In by Mark Goodman

A manufactured structural building component resembling the letter I, it is used as a floor joist and rafter. I-joists include two key parts: flanges and webs. The flange of the I joist may be made of laminated veneer lumber or dimensional lumber, usually formed into a width of 1-1/2 inches. The web or center of the I-joist is commonly made of plywood or oriented strand board (OSB). Large holes can be cut in the web to accommodate duct work and plumbing waste lines. I-joists are available in lengths of up to 60 feet long.

I-joist (Wikipedia)
A partially constructed floor built with I-joists

An engineered wood joist, more commonly known as an I-joist, is a product designed to eliminate problems that occur with conventional wood joists. Invented in 1969, the I-joist is an engineered wood product that has great strength in relation to its size and weight. The biggest notable difference from dimensional lumber is that the I-joist carries heavy loads with less lumber than a dimensional solid wood joist. As of 2005, approximately 50% of all wood light framed floors used I-joists. I-joists were designed to help eliminate typical problems that come with using solid lumber as joists.

The advantage of I-joists is they are less likely to bow, crown, twist, cup, check or split as would a dimensional piece of lumber. I-joists' dimensional soundness and little or no shrinkage help eliminate squeaky floors. The disadvantage is very rapid structural failure during a fire, reducing the time available for residents to escape, and increasing the danger to firefighters.

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