No masonry chimney shall be corbeled from a wall more than 150 millimeters nor shall a masonry chimney be corbeled from a wall which is less than 300 millimeters in thickness unless it projects equally on each side of the wall. In the second (2nd) storey of a two-storey building of Group A Occupancy, corbeling of masonry chimneys on the exterior of the enclosing walls may equal the wall thickness. In every case the corbeling shall not exceed 25 millimeters projection for each course of brick.