Which aspect of rafters prevents the walls from being pushed apart by the roof?

The aspect of rafters that prevents the walls from being pushed apart by the roof is the top plate. The top plate is a horizontal member that is installed at the top of the wall stud framing. It serves as a connection point for the rafters or ceiling joists to rest upon. By providing a secure anchor for the rafters, the top plate helps distribute the load of the roof down to the walls, reinforcing their structural integrity and preventing them from being pushed outward due to the weight and forces exerted by the roof structure.

In this way, the top plate plays a critical role in maintaining the stability of the wall system while supporting the roof's weight effectively. Other components, like the ceiling joist, pitch, and ridge board, while important for various aspects of roofing, do not primarily function to prevent the outward movement of the walls caused by roof load.