§ 257. Public stairs. 1. Every stair and fire-stair shall, except as otherwise provided in subdivisions three and four of this section and in section two hundred fifty-nine, be constructed as provided for fire-stairs in sections two hundred two and two hundred five for fireproof dwellings.

2. Every stair and fire-stair shall be at least three feet in clear width throughout, and at all floor levels shall have landings at least three feet eight inches in clear width.

3. Every stair and fire-stair shall be completely separated from every other stair and fire-stair 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. 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 or yard.

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 permanently occupied 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 permanently occupied 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.