§ 239. Tower fire-escapes and supplemental stairs. 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.