Par. 1. - Openings for doorways in party walls shall not exceed one hundred square feet each in area, and each opening shall have two sets of fire doors separated by the thickness of the wall, hung in a manner satisfactory to the commissioner, except that the aggregate width of all openings in any story shall not exceed fifty per cent of the length of the wall in which such openings occur. Openings, not exceeding one hundred and forty-four square inches, constructed and protected as shall be approved by a writing signed by the fire commissioner, and filed with the commissioner, may be permitted in any wall or floor.