| A little more information is needed to answer your question.
Location of the shed (snow load, wind, etc)
Type of roof; material, pitch
Spacing of the roof "beams" (actually joists)
timber? steel?
12 meters is a pretty decent span; at that span I would consider an engineered wood product (TJI or other) or roof trusses
To fully answer your question, one would need more details about the installation & desired performance.
Are talking about something like a Butler building or a very large garden shed?
cheers
Bob | 
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