"Ceiling" is interesting, because in eg our church roof, the external cladding (Under the tiles) is fixed to rafters, and the inside of those rafters is lined with what we'd call the "ceiling". So a ceiling must be a timber lining to the inside of structural members on to which the exterior cladding goes.After all, a boat hull is basically an inverted roof. Presumably it was in later house building, when houses had mezzanine or intermediate floors, that the bit next above your head became the "ceiling". Fascinating etymologies there.