The headroom in stairways shall be not less than 6 feet 8 inches (2032 mm) measured vertically from the sloped line adjoining the tread nosing PHOTO 5 or from the floor surface of the landing or platform on that portion of the stairway PHOTO 6.
Exceptions:
1. Where the nosings of treads at the side of a flight extend under the edge of a floor opening through which the stair passes, the floor opening shall not project horizontally into the required headroom more than 4¾ inches (121 mm) PHOTOS 7 & 8.
2. The headroom for spiral stairways shall be in accordance with Section R311.7.10.1 PHOTO 55 (p. 19).