Dimension of battens and roof beam

[kirtap01]

I am going to build a plastic roof to have extra storage space. The roof extends about 2.5m and is 4.20m wide. Can one then use rafters 45x45mm and roof beams 45x45mm or will it be too thin?

 

[Västerhaninge 17]

I have built a simple wood shed with 6m long spruce trunks, about 5cm in one end and 10cm in the other (used as beams). As battens, I used 2x2 inch studs, 3m long in each direction with 50cm center-to-center spacing (like a gable roof). On top of this, I have placed a tarpaulin. Works excellently in snow-scarce Stockholm

 

[anaitis]

Going to build a plastic roof to have additional storage space.
The roof extends about 2.5m and is 4.20m wide.
Can you then have rafters 45x45mm and a roof beam in 45x45mm or will it be too weak.

You need to tell more about how the support will be under the beam.
i.e., position of the vertical supports.