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 2x4 to the underside helps keep it from doing this. For this method to work, the 2x4 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 2x4 every 8 to 12 inches. Use a temporary 2x4 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 6x6 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.)