N.Y. Mult. Dwell. Law § 239
In fireproof tenements tower fire-escapes or stairs which are supplemental to the stairs required by law may be installed providing such tower fire-escapes or stairs shall be shut off from all other parts of the dwelling by brick walls or partitions of terra cotta blocks at least four inches thick, or hollow cement blocks at least four inches thick which have successfully withstood a three-hour standard fire test and been approved by the department, and have fireproof doors and assemblies with the doors self-closing at all openings. Whenever such supplementary stairs are provided they shall be constructed in accordance with such supplementary regulations as may be adopted by the department. Such tower fire-escapes or stairs shall not be used as service stairs and shall be kept adequately lighted at all times and free from encumbrance.
N.Y. Mult. Dwell. Law § 239