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To stiffen a deck, you have to be able to get to the framing underneath. You can add another beam, along with posts, to support the joists. However, this is a big job. We recommend that you first add rows of solid blocking every 3 to 4 ft. along the span. Run the first row down the middle of the span, check the deck for bounce, then add rows to further reduce it.
Photo 1. Snap lines for blocks every 3 to 4 ft. along the joist span. Measure and cut the blocks to fit tightly. Tap them into place in a staggered pattern.
Use treated lumber blocking that’s the same size as the joists (usually 2x8 or 2x10). Install the blocking in rows along a chalk line snapped at a right angle to the joists. You’ll have to measure and cut each block separately to get a snug fit, since the joists are never exactly the same distance apart. Staggering the blocking in a step pattern allows you to easily drive nails from both sides, rather than having to toenail.
Photo 2. Square each block to the joist and drive three 16d galvanized box nails through the joists into each end of the block. Repeat for each row.
Some otherwise solid decks tend to sway or wobble as you walk across them, especially decks resting on tall posts 4 or more feet above the ground. Angle-bracing the posts is one good solution to this problem, but the braces often look tacky. Instead, install an angle brace underneath your deck. It's a virtually invisible fix that all but eliminates sway.
If your longest 2x4 doesn't span the entire distance, don’t worry. Add a second one starting from the other corner and run it back alongside the first, nailing it to at least two of the same joists. Have a helper hold the 2x4 in place while you drive the first nails. Driving 16d galvanized nails upward will give your hammer arm a workout!
Photo 1. Cut and nail a treated 2x4 diagonally from corner to corner under your deck. Drive two 16d galvanized nails at each joist.