BDCS - I-Joist

Typical I-Joist Construction on a Flooring System

(From Wikipedia) An I-Joist (or an engineered wood joist) is a product designed to reduce many of the problems that occur with using conventional wood joists. It is a manmade engineered wood product which has incredible strength and durability 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. Approximately 50% of all wood light framed floors in the United States are framed using I-joists.

An I-joist comprises 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 water-resistant glue by pressing 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, or 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 (145mm - 600mm) and are available up to 80 feet long, although 12-13m is more common. The intended use for an I-joist is for floor joists, wall studs and roof rafters in both residential and commercial construction.

I-Joists Used for the Construction of a Ceiling

I-joists will not bow, crown, twist, cup, check, or split as would a dimensional piece of lumber. Also, I-joists are designed to help eliminate squeaky floors by being more dimensionally sound and featuring 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 too fast. Both can lead to an uneven or squeaky floor.

Below is an example of how I-joists are typically framed into floors (which they are used mostly for).  I-joists can also be used for walls and lateral bracing systems.