Bye-Bye, Bounce


Photo by Richard Howard

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. Tom recalls, “It was nerve-racking.” His father, a carpenter and builder, eventually went to the basement to reinforce the floor joists.

Tom’s home was built in 1765. However, bouncy floors don’t necessarily mean that the house is old. Even new, code-compliant floors that are structurally sound can deflect or flex more than is comfortable. There are many ways to stiffen a floor. The method Tom chooses depends on both practicality and 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.

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These are Tom’s top-rated solutions to bouncy floors. They are shown in order of effectiveness. To stiffen upper floors, you will need to remove some or all the ceilings below. Tom’s childhood kitchen is what inspired him to ensure that every new floor he constructs is solid. The following methods will help you fix your bouncy floors.

Photo by Richard Howard

4 Ways to Stop the Bounce

Sistering

Doubling the thickness of joists by adding material to their sides increases strength and stiffness. Tom attaches two pieces of lumber to the joists. If there is enough bounce, he might use engineered or laminated veneer lumber beams (LVL). A flitch beam is stronger. It uses a steel plate that is bolted between the old and new joists. For sistering I-joists see page 4. Pairs of 12d nails are used to fasten the new lumber to the old. Before putting in the new joist, Tom always applies a thin coat of adhesive to the top edge.

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Pros: This is the best way to preserve headroom.

Cons: Joist bays should be clear of obstructions such as plumbing and ductwork.

Stiffening the underside

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When a joist deflects, its bottom edge stretches slightly. To prevent this, add a 2×4 to its underside. To make this work, the 2×4 must run the full length the joist. Tom glues them together using construction adhesive. Tom then sinks a 12d nails through each 2×4 every 8 to12 inches. You can support the joist with a temporary 2×4 beam at the span’s midpoint until the adhesive cures, which usually takes 24 to 48 hours.

The pros: It won’t cause any interference with wiring or plumbing in joist bays.

Cons: This reduces headroom. The bottom edges of the joists must not be obstructed by wires, pipes, or ducts.

Adding mid-span blocking

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 adds a row to the joist bays in the middle of the span whenever it exceeds 9 feet. He places the blocking along a chalk line, so that he can drive three to four 16d nails through each block’s ends and the adjacent joist.

Pros: Very easy to do.

Cons: Has little effect on bounciness.

Adding a beam

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Placing a beam perpendicular to the joists at mid-span effectively shortens their length and eliminates flex. Tom creates the beam from two 2x8s, or 2x10s that are glued together and nailed together using 10d nails in an irregular pattern of 12 inches. Concrete-filled steel lally columns and 6×6 pressure treated posts will replace the temporary 4x4s. For beams of 2x8s, place permanent posts 8ft apart. If the beam is of 2x10s, spacing permanent posts 10ft apart.

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Pros: This is the most effective method of stopping bounce.

Cons: Beams can be intrusive and take up headroom. Although it is possible to “let-in” a beam flushing with the joists (but that’s a complicated project best handled by a contractor).

Photo by Richard Howard

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. For spans of up to 20 feet, building codes will specify minimum joist spacing and depth (usually 12 or 16 inches in center). However, those requirements are meant to prevent plaster ceilings cracking and not eliminate springy floors. Steve Frederickson is a registered professional engineer. He says, “If you build according to code minimums then you most likely will experience some bounce.” No matter how wide the floor is, it should not deflect more than one-half inches.

Tom will often go beyond code to accomplish this goal. Tom frames the room with deeper joists and spacing them closer together. He also uses bridging or blocking to increase stiffness. He says that the dining-room chandelier won’t move when the children are playing in the bedroom.

Photo by Richard Howard

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. However, I-joists can bounce if they are asked to span too far. I-joists can be used in the same way as solid-lumber floors. Tom uses plywood to stiffen the undersides of I-joists.

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For sistering, cut ¾-inch plywood into long strips the same width as the joist’s web. Adhere them to the web. You should ensure that you stagger the web’s end joints. Although it is more time-consuming and expensive, adding layers of plywood to the joist will make it stiffer.

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. To reduce the weight of the plywood, wedge 2x4s between it and the crawl-space or basement floor below. The adhesive will cure in about a day. Tom states, “Adding that extra layer makes all the difference.”

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