Final answer:
A tie beam or ceiling joist is the horizontal framing member that prevents opposing rafter pairs from spreading apart, maintaining the roof's integrity and the building's structural solidity.
Step-by-step explanation:
The horizontal framing member that prevents opposing rafter pairs from spreading apart is called a tie beam or ceiling joist. This structural component is crucial in roof construction as it maintains the integrity and shape of the roof by resisting the outward force exerted by the roof rafters.
The tie beam is placed perpendicular to the rafters and is often part of the ceiling framework, directly connecting the lower ends of the opposite rafters together to form a triangle, which is a very stable shape structurally. This ensures the walls do not bow outward under the weight of the roof, thus maintaining the structural solidity of the building.