Which roof framing members serve the purpose of reducing rafter spread?

Collar ties are horizontal framing members that are installed near the peak of a roof, connecting opposing rafters. Their primary purpose is to help stabilize the roof structure and reduce rafter spread, which can occur under heavy loads such as snow or during high winds. By resisting outward movement of the rafters, collar ties play a crucial role in maintaining the integrity of the roof over time.

In contrast, crown plates are typically used to connect roof framing members to the wall structure, but they do not specifically address rafter spread. Ridge boards serve as a support for the upper ends of rafters and provide a connection point but do not actively prevent rafter spread. Purlins are horizontal members that support the roof covering and are installed perpendicular to the rafters, but they also do not have a direct role in preventing rafter spread. Thus, collar ties are specifically designed for the purpose of controlling rafter movement, making them the most suitable choice among the given options.