How to Support a Leaning Deck

Check to see where the deck is not level to determine which, if any, of the posts are compromised. Wedge a post under the deck next to the failing post to support the weight of the deck while you service the failing post. Don't let anyone walk on the deck while this is happening. Using a circular saw, cut the failing post about a foot up from the bottom. Using the ratchet, unbolt the old post from the deck.
Ensure that the posts are the correct size given the height of the deck. Decks taller than 8 feet, for instance, should have posts that are 6-by-6 instead of 4-by-4. Cut a post of the correct size to the height of the deck. Bolt it in place by drilling two holes through the post and the framing. Put a bolt through both, a washer around the other side of the bolt and tightening a nut snugly after the washer.

Check the ledger board that connects the deck to the house. Ensure that you have a sufficient number of lag screws connecting the board to the house. Add lag screws with washers if necessary by drilling through the ledger board with a bit that is slightly thinner than the screws you will use. Put a washer around each lag screw and tighten it into the hole you just drilled.

Check the joist hangers. Remove any rusted nails from the hangers and add nails in any holes that are lacking nails. Only use nails meant for joist hangers.

Check if any of the decking boards are spongy, rotten, worn or otherwise corroded. Use a pry bar to remove any flawed decking boards, cut new boards to the size of the old boards and nail them into place by putting two nails through the decking board into the joist at each point the board crosses a joist.

Cut 2-by-6 planks to the diagonal length of the deck. If the length is longer than the standard 8-foot beams, use more than one. Put nails through either of the far ends of the plank into the joists, at least three in either side. Hammer in two nails at each point this plank crosses a joist under the deck to brace the deck against sway.

Reseal the deck with a sealant meant to protect against water damage.