N.Y.C. Admin. Code § 27-866: § 27-866 Corbelling of chimneys.

Walls less than twelve inches thick shall not be corbelled. The maximum horizontal projection in any corbel shall be one inch for each two inches of vertical projection and in all cases the total projection shall not be greater than one-third of the minimum thickness of the wall to be corbelled. In no case shall the total projection be more than six inches. No masonry chimney shall be corbelled from hollow or cavity wall masonry units.