Deck railing posts, usually made of heavy 4-by-4 inch lumber, may tend to wobble a bit, an undesirable feature where safety is concerned. Posts often benefit from reinforcement, especially those located on an end joist, which tend to wobble the most. You can reinforce deck railing posts during initial construction or as a retrofit if problems become more noticeable over time.
Investigate possible causes of wobbly deck railing posts, recommends the Simpson Strong-Tie Company in "The 5 Warning Signs of an Unsafe Deck" (see References, below). In older decks, check for missing or loose nuts on the galvanized bolts holding the post to the joist. Replace or tighten as necessary.
Add ½-inch galvanized carriage bolts and matching washers and nuts if the post is held by nails only. Clamp the post in place, remove the nails, drill ½-inch pilot holes 1 inch from the top of the joist edge and the bottom of the post for maximum reinforcement, and bolt the post in place.
Add blocking between joists for either new or retrofitted construction. Cut 2-by-8 pressure-treated lumber to a length that will fit snugly between the joists ( typically around 14½ inches for joists installed 16 inches on center). It's best to measure carefully, cut the blocking a hair long, and trim it back gradually on a chop saw until it fits snugly, requiring a mallet to fit between the joists.
Place blocking on either side of the deck posts. Nail the blocking in place with galvanized nails.
Add deck tension ties for maximum reinforcement of deck railing posts (see Resources for manufacturer). Bolt the post through the rim joist and into a hole in the base of the deck tension tie, which resembles a large joist hangar. Nail the deck tension tie to a joist using the eight holes provided. For posts on the end joist, bolt the post into the base of the deck tension tie and nail the tie to a piece of 2-by-8 blocking.