If your deck was not constructed with the highest-quality treated wood, you may need to replace deck beams and repair deck joists. Recognize that rebuilding deck joists can be incredibly challenging and time-consuming. Sadly, if the rot is advanced and the joist has lost significant strength, rotten joist restoration may necessitate extensive deck surgery.
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Why Do Deck Joists Rot?
Wet timber that won't dry out rots. Start with the best-treated lumber to avoid deck joist rot. Consider buying treated lumber rated for direct ground contact or burial when building a deck. This wood has the most copper preservatives.
When a deck drainage system is placed poorly below the joists, treated lumber rots. Under-deck drainage systems can cause harm in three ways.
First, it traps moisture. The deck's drainage system causes extreme humidity. Drainage system generates an elevated pool.
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Debris can collect on drains. This decomposes into a moist organic sludge. Only a quarter-inch gap between the top boards and where water exits the drainage system provides ventilation.
Summer heat heats trapped, moist wood and increases decay. Moisture + heat - ventilation equals decay over time. When homeowners paint the exposed joists (on three sides), the wet wood on top can't escape the painted wood, causing rot.
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Factors that Can Speed Up Deck Joist Rot
Several causes accelerate deck joist decay. First, deck joist tape isn't applied before decking. Deck fasteners were causing rot, thus this tape was created. After applying this tape on joists, you can skip repairing rotting deck joists.
Decking fasteners caused microcracks in deck joists. Under-deck water would penetrate fractures and swell treated lumber. This movement grew the crack slowly. More water enters larger cracks.
Lack of tape also caused deck joist deterioration. The decking may seem to protect the joist from getting wet, but the short area between the two pieces of wood contributes to it. Water under the decking evaporates slowly due to capillary attraction. The sun may heat decking to over 120F, accelerating decay.
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How to Prevent Deck Joist Rot
Buying treated lumber prevents deck joist rot. Again, choose treated lumber that can be buried. If you can't use joist tape, keep the decking gaps clear of debris. Remove debris from deck joists each fall. Clean the wood with a hard brush and running water. When mildew, mold, or algae appear on deck joists, clean them. Insects cause deck joist rot. All exposed treated lumber should always seem new. Deck joists shouldn't be painted or sealed. Paint and film-forming sealers limit treated lumber moisture loss. Trapped moisture will decay untreated lumber.
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Install a Deck Drainage System
Installing under-deck drainage can prevent deck joist rot. The ideal deck drains are above the joists. All joists and beams will be 100% protected from moisture. This method holds screws longer.
Choose an easy-to-install over-the-joist solution if you're building the deck yourself. Comparative chart. Ask a builder if the drainage is above the joists.
To finish the underside of a deck, add waterproof material to the joists that are protected by the deck drainage system. Add under-deck strip vents for further protection, especially in humid climates. Hot air vents reduce temperature extremes.
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Choose Pressure Treated Wood
Why does pressure-treated wood rot? Treated wood rots. Poor treatment plant quality control is an issue. Sometimes, treated lumber doesn't have enough preservatives. You can't evaluate your lumber's performance before installing it.
If you're at risk or inherited a treated wooden deck, you can apply DIY liquids. Deck joists can be treated with copper naphthenate solution.
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Properly Ventilate your Deck
The easiest approach to ventilate a deck is to keep all treated wood 18 inches away from the dirt. Next, resist the need to install a beautiful ceiling beneath the deck. Even if ventilation openings are built into the ceiling, a lack of circulating air on the deck might contribute to decay. You want any water that makes its way onto the deck joists to evaporate as soon as possible.
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How to Repair Deck Joist Rot
You can certainly fix or replace rotten joists yourself if you're handy. Additionally, you must detach the joist from the decking above, remove any joist hangers or tie-rod connectors, and detach it from the end band board. Whether it's a minor or major repair, you should know how to evaluate deck joists so you can determine whether to replace them.
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Step 1: Inspect the Deck and Determine What Needs to be Repaired
If you feel the need to repair rotting deck joists, follow the methods employed by insect exterminators. Use an ice pick or a long-shaft, flat-head screwdriver with a fine point to examine wood for decay. If wood rot is present, a small to moderate amount of pressure will allow an ice pick to easily penetrate the wood. Test your ice pick on rot-free wood to determine how much force is required to penetrate even a quarter inch. You will then recognize immediately whenever you encounter rotten wood.
The first location to inspect for rotted deck joists is where they come into touch with the decking above. Examine the joist from both sides along its whole length.
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Step 2: Remove Deck Boards to Expose Joists
You may wonder how deck joists are replaced. It is difficult to remove decking fasteners if the fasteners are nails that have been countersunk into the wood decking. Using a cat's paw to remove nails will inevitably cause harm to the wood's surface.
If the decking is installed using concealed fasteners and plastic pieces that fit between each plank, your work will be simplified. Use a reversed drill/driver to remove all fasteners.
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Step 3: Remove Rotted Wood
Use a chisel or a saw to remove the rotted wood. Cut away and remove all rotted wood.
Step 4: Beware Using Wood Filler
Realize that any filler you were considering applying to replace the rotting wood provides no structural benefit to the deck joist. In addition, filler frequently has a difficult time adhering to treated wood.
Using filler or epoxy to produce a repair that matches the appearance of the original floor joist may prove to be challenging. If you insist on using filler, make careful to apply copper naphthenate to the freshly cut or chiseled wood to prolong the usable life of the joist.
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Step 5: Reinforce the Joist with Sistered Material
The most effective method for repairing rotted deck joists is to sister new treated material to a deck joist with minor wood rot. If only the top 1 inch of the joist is rotten, the rotted deck joist can be repaired by screwing a treated 2x4 to the existing floor joist.
The 2x material must always be at least three times taller than the rot's depth. This indicates that a 3.5-inch 2x4 will suffice if only one inch of deck joist is deteriorated. However, extensive damage will necessitate replacement rather than repair of the rotten deck joist.
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Step 6: Replace Deck Boards
If a sistered repair was sufficient, it’s time to replace & secure the deck boards to the new 2x sister material. Cut the deck boards to size and secure them by using nails or hidden fasteners, depending on the material used.
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When to Replace Rotten Deck Joists
When at least 20 percent of the width of a deck's joists are decayed, it's time to replace them. You must now understand how to replace deck joists. With this much decay, the joist has lost a substantial amount of its strength, and it likely no longer complies with the construction code for span.
FAQs about Avoid, Prevent & Repair Deck Joist Rot
What’s the quickest, low-maintenance way to keep bugs from coming up through deck-board gaps?
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Install insect screen as a physical barrier: use standard 18×16 mesh for typical insects, or 20×20 “no-see-um” mesh if gnats/sand flies are a problem. You can staple it to the joists from below on an existing deck, or lay it on top of joists before decking so boards clamp it in place.
What mesh size should I choose?
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General use: 18×16 insect screen.
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Tiny biters (no-see-ums/gnats): 20×20 mesh, marketed as no-see-um screen. It still allows ventilation while blocking smaller flyers.
Should I seal or caulk the gaps between deck boards to stop bugs?
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No. Those gaps are there for drainage and airflow; sealing them traps moisture and debris and can accelerate decay. Keep the gaps open and use under-deck screen/skirting if you need a barrier.
Do certain outdoor lights attract fewer insects to my deck?
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Yes. Overall, LEDs attract fewer insects than many legacy bulbs; evidence on warm vs. cool white is mixed, but lower-blue/amber LEDs and good shielding/placement reduce attraction. If you can, use amber/“bug” spectrum LEDs and aim light downward, away from seating.
What’s the best way to cut down mosquitoes around the deck (besides sprays)?
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Eliminate standing water weekly (planters, toys, gutters, saucers).
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Use fans/air movement—mosquitoes struggle in higher air velocities.
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Keep vegetation trimmed and maintain drainage under the deck.
How do I discourage carpenter bees from drilling railings/soffits?
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Keep wood painted/finished, fill/plug old holes, and repair weathered areas; bees prefer bare or weathered wood. For active, extensive activity, consider a licensed pro for targeted treatments.
Can termites get into deck structures—what are the practical preventions?
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Yes—focus on construction and site hygiene: avoid wood-to-soil contact, keep an inspection gap, manage moisture, and store firewood away from the house/deck. For ongoing risk areas, consult a pro about EPA-registered termite treatments/monitoring.
Will composite/PVC decking help with insect damage?
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Usually, yes. Composite/PVC boards aren’t a food source for termites/carpenter ants, and many brands claim insect resistance (always check warranties). Substructure is still wood unless you choose steel/aluminum framing.
What about wasps and yellowjackets buzzing the deck during meals?
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Sanitation beats “repellents.” Keep food/drinks covered, seal trash tightly, and remove fallen fruit. Destroy problem nests early or hire a pro; decoy nests may help some people but evidence is limited.
Does closing off the deck perimeter help—or will it trap moisture?
- Skirting + mesh can deter pests and hide the under-deck area, but preserve ventilation (use louvered/lattice or vented panels) to avoid mold/wood decay. Pair with ground moisture control.
