I wanted to hide the plywood from he interior, what would I do? (You have to use timber trusses for a metal roof, yes?)

Would I place muslin on top of the trusses, then put the plywood, as this would allow me to plaster over the muslin and allow the trusses to remain exposed? 

I can use logs for that matter, right? 

jilly