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Construction glossary · Wood framing

What is i-joistin construction?

An I-joist is an engineered floor or roof framing member shaped like a capital I, with solid sawn or LVL flanges top and bottom and a thin OSB or plywood web between them. In framing it replaces dimensional joists for longer, straighter, lighter floor spans with consistent depth and no crown.

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Updated June 2026 · Reviewed by the Ruh construction team

Depths 9.5 to 16 inWeb 3/8 in OSBSpacing 16 or 19.2 in ocFloor spans Up to 24 ft

I-joist section, flange-web-flange profile

Top flange, LVL or solidOSB web, glued in grooveBottom flange takes tensionI-shape resists bending3/8 in OSB web9-1/2 to 16 in deep
The definition

I-joist, explained.

An I-joist combines two flanges and a vertical web glued into an I cross section that puts material where bending stress is highest and keeps the member light. Common depths are 9.5, 11.875, 14, and 16 in, and they are spaced at 12, 16, 19.2, or 24 in on center depending on span and load. They run long and straight, which is why they dominate residential and light commercial floor systems. The web is thin, so manufacturer rules govern where and how large you can cut holes for plumbing and HVAC, and web stiffeners are required at certain bearings and point loads. I-joists need rim board or blocking panels at the ends to transfer load and prevent rollover, and squash blocks where posts bear above. They bear on top plates or hangers and are not interchangeable with sawn lumber spans. For estimating, I-joists are counted by piece at a given depth and length, with rim board, hangers, web stiffeners, and blocking panels taken separately.

How it is measured

I-joists are taken off the floor framing plan by reading the called-out depth, series, spacing, and direction of span. Count pieces by dividing the framed width perpendicular to the joists by the on-center spacing in feet, then add one for the starting joist. Pull each joist length from the plan dimensions plus required bearing. Read the framing notes for web stiffener locations, blocking panel or rim board at the perimeter, and hole-cutting limits, since those are separate quantities. Note doubled joists under parallel walls and around stair and floor openings. Lengths and depths drive pricing, which is per piece or per linear foot by depth.

Worked example

A worked i-joist example.

Worked example

A floor uses I-joists at 19.2 in on center spanning 26 ft, in a bay 24 ft wide measured perpendicular to the joists. Convert spacing to feet: 19.2 in / 12 = 1.6 ft. Joist count across the bay is 24 ft / 1.6 ft = 15 spaces, plus one for the first joist = 16 joists. Each joist is 26 ft long plus bearing, so order 26 ft members; total joist material is 16 x 26 ft = 416 linear feet at the called-out depth. At an illustrative US price of $3.10 per linear foot, the joists are 416 lf x $3.10 = $1,289.60. Add a doubled joist under a parallel partition, raising the count to 17 and material to 17 x 26 = 442 lf x $3.10 = $1,370.20. The estimator confirms depth, series, and span against the framing plan before ordering rim board and hangers.

How Ruh handles it

How Ruh handles i-joist.

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I-joist: frequently asked questions

What spacing do I-joists use?+

Common spacings are 12, 16, 19.2, and 24 in on center, chosen from the manufacturer span table for the depth, series, and load. The 19.2 in spacing gives five joists per 8 ft, which suits many floors and trims piece count versus 16 in.

Can I cut holes in an I-joist web?+

Yes, but only within the manufacturer hole chart, which sets allowable size and location along the span and keeps a clear zone near bearings. Never cut or notch the flanges. The hole rules ship with the product and must be on site.

Why use I-joists instead of dimensional lumber?+

I-joists span farther, run straighter with no crown, weigh less, and come in consistent depths, which speeds layout and reduces squeaks. They cost more per piece but often pencil out on longer spans where 2x sawn joists would be undersized.

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Related terms

TrussJoistLVLSubfloorRim joist (band joist)

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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.

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