Ceiling joists are a number of horizontal beams, running parallel from one wall to the opposite wall of the structure. The beams are attached to the top of the wall at what’s known as the wall plate. A ceiling joist at the top floor of a structure may also support a flat roof, as well as the ceiling. If the roof of the structure is sloped, or pitched, each joist support is attached to a roof rafter, the angled roof support beams. This joist-and-rafter configuration prevents the sloped roof construction from pushing the walls of the structure outward. (source)