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For homeowners

Types of floor joists: which one fits your floor?

Choose Sawn lumber joist for floor spans of typically 12 to 16 ft (USDA Forest Service Wood Handbook); Wood I-joist for running electrical or small plumbing lines through prescored knockouts in the web (Roseburg, one maker's I-joists); Open-web floor truss for a floor framed to increase rigidity and span, eliminating some interior load-bearing walls (HUD Residential Structural Design Guide).

The right choice depends on the span between supports, whether pipes, wiring and ducts have to pass through the joists, and whether the framing stays dry and clear of concrete or masonry.

How we know: By InstaBids editorial team with AI research agents. · Sources read: 22 October 2026 · Updated 27 October 2026

The USDA Forest Service Wood Handbook names the floor joists commonly used: lumber, prefabricated wood I-joists, or parallel chord trusses.
TypeBest forMain advantagesMain drawbacks
Sawn lumber joistfloor spans of typically 12 to 16 ft (USDA Forest Service Wood Handbook)readily available tables in residential building codes give maximum allowable spans by species, grade, size and spacing (HUD Residential Structural Design Guide)cutting framing members to install plumbing lines and heating ducts should be minimized (USDA Forest Service Wood Handbook); bridging may be needed for 2x10 and 2x12 joists with clear spans past about 16 and 18 feet (HUD Residential Structural Design Guide); at clear spans over 15 feet, the code span tables' usual deflection limit has caused nuisance vibrations (HUD Residential Structural Design Guide)
Wood I-joistrunning electrical or small plumbing lines through prescored knockouts in the web (Roseburg, one maker's I-joists)the USDA Forest Service Wood Handbook says their "longer span capabilities may eliminate the need for interior supports"; one maker (Roseburg) does not require mid-span blocking or bridging for its I-joists in floorsto be supported by the manufacturer, they must be installed exactly to the manufacturer's instructions (HUD Residential Structural Design Guide); except for cutting to length, the flanges are never cut, drilled or notched (Roseburg, one maker); never installed where permanently exposed to weather or in direct contact with concrete or masonry (Roseburg, one maker)
Open-web floor trussa floor framed to increase rigidity and span, eliminating some interior load-bearing walls (HUD Residential Structural Design Guide)the web areas are designed for easy passing of plumbing, electrical and heating ducts (USDA Forest Service Wood Handbook); strongbacks installed through the trusses near the center of the span are effective in reducing floor vibration (HUD Residential Structural Design Guide)to be supported by the manufacturer, they must be installed exactly to the manufacturer's instructions (HUD Residential Structural Design Guide); where there is evidence of moisture, the metal plate connectors need checking for corrosion and loss of embedment (HUD inspection guide, 2000)

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If your priority is...
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floor spans of about 12 to 16 ftSawn lumber joist
a floor sized from the span tables in residential building codesSawn lumber joist
leaving out some interior load-bearing wallsOpen-web floor truss

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