A joist, in architecture and engineering, 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. It 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. There are approved formulas for calculating the depth required and reducing the depth as needed; however, a rule of thumb for calculating the depth of a wooden floor joist for a residential property is half the span in feet plus two inches; for example, the joist depth required for a 14-foot span is 9 inches. Many steel joist manufacturers supply load tables in order to allow designers to select the proper joist sizes for their projects.
Engineered wood products such as I-joists gain strength from the depth of the floor or the height of each joist. A common saying in the industry is that deeper is cheaper, referring to the lower-quality cost-effective joists 14 inches and above.
Bandsill is another term for joist used by construction workers and home inspectors in the southeast U.S.