A joist is a horizontal supporting member used in construction to provide structural support to floors, ceilings, or roofs.  Joists are typically made of wood, steel, or engineered materials like engineered wood or composite materials, joists are arranged parallel to one another and spaced at regular intervals to form the framework of a floor or ceiling structure.

Joists bear the load of the structure above them and transfer it to the vertical structural elements, such as beams or load-bearing walls. Joists provide the framework onto which flooring or ceiling materials are installed, serving as a critical component in the overall stability and support of a building's structure.  A variation of the joist is the I-Joist which is made of composite materials.