What is the primary function of collar ties in roof construction?

The primary function of collar ties in roof construction is indeed to prevent roof sagging. Collar ties are horizontal members that connect opposing rafters in the upper third of a roof. They help maintain the structural integrity of the roof by resisting the downward force exerted on the rafters from roof load, such as snow, rain, and the weight of the roofing material itself. By countering this downward force, collar ties effectively halt potential sagging and help keep the roof shape stable over time.

While supporting the roof joists is a function associated with structural members, collar ties specifically address the forces acting on the rafters, rather than the joists directly. Ventilation is also important in roof construction but is not facilitated by collar ties; instead, it is typically achieved through different components like vents or ridge caps. Decorative elements are not the primary concern of collar ties, as their design and placement focus more on structural support and functionality rather than aesthetics. Thus, the correct answer reflects the integral role collar ties play in maintaining the roof’s structural stability.