Beefing Up I-joists

These days, many new floors are framed with I-joists, a type of engineered lumber that's a fraction of the weight of conventional lumber and capable of spanning greater distances (image 6, left). Still, if asked to span too great a distance, I-joists will bounce. As a rule, the same kinds of methods that take the bounce out of solid-lumber floors work for I-joists. But when Tom sisters or stiffens the underside of I-joists, he uses plywood.

For sistering, cut ¾-inch plywood into long strips the same width as the joist's web. Glue them to both sides of the web and nail with 4d or 6d nails. Make sure to stagger the end joints on either side of the web. The more layers of plywood, the stiffer the joist, but it's time-consuming and costly.

A quicker and cheaper solution is to attach full sheets of ¾-inch plywood to the bottom of the joists, creating what Tom calls a "giant, monolithic box beam." Starting at mid-span, apply construction adhesive to the bottom edges of the joists and fasten the plywood sheets—long edge perpendicular to the joists—with 8d ring-shank nails or 1¾-inch screws. Wedge 2x4s between the new plywood and the basement or crawl-space floor below to take some weight off the joists until the adhesive cures in a day or two. "Adding that extra layer makes a big difference," Tom says.

Finally, many pros have praised Luxor's IBS2000, an interlocking bridge system sized especially to fit between the flanges of I-joists for quick added stiffness.

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