I didn't look for this... but I heard on the radio that a Duke civil engineering prof chimed in on this "ceiling is the roof" quote.
The ceiling is the surface in the room at the top. The roof is the covering on the building.
In a dome, if you're inside, the ceiling is the surface above you. On the other hand, if you're outside, the roof is covering the building. But since the ceiling if you're inside and the roof if you're outside are the *same* object... then... the ceiling IS the roof.