Whenever anyone walked across the kitchen floor in the home where This Old House general contractor Tom Silva grew up, teacups trembled in their saucers and pots and pans rattled on the stove. “It was nerve-racking,” Tom recalls. Eventually his father, a carpenter, went down to the basement with Tom in tow and reinforced the undersized floor joists.
Tom’s house was built in 1765, but bouncy floors aren’t just an old-house problem. Even structurally sound, code-compliant new floors can deflect, or flex, more than feels comfortable. There are a variety of ways to stiffen a floor; the method (or methods) Tom chooses depend as much on practicality as on effectiveness. For example, sistering with new lumber—the fix Tom’s dad used—makes sense only if the joist bays are not cluttered with electrical cable, plumbing lines, and ducts.
Tom’s solutions, shown here in order of effectiveness, are easiest on the first floor, where joists are accessible from the basement or crawl space. Stiffening upper floors typically requires removing some or all of the finished ceiling below. Remembering his boyhood kitchen, Tom makes sure that any new floor he builds is rock-solid.
4 Ways to Stop the Bounce
Sistering (image 1, left)
Doubling the thickness of joists by adding material to their sides increases strength and stiffness. For joists made of sawn lumber, shown here, Tom attaches a 2x of the same length and width; if the bounce is severe enough, he may use an engineered or laminated veneer lumber (LVL) beam. Still stronger is a flitch beam, which uses a steel plate bolted between the old joist and its new “sister.” (For sistering I-joists, see page 4 of this article.) Fasten new lumber to old using pairs of 12d nails every 16 inches. Tom always runs a bead of adhesive along the top edge before putting the new joist in place.
Pros: The best choice for preserving headroom.
Cons: The joist bays must be free of any obstructions, such as electrical cables, ductwork, and plumbing, while you’re doing the work.
Stiffening the underside(image 2, left)
When a joist deflects, its bottom edge stretches slightly. Adding a 2×4 to the underside helps keep it from doing this. For this method to work, the 2×4 must be long enough to run the full length of the joist. Tom bonds the two together with construction adhesive and sinks a 12d nail up through the 2×4 every 8 to 12 inches. Use a temporary 2×4 post at the midpoint of the span to support the joist until the adhesive cures (usually in 24 to 48 hours).
Pros: Doesn’t interfere with plumbing or wiring in joist bays.
Cons: Reduces headroom; bottom edges of joists must be
free of wires, pipes, and ducts.
Adding mid-span blocking(image 3, left)
Blocking, short pieces of 2x stock the same depth as the joists, stops sideways deflection and ties the joists together so they can effectively share floor loads. Tom installs a row of blocking in the joist bays at mid-span whenever a span exceeds 9 feet. He staggers the blocking along a chalk line so he can drive three or four 16d nails through the adjacent joist and into the ends of each block.
Pros: Relatively easy to do.
Cons: Has the least effect on bounciness.
Adding a beam(image 4, left)
Placing a beam perpendicular to the joists at mid-span effectively shortens their length and eliminates flex. Tom makes the beam out of two 2x8s or 2x10s glued and nailed together with 10d nails in a staggered pattern 12 inches on center. Concrete-filled steel lally columns or 6×6 pressure-treated posts will replace the temporary 2x4s shown here. Space permanent posts 8 feet apart if the beam is made of 2x8s; 10 feet apart if made of 2x10s.
Pros: The most effective way of stopping bounce.
Cons: Posts are intrusive, and the beam eats up some headroom. (It’s possible to “let in” a beam flush with the joists, but that’s a challenging project best tackled by a contractor.)
Building Without Bounce
The best way to limit annoying flex in a floor is to make sure that joists are sized correctly before a house or addition is built. Building codes specify the minimum joist depth and spacing (typically 12 or 16 inches on center) for spans up to 20 feet, but those requirements are intended to prevent plaster ceilings from cracking, not to eliminate springy floors. “If you build to code minimums, you most likely will have some kind of bounce,” says Steve Frederickson, a registered professional engineer. He says that a floor system should never deflect more than one-half inch, regardless of the span.
Tom often goes well beyond code to meet that goal (image 5, left), framing with deeper joists, spacing them closer together, and using bridging or blocking for added stiffness. That way, he says, “the dining- room chandelier doesn’t sway when the kids are jumping up and down in the bedroom.”
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.