§ 178. Lighting and ventilation of stairs. Where the stair and public
halls are not provided on each story with windows opening to a street,
yard or court, a skylight equipped with ridge ventilators having an
opening of forty square inches or more shall be provided in the roof
over the stair, except as otherwise provided in paragraph g of
subdivision three of section one hundred seventy-one. The glazed roof of
such skylight shall not be less in area than nine square feet nor less
than two feet in its least dimension, provided the ventilating and glass
areas in such skylight are not less than herein required; except that in
dwellings converted before April eighteenth, nineteen hundred
twenty-nine, where a skylight smaller than such prescribed dimensions
was in existence on such date, no structural change shall be required,
but a ventilating skylight fitting the existing skylight opening in the
roof beams shall be deemed sufficient.