§ 266. Exterior enclosed fireproof stairway. "Exterior enclosed fireproof stairway" means a stairway conforming to the following requirements:

1. It shall be completely enclosed from top to bottom by walls of fireproof material not less than eight inches thick extending from the sidewalk, court or yard level to the roof, and above the roof so as to form a bulkhead.

2. It shall in all other respects except as herein provided conform to the requirements of section two hundred and seventy in regard to the construction of stairways and their enclosures, except that stairways and landings in an exterior fireproof enclosed stairway not exceeding one hundred feet in height may be constructed of other than incombustible material to be approved by the board.

3. There shall be no opening in any wall separating it from the building.

4. Access shall be provided to the stairway from every floor of the building by means of an outside balcony or vestibule of steel, iron or masonry. Every such balcony or vestibule shall have an unobstructed width of at least forty-four inches and shall be provided with a fireproof floor and a railing of incombustible material not less than three feet high. Access to such balconies from the building and to the stairway from the balconies shall be by means of firedoors. The level of the balcony floor shall be not more than seven inches below the level of the door sill of the building. The doors shall swing outward onto the balcony and inward from the balcony to the stairway, and shall be provided with locks or latches with visible fastenings requiring no key to open them in leaving the building. The balconies shall be open on at least one side, upon an open space not less than one hundred square feet in area.

5. The landings shall be of such width that the doors in opening into the stairway shall not reduce the free passageway of the landings to a width less than the width of the stairs.

6. Such stairway shall be provided with a proper lighting system furnishing adequate light and shall be so arranged as to ensure its reliable operation, when through accident or other cause the regular factory lighting is extinguished.