Joist

In architecture and engineering, a joist is one of the horizontal supporting members that run between foundations, walls, or beams to support a ceiling or floor. Typically, a joist has the cross section of a plank. Joists are often supported by beams laid out in repetitive patterns.

Strength

Joists need to be designed large enough to support the anticipated load over a long period of time and in some countries such as the United States to meet standards for stiffness. 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 called dwang, blocking, or strutting 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.