I Joist selection for large span

I Joist selection for large span

I have an addition planned for a homeowner who specifically wants I-joists. I've received conflicting advice from my lumberyard and the big box stores regarding the appropriate joist size.

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The big box stores are recommending Boise Cascade I-joists. The homeowner desires a floor that feels as solid as possible with minimal deflection.

The structure in question measures 20ft 6in long by 28ft wide. I plan to space the I-joists 12 inches on center across the width of 28ft, totaling 26 I-joists. I intend to use 11 7/8 inch I-joists, spanning 26ft 6in, including the foundation wall, outer edge to outer edge.

The homeowner plans to install carpet, possibly wood flooring, and a large aquarium along with standard furniture and chairs in this sunroom. The upstairs floor will mirror the first, comprising a master bedroom with typical bedroom furnishings, such as a heavy bed set.

I've been told that a BCI 90, with a 3.5 inch top/bottom cord, would suffice. Others suggest a BCI minimally or even a BCI with various web stiffeners. The opinions vary significantly, with one source claiming the manufacturer mandates a 3.5-inch cord. This confusion leads to a considerable price difference, nearly double, between BCI and BCI 90.

Does anyone have a definitive answer on this issue? One source claims the manufacturer insists on a 3.5-inch cord, and I'm unsure who to trust.

If at first you don't succeed, try using a hammer next time; sometimes, everything needs a little extra persuasion. -ME

Floor Joist Spacing

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What Is Floor Joist Span and Spacing?

Floor joists are structural elements used in constructing floors, spanning open spaces between load-bearing walls and beams. They run parallel and equidistant from each other, connecting to vertical members like studs and support beams, thus distributing weight across the structure.

Individual joists must support the building's anticipated load over time. Choosing the correct joist depth for a project is crucial, as it ensures critical stability for the floor or ceiling system.

Joists are typically spaced one or two feet apart. The necessary joist depth increases as the spacing between joists grows, which helps reduce stress and prevents deflection over time. A common sizing rule for joist depth is to take half the intended span in feet, add two, and use that number in inches. For instance, a 12-foot span would require an 8-inch joist:

12 ft. / 2 = 6

6 + 2 = 8-inch joist depth

Blocking the gaps between joists with smaller wood pieces can decrease stress and enhance overall floor framing stability. A floor frame gains strength if constructed from engineered wood joists rather than standard lumber. Engineered and prefabricated options, like I-joists, offer increased depth and are manufactured from higher quality materials.

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