I-Joist
An I-joist is an engineered wood floor or roof joist shaped like the letter "I" in cross-section, with top and bottom flanges made of solid lumber or LVL connected by a vertical web of OSB or plywood. I-joists are lighter, straighter, and span farther than dimensional lumber joists.
In Detail
The I-shape is structurally efficient because it places material where it does the most work: the flanges resist bending forces (tension on the bottom, compression on the top) while the web resists shear forces and holds the flanges apart. This means an I-joist uses less total wood material than a solid member of the same span capacity.
I-joist flanges are typically made from machine stress-rated (MSR) lumber or LVL, depending on the manufacturer and performance level. The web is made from oriented strand board (OSB) that is glued into a routed groove in each flange. The web-to-flange joint is the critical connection and is engineered to transfer shear forces reliably over the life of the structure.
Standard I-joist depths are 9-1/2", 11-7/8", 14", 16", and 20". Deeper joists span farther and deflect less. I-joists are manufactured in lengths up to 60 feet, allowing clear spans that would be impossible with dimensional lumber. They are the dominant floor framing member in production residential construction.
Related Terms
Laminated Veneer Lumber (LVL)
Laminated Veneer Lumber (LVL) is an engineered wood product made by bonding thin wood veneers together with all grain running parallel, creating a beam material that is stronger, straighter, and more dimensionally stable than solid sawn lumber of the same size.
Rim Board
A rim board (also called band board or rim joist) is the structural member installed around the perimeter of a floor frame, standing on edge on top of the wall below and closing off the ends of the floor joists. Rim board transfers vertical and lateral loads between floors and provides a nailing surface for exterior sheathing.
Deflection Limit
A deflection limit is the maximum amount a structural member (beam, joist, or rafter) is allowed to bend under load, expressed as a fraction of the span length. Common limits are L/360 for floors (1 inch of deflection per 360 inches of span) and L/240 for roofs. Deflection limits prevent bouncy floors and cracked finishes.
Related Materials
I-Joists (Engineered Wood Joists)
I-joists are engineered wood floor and roof framing members with an I-shaped cross section — LVL or solid lumber flanges bonded to an OSB or plywood web. They are the dominant floor framing material in new residential construction, offering long spans, dimensional stability, and lightweight handling.
LVL (Laminated Veneer Lumber)
Laminated veneer lumber (LVL) is a high-strength engineered wood product made from thin wood veneers bonded with structural adhesive under heat and pressure. It is the most commonly specified engineered beam and header material in residential and light commercial construction.