I-joist

= I-Joist =

An engineered wood joist or more commonly known as an “I-joist” is a product designed to eliminate many of the problems that occur with using conventional wood joists. Invented in 1969, the I-joist is a man made engineered wood product which has incredible strength in relation to its size and weight. The biggest notable difference, other than their look, compared to dimensional lumber is that the I-joist is designed to carry heavy loads over long distances while using less lumber than a dimensional solid wood joist of a size necessary to do the same task (Vogt). As of 2005, approximately 50% of all wood light framed floors were framed using I-joists (Allen).

Design and Manufacture

An I-joist is comprised of two main parts, the web and flange. The web is sandwiched between a top and bottom flange, creating the “I” shape. The flange can be made from laminated veneer lumber or solid wood finger-jointed together for ultimate strength. It is then grooved on one side to receive the web. The web is typically made from plywood, laminated veneer lumber, or oriented strand board. After cutting the webs and flanges to the specified widths and lengths, they are assembled with waterproof glue by pressuring the web into the top and bottom flange. After initial assembly, the I-joist is then end-trimmed and allowed to cure in an oven at room temperature to approximately equilibrium moisture content. The sizes manufactured vary on the I-joists intended load and span. Their depths can range from 9 ¼ inches to 24 inches and are available up to 80 feet long. The intended use for an I-joist is for floor joists and roof rafters in both residential and commercial construction (Vogt).

Installation

I-joists were designed to help eliminate typical problems that come with using solid lumber as joists. The beauty in I-joists is they will not bow, crown, twist, cup, check, or spilt as would a dimensional piece of lumber might. Also, I-joists are designed to help eliminate squeaky floors by being more dimensionally sound and little to no shrinking. Although the I-joist may seem like a perfect choice as a building material, if it is installed incorrectly or altered the I-joist can fail. By reading the manufacturer’s instructions with the I-joist, most mistakes can be caught in advance. The most common mistake typically is with subcontractors misplacing or improperly sizing holes in the web. Doing so can greatly compromise the strength of the joist and the ones around it potentially leading to structural failure in those joists. Some of common mistakes made with installing I-joists are: cutting or chiseling flange in any way, improperly sized joists hangers, improper nailing into rim joist and joist hangers, and wrong sized nails. Another situation to look out for is matching the rim joist depth to the I-joist size. Improper matching can lead to too much strain on the I-joist. To solve the problem the rim joist can be made from rips of ¾ inch plywood, or engineered rim joist matching the I-joist in depth. A similar situation occurs where the I-joist crosses a main beam. Installing squash blocks (2x4 materials 1/16 inch higher than the I-joist) alongside the I-joists helps take the load of the above levels off the I-joist themselves and onto the main beam. Problems that arise while installing the sheathing are missed nails and having the glue set to fast. Both can lead to an uneven or squeaky floor(Eck).

References

Allen, Edward, and Joseph Iano. Fundamentals of Building Construction. Vol. 4th. Hoboken, NJ: John Wiley & Sons Inc, 2004.

Vogt, Floyd. Carpentry. 4th ed. Clfiton Park, NY: Thompson Delmar Learning, 2001.

Eck, Curtis. “Wood I-Joist Do’s and Don’ts.” Journal of Light Construction (Sept. 1995).