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Construction glossary · Wood framing

What is joistin construction?

A joist is a horizontal framing member, usually 2x lumber or an engineered member, that supports a floor or ceiling by spanning between bearing walls or beams. Floor joists are sized by their span and spaced at 16 in or 24 in on center to carry the subfloor and the loads above.

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Updated June 2026 · Reviewed by the Ruh construction team

Common spacing 16 in. o.c.Count rule (span ft x 0.75) + 1Typical size 2x8 to 2x12Live load 40 psf residential

Floor joists at 16 in. on center

16 in. o.c.Across floor spanHorizontal floor member
The definition

Joist, explained.

A floor joist is one of the repetitive horizontal members that frame a floor, running between supports such as foundation walls, bearing walls, beams, or girders. In light-frame work joists are commonly 2x8, 2x10, or 2x12 dimensional lumber, or engineered members such as I-joists where longer clear spans or consistent depth are needed. The joist depth and the lumber grade and species set how far it can span for a given spacing and load: a 2x10 spans farther than a 2x8 at the same spacing, and tighter spacing increases capacity. Spacing is typically 16 in on center, with 24 in OC used where the depth, grade, and subfloor allow it. The joists carry the subfloor sheathing above and any walls and live load it supports. Ends bear on plates or hangers, a rim joist or band joist closes the ends, and blocking or bridging is added at intervals and under parallel partitions to keep the joists from rolling and to share load. Count and span come straight off the floor framing plan.

How it is measured

On the floor framing plan, find the joist direction, the span, the size, and the OC spacing. Joists run perpendicular to their span, so count them along the wall that runs parallel to the span direction: that wall length in feet times 12, divided by the spacing, plus one. Joist length equals the span plus required bearing at each end, then rounded up to a stock length. Add doubled joists under parallel partitions and at openings, plus the rim or band joist around the perimeter and the blocking or bridging rows. Report joists as a size, length, and piece count, with rim and blocking listed separately. Confirm the size and span against the structural plan rather than scaling off the drawing.

Worked example

A worked joist example.

Worked example

Take a 24 ft by 14 ft room with 2x10 joists at 16 in OC spanning the 14 ft direction. The joists run across the 14 ft span, so they are spaced along the 24 ft wall: 24 ft x 12 = 288 in, divided by 16 = 18 spaces, plus 1 = 19 joists. Add a doubled joist under one parallel partition: +1 = 20 joists. Each joist spans 14 ft plus bearing, so a 16 ft 2x10 covers it with room for bearing and trim. Illustrative US pricing at $28 per 2x10x16: 20 joists x $28 = $560 for the field joists, before the rim joist, blocking, and subfloor. Confirm the 2x10 carries 14 ft at 16 in OC against the span table or structural plan.

How Ruh handles it

How Ruh handles joist.

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Joist: frequently asked questions

How do I count floor joists?+

Take the length of the wall that runs parallel to the joist span, in feet, times 12, divided by the OC spacing, plus one. Then add doubled joists under parallel partitions and at openings, plus the rim joist and any blocking rows.

Which direction do floor joists run?+

Joists run across their span, perpendicular to the supports they bear on, so they span the shorter distance between bearing walls or beams. The floor framing plan shows the direction with a span arrow or joist symbol; do not assume it from the room shape.

How far can a 2x10 floor joist span?+

It depends on the spacing, the lumber species and grade, and the load. As a rule a 2x10 spans farther than a 2x8 and tighter spacing spans farther than wider. Always confirm the allowed span against the applicable span table or the structural plan rather than a rule of thumb.

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Related terms

TrussI-joistRafterGlulam (glued laminated timber)SubfloorSleeperBlockingRim joist (band joist)

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