You might try capturing the section profile of that roof (or draw it from scratch) and use it to define a custom profile for the Beam tool. Draw a 'beam' and set the angle to the incline you wish for your roof.
Cheers,
Karl
[Years ago, all of us used to always point out that ArchiCAD required 'lateral thinking' to do what you want. These days, we don't even seem to say that any more...it just seems assumed!]
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