By 'flat' roof I assume you mean a single plane roof.
They only have one surface and therefore only one pitch.
Multi-plane roofs can have any number of surfaces and only show the original pitch as it was created.
In the settings (not info box) you will a warning to say there are multiple pitches.
If you click on a pivot line of a multi=plane roof and go to the 'Custom planes' option in the pet palette, you will see the custom pitch.
One of the forum moderators.
Versions 6.5 to 25
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