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.