How to Replace Deck Joists – Reasons, Symptoms & Maintenance

If you have a deck, you might have noticed some problems with its support beams called “deck joists.”

Don’t worry; it’s a common issue many deck owners face. But I am here to help you find the solution!

In this guide, I will show you how to replace deck joists effectively, making your deck strong again.

I will also talk about the signs that tell you when your deck joists need replacement and how you can prevent these problems in the future.

Whether you’re an experienced DIY person or new to decks, I will give you all the information you need to fix the issue and have a safe and beautiful deck once more.

Let’s start making your outdoor space a place for relaxation and good times!

How to Replace Deck Joists? DIY Guide

Over time, joists may become weakened due to weather exposure, rot, or pests, compromising the stability of the entire deck.

This step-by-step guide will walk you through the process of replacing deck joists effectively and safely.

Step 1: Inspection and Preparation

  • Look for signs of damage, rot, or decay in the joists.
  • Common indicators of compromised joists include soft spots, visible mold or mildew, discoloration, and sagging areas in the deck.
  • Pay special attention to areas prone to moisture exposure, such as near downspouts, under potted plants, or close to the ground.
  • Once you’ve completed the inspection, mark and identify the joists that show signs of damage or rot.
  • Use chalk to mark these areas clearly for easy reference during the repair process.
  • Using a tape measure, determine the length, width, and height of the damaged joists.
  • Visit your local hardware store or lumberyard to purchase new joists that match the dimensions of the damaged ones.
  • Opt for pressure-treated or weather-resistant wood to ensure the longevity of the replacements.

Step 2: Clearing the Deck

  • Clear the entire deck of all furniture, potted plants, grills, or any other obstacles that may hinder the repair process.
  • Store these items in a safe location away from the work area.
  • Use a pry bar to carefully remove the decking boards that are attached to the joists you need to replace.
  • Start from one end of the deck and work your way systematically to the other end.
  • If the decking boards are nailed down, gently lift and wiggle them to loosen the nails before prying them off.
  • In the case of screws, use a drill or screwdriver to unscrew them carefully.
  • With the decking boards removed, inspect the joists and the surrounding area for any debris, old nails, or screws left behind from the previous decking.
  • Use a broom or a brush to sweep away any dirt, leaves, or loose particles from the surface of the joists.
  • Check for any remaining nails or screws and remove them using a hammer or a screwdriver.
  • Make sure the surface is clean and free from any potential hazards that could interfere with the installation of the new joists.

Step 3: Supporting the Deck

  • Based on the location of the damaged or rotten joists, determine the sections of the deck that require additional support.
  • These are the areas where you’ll place temporary bracing or jacks.
  • Use sturdy wooden boards or adjustable metal braces as temporary supports.
  • Ensure the bracing material is strong enough to securely hold the deck’s weight.
  • Hydraulic jacks or screw jacks are also suitable for providing support.
  • Position the temporary bracing diagonally or vertically between the ground or foundation and the underside of the deck’s structure.
  • For diagonal bracing, cut the wooden boards to the appropriate length and angle, ensuring they reach from the ground to the underside of the deck at a diagonal orientation.
  • For vertical bracing, measure the distance between the ground and the underside of the deck and cut the wooden boards accordingly. Secure them vertically under the deck.
  • If using jacks, place them strategically under the damaged joists, ensuring they are positioned directly underneath the load-bearing points of the deck.
  • Slowly and steadily raise the jacks to lift the deck until it is level and fully supported.
  • Be cautious about lifting the deck slowly and evenly, as this may cause additional damage.
  • Once the temporary bracing or jacks are in place, check that the deck is stable and level.
  • Make any necessary adjustments to ensure an even and secure support system.
  • Use appropriate fasteners or braces to secure the temporary supports in place.
  • Ensure they are firmly attached to both the ground/foundation and the underside of the deck.

Step 4: Removing Damaged Joists

  • Examine the damaged joists and identify the areas that need to be replaced.
  • Mark these sections clearly with a pencil or chalk to guide your cuts.
  • Using a circular saw, carefully cut the damaged joists along the marked lines to create manageable sections that can be easily removed.
  • Take your time and ensure the cuts are straight and precise to avoid any accidental damage to neighboring deck components.
  • Once the joists are cut into manageable pieces, carefully lift and remove them from the deck frame.
  • Depending on the extent of damage, the sections may come out easily or require a bit of prying.
  • After removing the cut sections, inspect the deck frame for any remaining nails, screws, or other fasteners that were holding the damaged joists in place.
  • Use a hammer or a screwdriver to remove any lingering fasteners to prepare the area for installing the new joists.
  • While the damaged joists are removed, take the opportunity to inspect nearby deck components, such as the ledger board and beams, for any signs of damage or wear.
  • Address any minor issues you find to ensure the structural integrity of the entire deck.

Step 5: Measuring and Cutting Replacement Joists

  • Measure from one end of the damaged joist’s supporting points to the other, making sure to account for any overhang that extends beyond the beam.
  • Verify that all replacement joists are of the same length, as even small variations can lead to an uneven deck surface.
  • Use a pencil or chalk to mark the measured length on the new joists.
  • Use a sawhorse or a sturdy workbench to support the joist during cutting.
  • Depending on the type of joist material, use a circular saw, miter saw, or hand saw suitable for cutting wood or composite materials.
  • Align the saw blade with the marked line on the new joist.
  • Start the saw and guide it smoothly along the marked line, making a straight and accurate cut.
  • Repeat the measuring and cutting process for each replacement joist required for the deck.
  • If there are any rough or splintered edges on the cut joists, use sandpaper to smooth them down.

Step 6: Installing the Replacement Joists

  • Carefully lift the newly cut replacement joists and place them into their designated positions on the deck frame where the old joists were removed.
  • Ensure the replacement joists are aligned with the existing joists and positioned correctly according to the deck’s original layout and design.
  • Use a level to verify that the replacement joists are perfectly level horizontally. Adjust their position if needed to achieve a level surface across the deck.
  • Once the replacement joists are in their correct positions and are level and aligned, it’s time to secure them to the deck frame.
  • Use either nails or screws, depending on your preference and the type of joist material.
  • For pressure-treated wood joists, use galvanized or stainless steel nails or screws.
  • Nails should be driven at a slight angle through the face of the joist into the ledger board and the beam or header.
  • For composite or hardwood joists, use stainless steel screws or specialized fasteners recommended by the manufacturer.
  • Place fasteners at regular intervals, typically about every 16 inches, along the length of the replacement joists.

Step 7: Replacing Decking Boards

  • Replace any severely damaged or rotten boards with new ones to maintain the deck’s integrity.
  • Organize the replacement decking boards in the order they will be installed.
  • Carefully fit the first replacement decking board onto the newly installed joists.
  • Align it properly with the adjacent boards and ensure it is flush with the edge of the deck.
  • For wood decking, use galvanized or stainless steel deck screws.
  • For composite decking, use specialized screws or clips recommended by the manufacturer.
  • Ensure there is consistent spacing between each decking board.
  • To prevent warping, ensure the replacement decking boards are tight enough when securing them.
  • Leave a slight gap between the boards to allow for natural expansion and contraction due to changes in temperature and humidity.

Step 8: Inspect and Test the Deck

  • With the replacement joists installed and the deck structure stable, it’s time to remove the temporary bracing or jacks used to support the deck during the repair process.
  • Gradually lower and remove the bracing or jacks, ensuring a smooth and controlled process to avoid sudden weight shifts.
  • Check the ledger board, beams, deck posts, and other critical parts of the deck for signs of damage, rot, or wear.
  • Check that all the fasteners used to secure the decking boards and replacement joists are tightly in place.
  • Once the inspection is complete, perform a stability test by walking across the entire deck surface.
  • During the inspection, also verify that the deck has proper drainage. Water should not be pooling or accumulating on the deck’s surface.

Step 9: Apply Protective Coating

  • Select a high-quality weather-resistant sealant or wood preservative designed explicitly for outdoor use and suitable for the type of wood used in your new joists.
  • Before applying the protective coating, ensure the surface of the new joists is clean and free from any dust or debris.
  • Wear appropriate safety gear, such as gloves and safety goggles, during the application process to protect your skin and eyes.
  • Use a paintbrush, roller, or sprayer to apply an even coat of protective coating to the entire surface of the new joists.
  • Avoid walking on the coated joists or exposing them to moisture until they are fully dry.

Why do Deck Joists Rot?

A deck is a wonderful addition to any home but can face issues like rotting joists that weaken its structure.

Let’s look at the main reasons why deck joists rot and how to prevent them.

Moisture and Water Infiltration

Moisture is the arch-nemesis of wood, and deck joists are no exception.

The constant exposure to rain, snow, and humidity can lead to water infiltration into the wood fibers of the joists.

As moisture seeps in, it creates a breeding ground for mold, mildew, and fungi, gradually deteriorating the wood over time.

Prevention Tip

Regularly inspect and maintain the deck’s drainage system, ensuring that water flows away from the joists and does not accumulate on the deck’s surface.

Additionally, consider using pressure-treated or rot-resistant wood for the joists, as it offers better protection against moisture.

Lack of Proper Ventilation

Proper airflow is crucial to preventing rot in deck joists.

When there is limited ventilation under the deck, trapped moisture cannot escape, leading to condensation and an ideal environment for rot-causing organisms to thrive.

Prevention Tip

Install adequate ventilation to promote air circulation beneath the deck.

This can be achieved by using lattice panels, installing vents, or raising the deck slightly off the ground with support structures.

Inadequate or Missing Flashing

Flashing is a crucial component that protects the vulnerable areas of your deck, such as where it connects to the house or around posts.

When flashing is improperly installed or missing altogether, water can seep into these junctions, accelerating the rotting process.

Prevention Tip

Ensure proper flashing is in place during deck construction or when conducting repairs.

Use durable materials such as metal or rubberized flashing to create a watertight seal in critical areas.

Symptoms that Deck Joists Need Replacement

Over time, joists can deteriorate due to various factors, compromising the structural integrity of the entire deck.

It’s essential to be vigilant and recognize the warning signs indicating that your deck joists may need replacement.

Sagging or Uneven Deck Surface

If you notice that your deck’s surface is sagging or appears uneven, it could be a sign of weakened or damaged joists.

Sagging occurs when the joists lose their strength, and the deck starts to sink or become lopsided.

Visible Rot or Decay

Inspect the joists regularly for signs of rot or decay.

Rotting wood appears discolored, soft, and crumbly, indicating a significant issue that requires immediate attention.

Excessive Movement or Bouncing

When walking on the deck, if you notice excessive movement or bouncing, it could be due to weakened joists unable to bear the load.

Warped or Cupped Decking Boards

Warped or cupped decking boards can be a result of joist issues. The joists may not be providing sufficient support, causing the decking to deform.

Mold or Fungus Growth

The presence of mold, mildew, or fungus on the surface of the joists indicates excessive moisture, which can lead to rotting over time.

Wood-Piercing Insects

Infestation by wood-boring insects like termites or carpenter ants can damage the joists, making them structurally unsound. Using wood preservatives can help you get rid of this issue.

Water Pooling on the Deck

If water pools on the deck surface instead of draining away, it may indicate poor joist condition or improper deck construction.

Deck Railing Instability

An unstable or wobbly deck railing might indicate weak joists, as they support the railing posts.

If you observe any of these symptoms, it’s essential to take prompt action. Ignoring weakened or damaged joists can lead to more extensive and costly problems in the future.

Bottom Line

Great job! Now you know all about deck joists and how to handle their problems.

By spotting signs of weak deck joists early and acting fast, you can stop small problems from becoming big ones.

Regular maintenance and smart choices, like good ventilation and quality materials, will protect your deck from rot and decay.

A well-kept deck adds beauty to your outdoor space and provides a safe place for wonderful memories.

With your newfound knowledge, you can confidently handle any deck challenges that come your way.


How do you reinforce rotted deck joists?

To reinforce rotted deck joists, follow these steps:

  • Cut a Sister Joist: Cut a new joist from lumber that matches the dimensions of the damaged joist. This new joist is called a “sister joist.”
  • Treat the Sister Joist: Apply the waterproof sealer to all sides of the sister joist to safeguard it against rot.
  • Secure the Sister Joist: Place the sister joist flush over the old, rotted joist. Fasten it in place using 10d nails or screws every 2 feet to ensure a strong connection.

How long will deck joists last?

Typically, well-made deck joists and foundations can last for over a decade.

However, the exact lifespan depends on your local weather conditions and how well you care for them.

What is the minimum distance between deck joists?

The minimum distance between deck joists should be 40 centimeters, and structural posts should be positioned no more than 180 centimeters apart.