rafter tie

a horizontal member near the bottom of a trussed rafter system attached to opposing rafters to prevent their spreading and pushing the wall out of alignment. If ceiling joists are installed parallel to and tied to the rafters, rafter ties will not be needed. Rafter ties are used when the ceiling joists run perpendicular to the rafters and are typically installed in the bottom third of the roof framing. See also Collar Beam, Collar Tie or Tie Beam.