Floor Joists
Home Building Glossary · Last updated 2026-07-01
What Are Floor Joists?
Floor joists are the horizontal structural members that carry your floors — spanning between foundation walls, beams, and bearing walls, topped with subfloor sheathing. New Twin Cities homes use one of three systems: traditional dimensional lumber (2×10s/2×12s), engineered I-joists (plywood web with lumber flanges), or open-web floor trusses.
How Joist Systems Work
Joists are sized by span tables and deflection limits — code allows L/360 deflection (span ÷ 360), but floors built to the code minimum feel bouncy under a kitchen island or a piano. The three systems trade off differently:
- Dimensional lumber is economical for short spans but limited (a 2×12 tops out around 18 feet) and, being solid wood, it shrinks, crowns, and squeaks as it dries.
- I-joists span farther (up to ~26 feet in deep profiles), arrive straight, stay straight, and dominate metro new construction. Mechanical penetrations are allowed only through pre-scored web knockouts and sized holes — field-notching flanges is a structural sin.
- Floor trusses cost the most but solve two problems at once: long clear spans for open plans, and an open web that lets HVAC trunks, plumbing, and wiring run through the floor rather than below it — which is how you get 9-foot finished basement ceilings without bulkheads (see Basements 101).
Why It Matters for Your Minnesota Build
Joist choice quietly shapes the spaces Minnesotans care most about: the open great room (long spans mean engineered products or strategic beams) and the finished basement (trusses or deeper I-joists mean fewer soffits chopping up the ceiling). On a custom build, ask your builder two questions: “What deflection are the floors designed to?” — better firms design to L/480 or stiffer for tile and long spans — and “How will mechanicals run through the floor system?” The answer predicts whether your basement ceiling will be clean drywall or a maze of bulkheads.
One more cold-climate note: floor systems over unconditioned space (bonus rooms over garages, cantilevered floors) are notorious cold-floor complaints in Minnesota. The fix is framing and insulation detail at design time — closed-cell foam in the cantilever, air-sealed rim areas — not a rug afterward.