§ 142. Sub-curb uses. 1. When such a dwelling is more than eighty feet in height measured from the lowest point of the yard or of any curb on which any part of the dwelling faces, whichever is lower, and any room in such dwelling below the level of the highest curb which any part of such dwelling faces is occupied for living purposes as permitted in paragraph f of subdivision one of section thirty-four, the yard at the level of such highest curb shall be set back so as to be at least eight feet greater in depth than the yard below such level unless the yard opens directly upon a street.

2. If such a dwelling is erected upon an interior lot running through from street to street, the wall of the dwelling facing the lowest street shall, at the level of the highest curb on which any part of such dwelling faces, be set back at least eight feet.

3. The setback of the yard or wall required by each of the two preceding subdivisions shall be a fire-terrace. Nothing herein contained shall be construed to permit a yard of smaller dimensions below the level of such fire-terrace than required by paragraph f of subdivision one of section thirty-four.

4. From each apartment below the level of such highest curb at least one means of egress shall lead directly to such fire-terrace in a manner approved by the department. Such fire-terrace shall be protected by a guard railing approved by the department, and shall afford safe and unobstructed access either directly to a street or to a covered fireproof passage at least three feet in width and seven feet in height leading directly and without obstruction to a street.

5. In all portions of such dwellings below the level of the highest curb all structural members, partitions, furrings and ceilings shall be constructed of incombustible materials.

6. Apartments may also be occupied for living purposes in cellars and basements in accordance with the provisions of subdivision six of section thirty-four.