I am going to build a carport and am considering using formplyfa instead of tongue and groove boards as the underlay on the roof joists. The reason is that since the tongue and groove will be visible from underneath and the risk of black mold is significant, I would avoid having to paint and maintain the surface in the future. Initially more expensive, yes, but it would save time and money in the long run. Is it a good or bad idea? What I'm hesitating about is whether the panel seams between the roof joists risk shearing differently, since they won't interlock as there's no tonguing. I'm planning to have felt and sedum as the roof surface.
The requirement today is 23mm råspont if you are going to have ytpapp. If you use formplyfa, it will need to be 22mm and you will need to bridge the joints between the roof trusses. I think it will look very silly and unsuccessful. I say, use råspont.