Floors are most generally con, structed single; that is, simply a series of parallel beams, each spanning the width of the building, as seen at Fig. 39, Occasionally floors are constructed double, as at Fig. 40; and sometimes framed, as at Fig. 41; but these methods are seldom practised, inasmuch as either of these requires more timber than the single floor. Where lathing and plastering is attached to the floor-beams to form a ceiling below, the springing of the beams, by customary use, is liable to crack the plastering. To obviate this in good dwellings, the double and framed floors have been resorted to, but more in former times than now, as the cross-furring (a series of narrow strips of board or plank nailed transversely to the underside of the beams to receive the lathing for the plastering) serves a like purpose very nearly as well.