Replace broken deck boards
1. Cut out the bad board
The most difficult part of this deck repair task is cutting out the damaged section cleanly. Don't try to cut directly over a joist. Instead, cut to one side and screw on a cleat to support the new decking. It's a fairly hefty cut for a jigsaw, so use a sharp, stiff blade to keep your cuts as straight and smooth as possible.
Draw a square line on the decking to one side of a joist below. Cut the deck board with a jigsaw. Pull the decking nails with a cat's paw.
2. Add cleats to support the new board
3. Install the new board
Strengthen wobbly posts
1. Drill bolt holes
2. Drive in the carriage bolts
Most posts are held fairly plumb by the railing, but check them anyway with a level and tap in shims to straighten them if necessary. Don't over-tighten the bolts; the heads will sink deep into soft wood without much effort.
Stiffen a bouncy deck
1. Tap in snug-fitting blocks
2. Nail blocks in place
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.
Replace loose, popped nails
1. Start with a diagonal cutter
2. Yank the nail with cat's paw
3. Replace the nail with a screw
Take out the sway with an angle brace
Cut and nail a treated 2x4 diagonally from corner to corner under your deck. Drive two 16d galvanized nails at each joist.
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!
Thinking of building an all new deck? Here is a great collection of tips from a pro.