Joists are the parallel horizontal members that hold up a floor or ceiling. Floor joists span between foundation walls, beams or bearing walls and carry the subfloor; ceiling joists carry the ceiling below an attic and, on many roofs, tie the bottoms of the rafters together so the walls don’t spread. The rim joist (band joist) caps the ends of the joists around the perimeter. Joists are usually 2x8–2x12 dimensional lumber or engineered I-joists, spaced 16” on center (12”, 19.2” and 24” are also common).
Joist size comes from the IRC span tables (R502.3.1 for floors, R802.4 for ceiling joists), based on species and grade, spacing, and the design live and dead loads. Floors are designed to a deflection limit of L/360 — a 12 ft span may sag no more than 0.4” under full live load — which is why floors that meet strength requirements can still feel bouncy if they are near the maximum span. Blocking or bridging at mid-span stiffens the floor and keeps joists from rolling.
Plumbers and electricians must follow the notching rules for sawn joists (R502.8.1): notches no deeper than 1/6 of the joist depth and never in the middle third of the span; bored holes no larger than 1/3 of the depth and at least 2” from the top or bottom edge. Engineered I-joists follow the manufacturer’s hole chart instead.