Definition
A joist is a horizontal structural member that supports floors, ceilings, or roofs. Joists run parallel to each other between walls or beams and transfer loads to the vertical supports. Common types include dimensional lumber (2x8, 2x10, 2x12), engineered I-joists, and open-web trusses. Joist spacing (on-center) is typically 12, 16, or 24 inches depending on span, load requirements, and flooring material.
Formula
Standard joist spacing is 16 inches OC for most residential floors. For heavy tile floors, use 12 inches OC. For lightweight carpet or decks, 24 inches OC may be acceptable. Span limits vary by joist size, wood species, and spacing-2x10 southern pine at 16 inches OC spans approximately 16 feet for residential floors (40 psf live, 10 psf dead load).
Example
Joist Selection Example
For a 14-foot span supporting a bedroom floor, 2x10 southern pine joists at 16 inches OC are typically sufficient. For the same span with ceramic tile flooring, use 2x12 joists at 12 inches OC to reduce deflection and prevent tile cracking. Always consult local building code span tables and consider using I-joists for longer spans or improved performance.