Joist
Affiliate link — we may earn a commissionA joist is a horizontal framing member that supports a floor or ceiling. Floor joists run between bearing walls or beams, spaced 16 inches on center (sometimes 12 or 24). In residential construction, joists are typically 2x8, 2x10, or 2x12 lumber, or engineered I-joists (TJIs). The joist size and spacing determine the floor's load capacity and stiffness. When you're working on a floor, the joists are the structure you screw subfloor sheathing into. When you're running wire or pipe through a floor, you're drilling through joists.
Why It Matters
Knowing where the joists are tells you where the structure is. Heavy items (bathtubs, safes, pianos) need to land on joists or have the load spread across multiple joists. Drilling or notching joists in the wrong spot weakens the floor. There are code limits on how big a hole or notch you can put in a joist relative to its depth.