Are you able to elaborate a little more on what issues you’re facing? Is it the fact that it’s next extending back all the way.
Theoretically it should just be a matter of running the full horizontal extent you need and then cropping the base and then cropping the top.
More info here: https://helpcenter.graphisoft.com/user-guide/76498/
I have always found when using the crop to roof tool to select both the wall and the roof you are trying to crop to force archicad to crop to that roof. If you only have the wall select when you use the crop to roof tool it makes an assumption in the roof you want and it choose the lowest one.
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