joist

A joist is a horizontal structural member that forms part of a floor or ceiling framing system. It supports loads from above and transfers them to supporting beams, walls, or girders. In typical wood-frame construction, joists run parallel to one another and perpendicular to the primary supports, creating a grid that provides a flat, stable surface.

Materials and types vary. Solid sawn lumber such as pine, spruce, or fir remains common for traditional joists, often in nominal sizes like 2x6, 2x8, 2x10, or 2x12. Engineered options, including I-joists and laminated veneer lumber (LVL) joists, are widely used for longer spans or higher loads because they can offer greater strength with lighter weight and more uniform performance. Joists are sized and spaced according to expected loads, spans, and local building codes.

Spacing and bearing are important design considerations. In residential construction, joists are typically spaced 16 inches on center, though 12 or 24 inches on center may occur in some projects. Ends must bear on solid supports such as load-bearing walls or beams, and many installations use joist hangers, blocking, or bridging to maintain alignment and resist twisting.

Durability and maintenance focus on moisture control and protection from decay and insects. Excessive moisture can reduce strength and lead to rot, while improper ventilation can foster mold. Holes and notches are limited to preserve capacity; cutting or drilling should follow code guidelines and manufacturer specifications.

Overall, joists are a fundamental element in floor and ceiling systems, balancing strength, span, and economy through material choice, sizing, and spacing.