§ 106. Cellar and basement stairs. A cellar or basement stair may be located inside the dwelling, but shall not be located underneath a stair leading to the upper stories unless it is a basement stair leading upward from a basement which is the main entrance story of the dwelling, or unless it is a stair leading downward from the entrance story which is separated by a fireproof arch from the stair leading upward from the entrance story. Except as otherwise provided in subdivision five of section fifty-two, all inside cellar or basement stairs shall be entirely enclosed with fireproof walls and be provided with fireproof doors and assemblies, with the doors self-closing, at all openings.