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 [:D] )
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.