How to Repair a Rotted Deck Joist

D eck integrity is non-negotiable for safety. When you notice a joist showing signs of rot—soft wood, crumbling fibers, or deep discoloration—you aren't just looking at an eyesore; you are looking at a structural failure point that needs immediate attention. Ignoring it puts your entire deck framing at risk, especially near ledger boards or beam supports where water tends to trap. Repairing a joist doesn't usually require tearing up the entire deck surface. By 'sistering'—securing a new, healthy board against the damaged one—you transfer the load to a sound member. The result is a deck that is stronger than it was before the rot set in, provided you take the time to treat the cut ends and ensure the new wood is properly rated for ground contact.

  1. Access the Damage. Remove any deck boards directly above the damaged joist to gain clear access. Use a reciprocating saw or a pry bar to lift the boards carefully, labeling them so they can be reinstalled in the same order.
  2. Assess the Scope. Remove any loose debris, mold, or rotting wood fibers from the existing joist with a wire brush. Inspect the ledger board and beam to ensure the rot hasn't spread to the supporting structure.
  3. Prep the New Board. Measure the length of the existing joist and cut a new piece of pressure-treated lumber of the same dimension to match. Apply a high-quality end-cut wood preservative to all newly cut ends to prevent future rot.
  4. Lock in Position. Place the new board flush against the side of the damaged joist. Ensure the top edges are perfectly level with the surrounding joists so the deck boards sit flat when reinstalled.
  5. Sister the Joists. Drill three-quarter-inch holes in a staggered pattern along the length of the joists. Secure the two boards together using half-inch galvanized carriage bolts, washers, and nuts, tightening them firmly to lock the boards into a single structural unit.
  6. Restore the Surface. Reinstall the original deck boards using high-quality exterior-rated deck screws. Ensure the boards are spaced correctly to allow for proper drainage and airflow.