"Height of building" means the vertical distance from the established sidewalk grade at the center of the front of the building to the highest point of the roof surface if a flat roof, to the deck line of mansard roofs and to the mean height level between eaves and ridge for gabled, hip and gambrel roofs. For buildings set back from the street line "height of building" may be measured from the average elevation of the finished grade along the front of the building. (1955 Code. §1111.08)