Ceiling joists usually:

Ceiling joists typically run across the narrow span of a building, providing structural support to the ceiling and any materials attached to it, such as drywall or plaster. This orientation is crucial because it helps distribute the weight of the ceiling and any load on it evenly across the supporting walls. By spanning the shorter distance, ceiling joists maximize their ability to resist bending and deflection, which enhances the overall stability of the structure.

This positioning also contributes to the integrity of the building by preventing sagging and maintaining a level ceiling. Running parallel to the walls, which could lead to a weak structure if the load is not properly supported, is not the standard practice. Additionally, ceiling joists are designed to function cohesively as a continuous structural element rather than being spliced frequently, which could compromise their strength and stability.