I-joists are measured according to the length and depth of the joist itself; the measurement does not take the depth of the joist's flanges into account. Residential I-joists are typically sold in 9.5, 11.78, 14 and 16 inch depths, while commercial I-joists come in depths of up to 32 inches.
There is no standard length measurement for I-joists, because the length varies based on the dimensions of the ceiling or floor the joists are being used to build. I-joists are available in lengths far above those of traditional lumber joists. I-joists can come in lengths of as little as a few feet to extremely long lengths of 70 feet for industrial or commercial applications.
Flanges for I-joists are available in a number of materials and sizes based on the builder's needs. Laminated veneer lumber, or LVL, is a common engineered wood material for I-joist flanges. LVL flanges are usually sold in widths and depths of 1 1/2 inches. However, most I-joists are manufactured with traditional lumber flanges. Lumber flanges are weaker than engineered wood flanges, so they require larger sizes. These flanges are typically two inches wide by either three or four inches deep. While LVL flanges are smaller and stronger than solid wood, they are also more expensive than traditional lumber flanges.