joists

Joists are horizontal structural members that form the floor or ceiling framework of a building. They span between bearing walls, beams, or other supports and transfer loads from the floor or ceiling to vertical supports. In residential construction, floor joists typically run perpendicular to the supporting walls and are spaced at 16 inches or 24 inches on center, depending on the design.

Materials and types: Joists can be solid sawn timber, engineered wood products such as laminated veneer lumber (LVL) or I-joists, or steel open-web joists. Timber joists are common in light construction and shorter spans, while engineered wood and steel joists enable longer spans, greater stiffness, or reduced depth.

Sizing and spans: The depth, width, and spacing of joists depend on species or material, grade, load, and span. Codes and span tables from the IRC/IBC guide proper sizes. Typical residential floor joists are 6–12 inches deep, with deeper sections used for longer spans.

Installation and continuity: Joists are installed perpendicular to supports, with solid bearing on ends. They support a subfloor or ceiling finish, which is fastened to the top or bottom edge. Notches and bore holes must follow code limits; holes are usually limited to a fraction of the joist depth and must avoid critical zones near edges.

Bracing and blocking: Bridging or blocking between joists improves stability, reduces twisting, and helps distribute loads.

Maintenance and issues: Joists can be affected by moisture, rot, insect damage, or sagging. Proper moisture control, ventilation, and drainage reduce risk; damaged members should be repaired or replaced by a qualified professional.