Final answer:
Ridge boards are less substantial than rafters; rafters support the roof's weight while ridge boards provide a nailing surface without bearing the roof's load.
Step-by-step explanation:
When compared to rafters, ridge boards are typically less substantial structural components. Rafters are the sloping beams that extend from the ridge of the roof down to the walls of the building. They are one of the primary structural elements that support the weight of the roof. Ridge boards, on the other hand, serve as a nailing surface at the peak of the roof to which the rafters are attached. They are not designed to carry the load of the roof; rather, they help to maintain the proper angle and positioning of the rafters. Therefore, among the given options, the most appropriate answer to what ridge boards are typically is 'a. ridge boards'.