How to Repair or Replace a Rotted Deck Joist

DECK integrity begins with the joists, the hidden backbone that keeps your outdoor space safe and level. When rot takes hold—often caused by water trapping at the connection points or lack of flashing—it compromises the entire structure. Replacing a joist isn't just about appearance; it is a fundamental safety repair that prevents floorboard sagging and catastrophic structural failure. Done well, a joist repair is invisible and permanent. You are essentially transferring the load, clearing the decay, and re-establishing a rot-resistant connection. Approach this with patience, especially when working in tight crawl spaces beneath the deck, and prioritize galvanized hardware to ensure your new timber outlasts the rest of the frame.

  1. Support the Load First. Place a sturdy 4x4 post and a bottle jack on a firm surface directly under the joist you are repairing. Slowly raise the joist just enough to relieve the weight of the decking boards without lifting the entire deck off its perimeter frame.
  2. Clear the Way. Unscrew the deck boards directly above the damaged joist to gain clear access from the top. If the rot is isolated, you may only need to remove enough boards to reach the hardware at both ends of the joist.
  3. Remove Rotted Wood. Inspect the ledger and the outer rim joist for secondary rot. If the primary joist is too far gone, use a reciprocating saw to cut the nails or screws at the connection points and remove the damaged timber entirely.
  4. Fit the New Joist. Cut a new piece of pressure-treated lumber to the exact length of the old joist. If you are 'sistering'—bolting a new joist to the side of a partially rotted one—ensure the new wood is clear of any remaining decay.
  5. Lock It Down Tight. Install new hot-dipped galvanized joist hangers on both ends of the new joist. Use structural connector screws or heavy-duty galvanized nails meant for hangers, as common nails lack the necessary shear strength.
  6. Restore the Deck. Slowly release the tension on your bottle jack until the joist bears the weight of the deck. Reinstall the deck boards using new, corrosion-resistant deck screws.