A Rafter Tie serves a similar purpose as collar ties only they are utilized at the bottom of the joist and can double as a ceiling joist. The rafter ties help keep walls from spreading apart and act as a tension member to resist gravity pushing out from the weight of the roof. If the walls of a structure spread the roof may sag and a visible sagged ridge may indicate improper rafter tie performance or installation .  Rafter ties are almost always needed unless the ridge is structural and self-supporting, which is fairly rare. Rafter ties are most commonly 2×4 or 2×6 material.