Floor joists are the repetitive members that support a floor. They run parallel to each other, spaced 12, 16, or 24 inches apart, and span from wall to wall or beam to beam. They carry the floor load and transfer it to the structure below. A joist is not a beam. It is smaller and more numerous. But it works the same way: top in compression, bottom in tension, middle resisting shear. The floor above sits on the joists. The ceiling below hangs from them.

Joists come in several materials. Solid sawn lumber is common in older homes and light construction. Engineered wood products, like I-joists and open-web trusses, are common in new construction. They are lighter, stiffer, and can span longer distances. Steel joists are used in commercial buildings. The choice depends on the span, the load, and the budget. The joist size is determined by the span and the spacing. A longer span needs a deeper joist. A heavier load needs a closer spacing.

Joists must be installed correctly. They must be level, square, and properly fastened. They must be bridged or blocked to prevent twisting and to distribute loads. They must not be notched or drilled in the wrong place. A notch in the top or bottom of a joist weakens it. The building code specifies where holes and notches are allowed. A floor that bounces may have joists that are too small or too far apart. The joist is the skeleton of the floor. It is hidden. It is essential.