There are (IMHO) two different things:
1. The roof projection which is the base room surface.
2. The roof surface which means (at least to me) the sum of all the faces of the roof surface.
This could be really useful.
Probably a label with the total area AND a label with the sum of the selected roof faces.
I'll see. I need to learn how to calculate the area of the 3D polygon first.
Still a "Happy New Year!"