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Sisters for Bouncy Joists

Q: "My floors are supported by 2x10 joists. How do stiffen them to make the floors less bouncy?"


My floors are supported by 2x10 joists, 16 inches on center, above a 3-foot crawl space. I'd like to stiffen them to make the floors less bouncy. I was wondering if attaching smaller 2x8s or 2x6s would work. Trying to maneuver 2x10s into the crawl space won't be much fun.


Tom Silva replies: I can't think of anything that's fun to do in a crawl space. But in this case, I'd "sister" the sides of the joists with ¾-inch plywood rather than with solid-wood members. Plywood will be a lot easier than 2x10s and more effective than 2x8s or smaller stock.

Cut the plywood into 8-foot strips about ¼ inch narrower than the depth of the existing joists, so they'll fit easily between the floor and mudsill. Then, using polyurethane construction adhesive and 6d common nails spaced 8 inches on center along each edge, nail and glue the strips to one side of each joist along its entire length. You might want to rent a nailgun to keep your time in the crawl space to a minimum.

You should notice an improvement, but if the floors are still too bouncy after you've lived with them for a couple of days, glue and nail a second layer of plywood to each joist, either onto the first layer or to the joist's other side. Either way, make sure to offset the vertical seams by at least a couple of feet.


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