In tenement houses over four stories high or that are occupied by more than two families on any floor, every public hall must have at least one window opening directly upon the street, yard or court. If the window is not at the end of the hall with the plane at right angle to the axis of the hall, then there must be one window for every twenty feet in length, or fraction thereof, of the hall. Where there are recesses in such halls and the length of the recess exceeds twice its width, additional windows must be provided for such recesses. If any part of a public hall is shut off from any other part of the hall by a door or doors, it is considered a separate hall, and must be lighted and ventilated separately. At least one of the windows in the public hall must be not less than two feet six inches wide and five feet high. In stair halls the aggregate area of windows should not be less than eighteen square feet, and there must be provided for each story at least one window which shall not be less than two and a half feet wide and five feet high. In addition to windows there must be in every hall directly over the stairwell in the roof a ventilating skylight with ridge ventilators and fixed louvres, with an area of not less than twenty square feet.