How to find out orientation of the roof and the pitch angle of its skylight?

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The OpenStudio surface.outwardNormal() method will give you exactly what you want, a 3D vector of the surface normal. It is used in the Radiance forward translator to determine the orientation for a given window group's hemispherical sampling basis. e.g.

If you are using EnergyPlus, and if you've added the Output:Table:SummaryReports object to your idf file with either AllSummary or EnvelopeSummary selected, you can get this information from the *.htm file created by the program at the end of a simulation. Open it and lookup for "Report: Envelope Summary".