I-joists, an engineered lumber product for use as floor joists, ceiling joists, rafters and in other applications, have proven to be one of the most significant developments in the construction field in the last several decades. Strong, light, consistent and reasonably priced, I-joists have all but replaced solid-sawn lumber in most joist applications in today’s homes.
I-joists, however, have some different characteristics from solid lumber, and as such they need to be handled carefully during construction. Failure to properly install and brace the joists as they are being installed can result in serious injury, as well as damage to the joists and other structural members. So whether you’ve been framing with them for years or are just getting ready for your first installation, it’s important that you understand proper handling procedures.
An I-joist, like a steel I-beam, gets its strength from the combination of the vertical center section, called the web, and its connection with the upper and lower horizontal flanges. Viewed from the end, with the web upright, the joist resembles a capital letter “I,” and it is in this orientation that it has its strength. When the joist is laid on its side, with the center web horizontal so that it resembles an elongated letter “H,” it actually has very little strength and will bow quite easily between supports. For this reason, I-joists always need to be vertical. You can damage the joist if you store it horizontally, and you should NEVER lay one horizontally to use as a walkway or a ramp.
I-joists have a relatively narrow top and bottom flange, especially in relation to the height of the center web. As such, even as the I-joists are installed they are very wobbly from side to side until they are braced. You should NEVER walk on the tops of unbraced I-joists, and you should NEVER place a load on them. Either activity can easily cause the joists to tip over to the side, and once they do that and end up laying horizontally, they lose all their structural strength and send you and the load straight down to the ground.
As the joists are installed, begin bracing them immediately, using 1×4 lumber nailed across the tops of the joists. Remember that the 1x4s will have very little bracing effect unless they are firmly attached to something at one end, so start at a solid point such as a rim joist at the outer perimeter of the floor, a girder or ridge beam, or other solid and well-braced structural member.
Keep the 1x4s perpendicular to the run of the joists, with a couple laid at a 45-degree angle near the ends of the joists for additional lateral bracing. Keep the runs of 1×4 bracing about six feet apart, overlap the ends at least one joist space, and attach them to the tops of each joist with two 8d nails. The same is true for bracing I-joists when they are used as rafters.
Leave the 1×4 bracing in place as you begin installing the subfloor. As each run of subfloor approaches the bracing, remove the bracing by pulling the nails up through the lumber from above. Be careful not to damage the tops of the I-joists by trying to knock the bracing lumber off with a hammer.
In addition to the temporary bracing and permanent subfloor, I-joists may also utilize intermediate blocking, metal strapping, laminated or solid lumber rim joists, and other components as part of a carefully engineered structural package. Your I-joists will come with a set of instructions and specific details on assembly, bracing and blocking, so be sure to study the manufacturer’s detail drawings very carefully before and during framing operations. If you have any questions about how to install and brace the joists properly, contact either your lumber yard or the I-joist manufacturer before proceeding.
Remodeling and repair questions? E-mail Paul at firstname.lastname@example.org.