What version of Archicad and what operating system are you using?
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So you are selecting the roof and right mouse clicking and choosing 'Split into single roof planes'.
Then choosing 'Split anyway' in the resulting dialog.
This is working fine for me in 17 even when the roof is visible in a storey other than its home storey.
What are the exact steps you are trying?
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11