Q. Can I-joists be used as headers in walls?
A. Jim Anderson, P.E., with the Product Engineering, Codes and Standards group at Weyerhaeuser, responds: With careful design, detailing, and installation, I-joists could possibly be used for headers, but the practice is not recommended. I-joists are commonly used in floor systems today. They are strong, stable, and available in long lengths, and they produce stable floors. They are specifically designed to efficiently support typical floor loads. At first glance, I-joists may look attractive as low-cost, lightweight, stable members that could replace dimension lumber in wall-header applications. However, special detailing, design, and connections may make using them more laborious and expensive than using a sawn lumber or structural composite lumber (SCL) header.
The unique cross section of an I-joist utilizes LVL or Machine Stress Rated (MSR) lumber flanges to resist high bending forces that occur in the middle of the span (for uniform loads). At the ends of the member, reaction forces need to be transferred into the support through an OSB web. I-joists work well for the transfer of reaction loads in typical floors, but they are not designed to support heavy point loads from a roof girder truss above. In addition, the reaction capacities for I-joists were developed and tested with a minimum 1 3/4-inch end bearing length. This minimum required bearing length exceeds a typical 1 1/2-inch-thick jack stud used to support wall headers. Therefore, if an I-joist were used as a header, each end would require a minimum of two jack studs, which may not be desirable.
I-joists, which typically support a uniform load, can also support point loads from above, such as a truss reaction; however, if the load is too high, the flange may crush the web. There are published provisions that if point loads are 1,500 pounds or more, web stiffeners are required to reinforce the top flange. Accurate placement of web stiffeners could prove difficult if truss placement locations are unknown or if they are different from what was assumed.
Under most conditions for wall headers, multiple-ply I-joists would be required to support the load. Having two or more members support the load will increase capacity, but special detailing would be needed to connect the two I-joists together, increasing construction costs. The detail would require filler blocks between the webs of each of the I-joists. Depending on the series of I-joist (different width of flanges), the filler may be a 2-by or a combination of a 2-by and sheathing. The filler would have to be the full length of the joist and be properly nailed together. If the multiple header is flush to the exterior sheathing or to the drywall inside, you may need to install an additional filler to flush out the surface for gypsum attachment.
End connections would be difficult as well. Typical prescriptive nailing for the end of a header to a stud is four nails. It would not be possible to get this quantity of fasteners into the end of an I-joist. Finally, lateral stability (buckling to the side under load) of the top edge needs to be addressed. This is true of all structural members, but I-joists typically require more frequent bracing than a solid piece of wood. Typically, in a floor system, this is achieved with the floor sheathing attached every 6 to 12 inches.
Because of the lower capacities, bearing length requirements, web stiffener requirement at point loads, and connection of multiple pieces, premade loading charts are not usually available for this application. If you are looking for the benefits of engineered lumber, using shallower (less than 9 1/2 inches) structural composite lumber is easier to design and install for wall headers. If you still desire to use I-joists in a header application in a wall, I would contact the manufacturer or a design professional for assistance.