I'm not an architect but usually I draw walls (structure) first and insert the rooms after.
The "rooms" are 3D objects wile has high (the level high) and ceiling.
I want to get a roof for a building, not for a room
1. You could use AutoDimensioning to get the roof projection (a room 
)Suggestion: Read also at the binging of tread to see how it work.
2. Select the roof projection and use GenerateRoof.
...without creating a single room...
And how you calculate the area?? Manually??I'm still not an architect but a plan need to have dimensions and surfaces for every closed space.
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A computer program does what you tell it to do, not what you want it to do. Murphy's Law
When all else fails, read the instructions. Murphy's Law
If you don't like "AS IS", DIY. Dorin's law
