Manufactured structural building component resembling the letter 'I'. Used as floor joists and rafters. I-joists include two key parts: flanges and webs. The flange or from of the I joist may be made of laminated veneer lumber or dimensional lumber, usually formed into a 1 ½' width. The web or center of the I-joist is commonly made of plywood or o... Found on http://www.homebuildingmanual.com/Glossary.htm