Collar Ties Vs Rafter Ties

Collar ties and rafter ties are both horizontal roof-framing members, each with different purposes and requirements. Home inspectors should be familiar with these structural members and the differences between them, as they are not the same. 

In physics, tension is the pulling force exerted by a solid object on another object. Tension members are solid objects (or structural members) that are subjected to axial tensile forces, or tension. Collar ties and rafter ties are examples of tension members.

Tension Tie:

A tension tie is a structural member that is subject to net tension.

 Collar Ties:

A collar tie is a tension tie in the upper third of opposing gable rafters that is intended to resist rafter separation from the ridge beam during periods of unbalanced loads, such as that caused by wind uplift, or unbalanced roof loads from snow. Ridge straps are permitted to replace collar ties.

If collar ties or ridge straps are installed to connect opposing rafters, they must be located in the upper third of the attic space. They must be at least 1 inch by 4 inches (25 mm by 102 mm). They must be spaced no more than 4 feet (1219 mm) apart. They must be properly fastened.

In high-wind areas, uplift can tear a roof off of a house if it's not properly attached.

Collar ties and ridge straps should be installed on roof with a minimum slope of 3:12, a maximum wind speed of 100 miles per hour, and a maximum roof span of 36 feet. 

Rafter Ties:

A rafter tie is a tension tie in the lower third of opposing gable rafters that is intended to resist the outward thrust of the rafter under a load. In many situations, you'll find that ceiling joists installed parallel to the rafters are intended to function as rafter ties. The roof framing mock-up below shows a standard rafter tie.

Rafter ties are installed between opposing rafters, and they should be installed as close as possible to the top plate. 

Rafter ties help resist the outward thrust that rafters exert on the exterior walls. They help keep walls from spreading due to the weight of the roof. When the walls spread, the ridge board might sag. A sagging ridge is one indication that the roof structure may lack adequate rafter ties. A rafter tie forms the bottom chord of a simple triangular roof truss.

When ceiling joists run perpendicular to the rafters, inspectors may find rafter ties installed above ceiling joists as framing members every 4 feet running above the ceiling joists connecting opposing rafters.

Rafter ties should be at least 2 x 4 inches (51 mm × 102 mm).

In summary, collar ties and rafter ties perform different functions, but both are essential roof-framing members, and it's useful for inspectors to be aware of their differences in order to properly call out defects. Source