what is a joist in construction

A joist is a horizontal structural member used in construction to span an open space, often between beams that subsequently transfer loads to vertical members. Joists are either made of wood, engineered wood, or steel, each of which has unique characteristics. Typically, wood joists have the cross-section of a plank with the longer faces positioned vertically. However, engineered wood joists may have a cross-section resembling the Roman capital letter "I"; these joists are referred to as I-joists. Steel joists can take on various shapes, resembling the Roman capital letters "C", "I", "L", and "S". Joists must exhibit the strength to support the anticipated load over a long period of time, and in many countries, the fabrication and installation of all framing members including joists must meet building code standards. The wider the spacing between the joists, the deeper the joist will need to be to limit stress and deflection under load.