Joist
A horizontal framing member that supports the floor or ceiling decking of a structure.
Joists are the repetitive horizontal framing members—typically spaced 12, 16, or 24 inches on center—that span between beams, walls, or foundation sills to support the floor or ceiling above them. Floor joists carry the live load (furniture, people) and dead load (the weight of the floor assembly itself) across the span and transfer those loads to bearing walls or beams at each end.
Traditional joists are dimensional lumber (2×8, 2×10, 2×12) but engineered alternatives like I-joists (TJI) or open-web floor trusses are now common because they span farther with less deflection and allow mechanical systems to run through the web rather than notching solid lumber.
During renovations, damaged, notched, or undersized joists must be sistered (a new joist fastened alongside the old one) or replaced. Adding tile floors or heavy stone countertops may require joist reinforcement because these materials exceed the design live load of typical residential framing.
Real-World Example
The inspector flagged three notched joists in the basement where a previous owner had run drain lines; the contractor sistered each one with a new 2×10.