A Joist is one of the horizontal supporting members that run from wall to wall, wall to beam, or beam to beam to support a ceiling, roof or floor. Ceiling joists are the horizontal members that provide a structure to fix the ceiling, and support and fix the diagonal rafters that define the roof shape and are attached at the top to a ridge beam.
They may be made of wood, steel or concrete. Typically, a Beam is bigger than, and is thus distinguished from, a joist. Joists are often supported by beams and are usually repetitive.
The wider the span between the supporting structures, the deeper the joist will need to be if it is not to deflect under load. Lateral support also increases its strength. The flooring system spans between the joists, and the stiffness and strength of the flooring used will establish the joist spacing.