§ 150. Cellar and basement stairs. 1. Except as otherwise provided in
this section, there shall be no inside stair communicating between the
lowest cellar or the lowest story, if there be no cellar, and the floor
next above, but any stair communicating between such floors shall be
located outside the dwelling and if enclosed shall be fireproof in a
fireproof enclosure with fireproof doors and door assemblies, with the
doors self-closing, at all openings.

2. This provision, however, shall not apply to any stair not extending
through more than one story and leading from an entrance hall to the
upper stories where the walls enclosing such hall are fireproof and
unpierced except for openings to stairs, elevators, apartments, public
reception rooms, professional offices and the outer air. In no event
shall there be any other opening from such an entrance hall, and all
apartments and such professional offices opening therefrom shall be
entirely separated from any space within such dwelling used for any
other purposes by fireproof walls which shall be unpierced except for
means of egress into the entrance hall. The provisions of this
subdivision shall not apply to openings which are protected with
fireproof vestibules. Any such vestibule shall have a minimum
superficial floor area of fifty square feet and its maximum area shall
not exceed seventy-five square feet. It shall be enclosed with
incombustible partitions having a fire-resistive rating of three hours.
The floor and ceiling of such vestibule shall also be of incombustible
material having a fire-resistive rating of at least three hours. There
shall be two doors to provide access from the entrance hall and any
other space not used for openings to stairs, elevators, apartments,
public reception rooms and the outer air. Each such door shall have a
fire-resistive rating of one and one-half hours and shall be provided
with a device to prevent the opening of one door until the other door is
entirely closed. One of these doors shall swing into the vestibule from
the entrance hall and the other shall swing from the vestibule into
space which it serves. Such vestibule shall also be equipped with
sprinklers and with an independent exhaust duct having a minimum
cross-sectional area of one hundred forty-four square inches for each
one thousand cubic feet, or fraction thereof, of air content, and such
exhaust duct shall not be connected with any other ventilating system.

3. In dwellings three stories or less in height which are occupied by
two families or less on every story, any stair leading to the cellar may
be located inside the dwelling provided it is entirely enclosed with
fireproof walls and with fireproof doors and door assemblies at both the
cellar level and the story above, with the doors self-closing.

4. The provisions of subdivision one shall not prohibit or apply to an
inside stair extending from a space used for commercial purposes in the
cellar or lowest story to a store on the story next above, provided such
stair is of incombustible materials, has closed risers, is enclosed
between the two lowest floors with walls having a three-hour fire
resistive rating and has fireproof doors and door assemblies at top and
bottom, with the doors self-closing, and provided such commercial space
in the cellar or lowest story is completely enclosed with partitions
having a fire-resistive rating of at least two hours and there is no
opening between the store on the story next above and any entrance hall
or other public hall or public portion of the dwelling.