Section 148. Public stairs

1. Every stair, fire-stair and fire-tower shall,
  except  as  otherwise  provided  in  subdivisions three and four of this
  section and section one hundred forty-nine, be constructed  as  provided
  for  fire-stairs  in  sections  one hundred two and one hundred five for
  fireproof dwellings.
    2. Every stair, fire-stair and fire-tower shall be at least three feet
  in clear width throughout, and at all floor levels shall  have  landings
  at least three feet six inches in clear width.
    3.   Every  stair,  fire-stair  and  fire-tower  shall  be  completely
  separated from every other stair, fire-stair  and  fire-tower  and  from
  every public hall and shaft by fireproof walls, with fireproof doors and
  assemblies,  with  the  doors  self-closing and without transoms, at all
  openings, except that in dwellings two stories or less  in  height  such
  walls may be fire-retarded on both sides in lieu of being fireproof. The
  doors  giving access to such stairs shall not be held open by any device
  whatever.
    4. Except in the case of an interior enclosed stair separated from and
  directly accessible to the public hall by a self-closing fireproof  door
  and  except  as provided in subdivision five, there shall be provided to
  light and ventilate every stair at  every  story  a  window  or  windows
  opening  on a street, court, yard or space above a setback. At least one
  such window shall be at least two feet six inches  wide  and  five  feet
  high unless it opens on a street, in which case its minimum height shall
  be four feet. The aggregate area of such window or windows at each story
  shall  be  at least eighteen square feet. On the top story a ventilating
  skylight may be substituted for a window. At the entrance  story  or  at
  the  roof  level  a sash door, such as described in section thirty-five,
  opening to the outer air may be substituted for such window.
    5. In a dwelling occupied by two families or less on every story:
    a. If such dwelling is three stories or less in height, there  may  be
  provided  for  any stair, in lieu of windows, a stairwell sixteen inches
  or more in clear width extending from the entrance story to the roof.
    b. If such dwelling is a class A dwelling and is two stories  or  less
  in  height,  there  may be provided for any stair, in lieu of windows, a
  stairwell six inches or more in clear width. For the  purposes  of  this
  paragraph  a  section  of  a  class  A multiple dwelling may be deemed a
  separate multiple dwelling.
    c. If such dwelling is two stories in height and has no public hall on
  the first story, no stairwell or stair windows need be provided.