Section 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.