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.