An I-joist is a type of engineered wood product used in construction, designed to provide strength and stability for floors, ceilings, and roofs. It has an "I"-shaped cross-section, consisting of two flanges (typically made of solid wood or laminated veneer lumber) joined by a web (often made of oriented strand board or plywood). This design allows I-joists to be lightweight, cost-effective, and resistant to bending or warping, making them an efficient alternative to traditional solid wood joists.