I-joists were developed as a lightweight alternative to rolled steel joists (RSJs) and conventional timber joists/beams for applications in construction, including floors, roofs and walls. The I-Joist is comprised of three components, namely two flanges with machined slots into which a shear web is adhesively bonded (see image). The flangers are made from either solid softwood (often CLS) or Laminated Veneer Lumber (LVL) and shear web is made from oriented strand board or particleboard or plywood. The web spaces the flanges, which provide flexural rigidity under load, and takes the associated shear loads. Holes, properly sized and located, may be drilled in the web to take services such as pipes and cables.
I-Joists possess a superior strength to weight ratio in comparison with solid wood. They will resist splitting, bowing and twisting and are dimensionally stable. As a result there is less noise emitted from the floor as it flexes. A wide-span roof constructed from structurally engineered I-Joists is seen in the image above. The spacing of joists depends on the span and load conditions and a range of flange widths may be selected to meet design requirements for deflection.