"Building Height" means the vertical distance measured from the adjoining curb grade to the highest point of the roof surface if a flat roof, to the deckline of a mansard roof and to the mean height level between eaves and ridges for a gable, hip or gambrel roof; provided that where buildings are set back from the street line, the height of the building may be measured from the average elevation of the finished surface of the ground adjacent to the exterior walls of the building.