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Stiffening Engineered Floor Joists

Q: "How can I stiffen my floor joists?"

Tom Silva stiffening i-joists before installing a floor
Photo by Kolin Smith

The I-joists in my two-year-old house don't seem very strong. When I use the treadmill in one bedroom, the floor flexes so much that all the dresser drawers open by themselves. How can I fix this? —Tom Deiters, Cartersville, GA.


Tom Silva replies: It isn't necessarily a lack of strength that's causing this problem—I-joists are plenty strong—so you shouldn't have to worry about your floor caving in. More likely they just need to be made stiffer. This won't be too difficult if the joists are exposed and accessible from the basement or a crawl space, but if they are hidden by a ceiling in the room below, then you'll have to cut open some of the ceiling to expose the framing.

First, check with a local building official to make sure your I-joists were designed and ­installed properly according to your local codes. If they're not up to snuff, you'll need a contractor to make the necessary fixes. But if they were installed correctly you can stiffen them yourself in the following way.

Rip a sheet of 5⁄8-inch plywood lengthwise into strips as wide as the height of the I-joists. Fasten these strips to both sides of the four or five joists closest to your treadmill using construction ­adhesive and 6d nails. Drive the nails every 6 inches into the thick flanges at the top and bottom of the joists. Offset the nails on each side by 3 inches. Stagger the plywood joints on each side as well. If your joists' flanges are made of laminated veneers instead of sawn lumber, nail into them at an upward angle to prevent splitting.

Now give your treadmill a try and see what happens. If the floor is still too bouncy, add plywood to more joists. And if that doesn't do the trick, try one or more of the other methods shown in the I-joist floor-vibration retrofit form at APA-The Engineered Wood Association


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