Joist

A joist is a horizontal structural member, typically made of dimensional lumber, engineered wood, or steel, that spans between two supports and carries the load of a floor or ceiling directly above or below it. Joists are installed in parallel rows at regular spacing, most commonly 12, 16, or 24 inches on center, with plywood or subfloor sheathing fastened across the top to create a continuous walking surface. Every floor and most ceilings in residential construction rely on a system of joists to transfer weight to the walls or beams below.

Sizing joists correctly is essential because an undersized joist will deflect excessively under normal use, creating a bouncy or springy floor feel, while a severely undersized joist can crack or fail under load. The Floor Joist Calculator takes span length, spacing, and expected load to recommend an appropriate joist size and species, following standard span tables used throughout residential construction.

Joists ultimately rest on and transfer their load to beams, walls, or foundation elements, making them one part of a larger Load-Bearing system in a building. Where a joist's span is long or its load is heavy, engineers check its expected sag using Beam Deflection principles to confirm it stays within acceptable limits, often expressed as a fraction of the span such as L/360.

• Spacing and span determine joist size together. A shorter span or tighter spacing allows a smaller joist, while a longer span or wider spacing requires a deeper or stronger joist, which is why the Floor Joist Calculator asks for both values.
• Joists transfer load to load-bearing supports. Every joist eventually rests on a wall, beam, or ledger that is part of the building's Load-Bearing path down to the foundation.
• Deflection limits keep floors feeling solid. Excessive joist sag under load is checked against Beam Deflection standards, commonly limited to span divided by 360 for floors.
• Notching and drilling weaken joists. Cutting a joist for plumbing or wiring removes material and reduces its load capacity, so building codes limit how much and where a joist can be notched or drilled.
• Engineered joists span farther than solid lumber. I-joists and laminated veneer lumber can span greater distances than same-depth solid wood joists, which is useful for open floor plans without adding interior support walls.