How to replace damaged deck joists

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Introduction

Deck joists are the backbone of your deck, forming the framework that supports the surface boards. When joists become damaged from rot, insects, or moisture, the safety and stability of the entire deck are at risk. Learning how to replace damaged deck joists is an essential skill for keeping your outdoor space safe and long-lasting. If you’ve noticed sagging boards, soft spots, or structural weakness, this step-by-step guide will walk you through removing and replacing damaged joists with confidence.

Why Deck Joists Fail

Even pressure-treated lumber is vulnerable to wear over time. Common reasons for joist damage include:

  • Moisture exposure: Water infiltration causes rot, especially at ends near ledger boards or beams.
  • Insect damage: Termites and carpenter ants weaken wood fibers.
  • Improper ventilation: Trapped moisture underneath the deck accelerates decay.
  • Heavy loads: Hot tubs, grills, or heavy furniture strain undersized joists.
  • Age: Even well-built decks may see joist deterioration after 15–20 years.

Addressing damaged joists quickly prevents further structural problems and ensures your deck remains safe.

Step-by-Step Guide to Replacing Damaged Deck Joists

1. Inspect the Deck Structure

Before starting, examine the entire deck frame:

  • Look underneath for soft, cracked, or sagging joists.
  • Use a screwdriver to probe wood for softness—an indicator of rot.
  • Identify which joists need full replacement versus those needing reinforcement (sistering).

Mark all damaged joists with chalk or tape for easy reference.

2. Gather Tools and Materials

You’ll need:

  • Replacement pressure-treated lumber (same dimensions as original joists).
  • Galvanized or stainless steel joist hangers.
  • Deck screws or lag bolts.
  • Pry bar and hammer.
  • Circular saw or reciprocating saw.
  • Drill with screwdriver bit.
  • Measuring tape and level.
  • Safety gear (gloves, goggles).

Pro Tip: Always use pressure-treated or rot-resistant lumber to match the strength of existing joists.

3. Remove Decking Boards Above the Damaged Joist

Carefully remove the deck boards covering the damaged joist:

  • Unscrew or pry up fasteners holding boards in place.
  • Work slowly to avoid breaking boards you plan to reuse.
  • Stack removed boards neatly for reinstallation later.

This exposes the damaged joist for removal and replacement.

4. Support the Deck Temporarily

If the damaged joist is critical to deck stability, provide temporary support:

  • Use adjustable deck posts or jack stands under the frame.
  • Place them securely on solid ground.

This prevents sagging or collapse while you remove the joist.

5. Remove the Damaged Joist

  • Use a pry bar or hammer to remove nails or screws holding the joist to beams or ledger boards.
  • If the joist is wedged tightly, cut it into sections with a reciprocating saw for easier removal.
  • Clear all debris and old hardware from the opening.

6. Prepare and Install the Replacement Joist

Measure and cut the new joist to match the old one’s length. Ensure the ends are square for a snug fit.

  • Position the new joist in place between the ledger board and beam.
  • Secure with galvanized joist hangers at both ends.
  • Use deck screws or lag bolts to fasten the joist hangers firmly.
  • Check with a level to ensure the new joist is flush with the surrounding frame.

7. Reinforce Adjacent Joists (if needed)

If nearby joists show minor damage but don’t require full replacement, reinforce them with “sistering”:

  • Place a new joist of equal size directly alongside the old one.
  • Bolt or screw them together every 12–16 inches.
  • Ensure both joists rest fully on beams at each end.

This adds strength without a full replacement.

8. Reinstall Deck Boards

Once the joist is secure:

  • Replace the removed deck boards in their original positions.
  • Fasten with composite or galvanized screws to prevent rust.
  • If boards are damaged, replace them with matching decking material.

9. Inspect and Test

Walk across the repaired area to ensure stability. Check:

  • Boards no longer sag or bounce.
  • Joists are level with surrounding structure.
  • All fasteners are secure and rust-resistant.

Common Mistakes to Avoid

  1. Mistake: Using untreated lumber.
    Solution: Always choose pressure-treated or rot-resistant wood for durability.
  2. Mistake: Skipping temporary supports.
    Solution: Brace the deck while working to avoid sagging or collapse.
  3. Mistake: Relying on nails alone.
    Solution: Use screws or bolts with joist hangers for stronger connections.
  4. Mistake: Ignoring nearby damage.
    Solution: Inspect and repair adjacent joists to prevent future problems.
  5. Mistake: Poor alignment.
    Solution: Ensure new joists are level and flush to avoid uneven decking.

Extra Patio & Deck Tips & Hacks

  • Prevent Future Rot: Apply a wood preservative or flashing tape to joist tops before reinstalling decking boards.
  • Check Hardware: Replace corroded nails with galvanized or stainless steel screws.
  • Ventilation: Improve airflow under the deck with lattice panels or vents to reduce moisture buildup.
  • Related Guide: For surface board repairs, see our article on How to Repair Loose Composite Decking.

Conclusion

Replacing damaged deck joists may seem daunting, but with the right tools and careful planning, it’s a project most DIYers can handle. By removing decking, supporting the structure, and installing strong replacement joists, you’ll restore your deck’s safety and functionality.

A solid frame is the key to a long-lasting deck. Replace damaged joists today, and enjoy peace of mind every time you step outside.