To brace the underside of your deck, all you need to do is fasten a pressure-treated 2×4 diagonally from corner to corner on an angle. You'll need to drive in two 16d nails at every point in which the 2×4 meets the deck's joists. If the underside of your deck is too large to use a single 2×4, use two 2x4s.

When should you brace a deck?

Bracing may be required in decks which are built high up or on a slope to help stabilise the framework of the structure. Additional lengths of lumber are placed within the frame at support angles to increase the load capacity at the edge of the deck.

Does my deck need cross bracing?

Bracing is required on some decks, so check your local building code for specific guidelines. Even if it is not mandated, as a general rule it is wise to add bracing to decks with 4x4 posts more than 4 feet high and 6x6 posts more than 8 feet high. Freestanding decks over 3 feet high also should be braced.