You could use the 'hole' windows to make the cut. At least NL library has a collection of hole shape windows with curved arches etc. It will be tricky to setup to match the roof.
Alternatively you could make your own window-hole library part for the job. Look at the archicad wiki, this forum and the reference guide how to make your own window parts.
Erwin Edel, Project Lead, Leloup Architecten
www.leloup.nlArchiCAD 9-26NED FULL
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