While the International Residential Code (IRC) specifies allowable clear spans for floor joists of varying dimension and on-center spacing, building to code minimum is unlikely to deliver a finished floor that feels solid underfoot and won’t rattle the China cabinet.

“Just because a span chart says something is going to work doesn’t mean it’s going to feel good,” says Art Lewis, an engineer for Weyerhaeuser Trus Joist Engineered Lumber in Charlotte, N.C. “If you max out the span and on-center spacing for a given joist, it probably won’t perform well even if it passes code.”

That’s true whether it’s an old-school, solid-sawn 2-by floor frame or one built using wood I-joists and other structural engineered lumber materials. “Floor deflection is both objective and subjective,” says Alan Mooney, president of Criterium Engineers, a national construction consulting firm based in Portland, Maine. “You can follow the span charts in the IRC, but you also have to figure out the owner’s expectations.”

Go Beyond Code. While difficult to quantify, a home-owner’s expectations for floor stiffness likely involves doing more than code minimum will deliver. Fortunately, the IRC and engineered lumber suppliers provide span tables that show builders and framers how to upgrade their floor joist specs to achieve a stiffer floor frame.

Put simply, increasing the joist’s depth, reducing on-center spacing between joists, and/or shortening the joist’s span across allowable supports at either end will take out most, if not all, of a floor frame’s deflection under presumed dead (the weight of the structure) and live (furniture and foot traffic) loads. “With a little extra framing material, you can get a 50% improvement in the floor’s deflection ratio” or stiffness, says Mooney. “The difference in materials and labor cost is nominal, but the difference in feel is significant.”

So, too, can be the effect on finishes—as well as a homeowner’s perception of overall quality. Soft floors can crack unforgiving surfaces such as tile floors, while second-floor foot traffic can damage plaster-coated ceilings. “The problem is with the structure, not the finishes,” says Mooney, resulting in a recurring cycle of surface cracking and cosmetic fixes.

Striking a Balance. The code minimum for raised floors in occupied living spaces is expressed as a deflection ratio of L/360, essentially a deflection of 0.4 inch across a 12-foot clear span, assuming proper end-bearing (12 feet equals 144 inches. Divide 144 by 360 and you get 0.4 inch). The code further prescribes a maximum live load of 40 pounds per square foot (PSF) and a dead load of either 10 or 20 PSF on those spans.

For even less deflection, look for span tables that chart an L/480 floor deflection ratio. That upgrade prescribes a 0.3 inch deflection across a 12-foot span.

Of course, many rooms are more than 12 feet across, so joist manufacturers’ tables tell you the maximum span lengths under certain circumstances. To take Weyerhaeuser’s Trus Joist span tables as an example, 17 feet, 2 inches is the maximum span with an L/360 deflection ratio allowed by code for a floor that’s framed with the company’s 9-½-inch deep (nominal 2x10) series 110 TJI joists, spaced 16 inches on center. For L/480 deflection, the maximum span drops to 15 feet, 6 inches.

Those maximum distances vary based on joist strength, depth, and on-center spacing. According to TJI’s span table, switching to deeper 11-7/8 inch joists (a nominal 2x12), even when spaced at 19.2 inches on-center, enable you to go as long as 17 feet, 4 inches and still achieve L/480. Alternatively, using 9-½-inch-deep joists spaced at 12 inches on-center enables you to go up to 17 feet, 9 inches at L/480. (There also are tables for spacing 24 inches on center, an advanced framing technique that creates larger insulation cavities, reduces the amount of lumber in a structure, and probably saves money on labor, if not materials.) Of course, correctly installing the joists—as well as other components of the floor system—is also critical to achieving desired floor performance.

Don’t Neglect the Deck. In addition to joist span tables and various options therein, a slightly thicker and properly installed subfloor also will have a positive impact on deflection.

The span table for Weyerhaeuser's engineered Trus Joist TJI joists exemplifies the myriad options available to achieve desired spans for codeminimum and upgraded floor performance.
The span table for Weyerhaeuser's engineered Trus Joist TJI joists exemplifies the myriad options available to achieve desired spans for codeminimum and upgraded floor performance.

“A 7/8-inch or 1?1/8-inch deck goes a long way to reduce deflection in a code-minimum floor,” says Weyerhaeuser’s Lewis. “It’s a more expensive panel, but if you can reduce the number of joists with wider spacing [per the span tables], you might balance the installed cost of that system.”

In addition to thickness, an approved construction adhesive will help better secure the subfloor to the joists and in the tongue-and-groove joints of abutting panels. (Tip: Use only enough glue for a few panels at a time to ensure adhesion.)

Also, builders can improve performance by staggering the subfloor joints so they don’t run along the same joist length and by fastening the panels to the floor frame in the code-prescribed pattern with ring-shank nails or screws. “It creates an integral system that reduces soft spots in the floor,” says Lewis. To wit, simply nailing the subfloor to TJI joists reduces their allowable span by 6 inches, a testament to the impact of an integral system on joist deflection and overall floor performance.

Such attention to detail also is likely to significantly reduce the potential for nails and metal connectors to work loose over time, resulting in floor squeaks.

“The fewer fasteners and hardware you have, the less chance there is for noise,” says Mooney. And, if blocking is required by code for longer spans to manage lateral loads and perhaps boost stiffness, they should be face-nailed to the joists.

“Keep it simple but do it well,” Mooney says.

He also notes that the younger trees used to produce lumber today need more time to properly reach an equilibrium moisture content before installation, if one hopes to reduce the shrinking and twisting that can also cause nails to lose their grip.

Test Your Limits. Most builders and framers, says Mooney, simply follow the code-minimum span tables out of convenience and habit—and perhaps an aversion to spending money for some extra engineering of their framing plans.

Dealers, however, can leverage software from engineered lumber manufacturers to virtually build various combinations of joists and subflooring that meet or exceed code. Even better, each scenario can be “scored” for performance and presented with a cost based on the dealer’s pricing structure—down to the last hanger or tube of glue—before issuing a purchase order.

Perhaps even more so than knowing how to read a span table, such electronic tools can establish a dealer as far more than a materials supplier when it comes helping builders and framers reduce callbacks and retain profits.