Hi @Martina8,
what about something like this?
First, I simplified the way you are creating the projection for the beams because that was giving you redundant points that were creating problems later. But if you want to keep them that way because you want the lines to be tangent to the outer circle, feel free to keep exploring that option 
. In that case I would connect the outer circle to the Tangents input in the Rebuild components.
Second, what I am doing is creating the offset of the curves on the XY plane to project them later to your roof, that way you avoid all the mistakes that you are getting.
Third, once the curves are on the roof, as they are simpler, it is easier to create the beams.
pavilion_01.gh (23.0 KB)
I hope this helps!