opening in roof. How do I do it?

I have been trying to cut an opening in a roof but have given up.
I note some issue in past posts.
Am I able to put an opening in a roof slope?
S Hyde
2-4c  Brentwood UK
Open Building Designer CONNECT Edition Update 5  v.10.05.00.49
Microsoft Win 10 Pro Version 10.0.17134 Build 17134
Processor Intel(R) Core(TM) i7-4790 CPU @ 3.60GHz, 3601 Mhz, 4 Core(s), 8 Logical Processor(s)
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