Section 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.