Collar Ties Vs. Rafter Ties
In common rafter framing the rafters support the weight of the roof construction and snow or live load by pushing against the opposing rafters (at the ridge plate at the peak), pushing horizontally outward at the top of the supporting wall, and pushing vertically downward on the top of the supporting wall. The ceiling joists or ties not only provide vertical support for the weight of the ceiling, but they also resist the outward horizontal thrust of the rafters at the top of the supporting wall. That is, the ceiling joists or ties restrain the bottom of the rafter and the top of the wall from thrusting outward. The restraint by the ceiling joist or tie is usually provided through the opposing ceiling joist that is restraining the outward push from the bottom of the rafter on the other side of the roof.
If the opposing ceiling joists or ties are not adequately attached to each other at their lap, or if the ceiling joists or ties are not adequately attached to the bottom of the rafters, or if the ceiling joists or ties span in a direction different than the rafters, the bottom of the rafters will thrust outward. Generally, the outward thrusting of the rafters will cause the top of the supporting walls to lean outward, the peak of the roof to drop and sag, and the drywall ceiling to be stretched and cracked.
In some construction the ceiling joists or ties are raised to create a higher ceiling or clearance under the rafter framing system. Generally, if the ceiling joists or ties remain within the lower third of the vertical height of the rafter framing they are still considered ceiling joists or ties. In these cases the rafters and the connection of the ceiling joists or ties require additional analysis and an increased size for the additional stresses created by the higher location. The magnitude of the horizontal outward thrust also depends on the weight being supported by the roof framing, the span (length) of the rafters, and the pitch of the roof.
If the ceiling joist or ties are raised to the upper third of the vertical height of the rafter framing, they are considered collar ties. Some believe that collar ties provide restraint similar to ceiling joists or ties. They do not. Computations show that the higher locations renders them practically ineffective in restraining the bottom ends of the rafters. Collar ties are actually installed to hold the top of the rafter framing together in the case of uplift from wind pressures.
Some of you may be wondering what the joist or tie in the middle third of the vertical height of the rafter framing is called. I’ve heard them referred to as a “collar tie” or a “brace.” Whatever the name, it is still practically ineffective for restraining the bottoms of the rafters from spreading, unless it has been engineered.
If in doubt, contact your local building official or consult a structural engineer.