Rotted Deck Joists: Prevent & Repair Joist Rot | (2023)

Rotted Deck Joists: Prevent & Repair Joist Rot | (1)

Rotted Deck Joists: Prevent & Repair Joist Rot | (2)Save

If your deck wasn't built using the best-treated lumber, you might be adding replacing deck beams and deck joist repair to your list of things to do. Understand that replacing deck joists can be extremely difficult and time-consuming. Unfortunately, if rot is advanced and the joist has lost significant strength, rotten joist repair might require major deck surgery.

Why Do Deck Joists Rot?

The simple answer is: that any wood that regularly gets wet and can’t dry out will rot. It’s important to realize you can avoid having to repair deck joist rot by starting with the best-treated lumber you can get. When building a new deck, consider purchasing treated lumber that’s rated for direct ground contact or burial. This lumber contains the highest amount of copper preservatives you can get.

Treated lumber rot often occurs when a deck drainage system is improperly installed below the joists, so wood joists stay wet.

(Video) HOW TO replace a rotted deck joist

Under-deck drainage systems mounted below the joists can cause damage in three ways:

  • First, this type of system can trap moisture. The drainage system attached to the bottom of the deck joists creates an environment of exceptionally high humidity. The drainage system forms the bottom of an elevated swimming pool of sorts. Debris can collect on the top of the drainage system. This can decay, creating an organic goo that holds moisture. The only ventilation is through the quarter-inch (or less) space between the top boards and where the water exits the drainage system.
  • Second, in the hot months of summer, the temperature further cooks this trapped, wet wood and accelerates the rotting. Over time, this is the perfect formula for wood rot: moisture + heat - ventilation = destruction.

Another common source of rotted deck joists is when homeowners paint the exposed joists (on three sides), which can cause further damage because as the wood on the top gets wet, there’s no way for the painted wood to release the moisture.

Factors that Can Speed Up Deck Joist Rot

Several other factors can accelerate deck joist rot. First and foremost is the failure to install deck joist tape on top of deck joists before the decking is applied. This tape was developed once it became apparent that the fasteners that hold down decking were contributing to rot. Once you apply this tape to the top of joists, you can scratch replacing rotted deck joists off your to-do list.

The issue was that decking fasteners would create micro-cracks in the top of the deck joists. Water seeping under the decking would enter the cracks and cause the treated lumber to swell. This movement would slowly over time enlarge the crack. The larger the crack, the more water would enter.

The lack of tape also created a perfect storm for you having to repair deck joist rot. While it may seem that the decking protects the top of the joist from getting wet, it contributes to it because of the narrow space between the two separate pieces of wood. Capillary attraction pulls water under the decking, and it’s slow to evaporate. When the sun hits the decking, it can cause its temperature to rise to well over 120F creating accelerated rot conditions.

How to Prevent Deck Joist Rot

Deck joist rot prevention begins with purchasing the best treated lumber. Once again, use treated lumber rated for direct ground contact or burial.

If your deck is already built and you can’t install joist tape, do your best to keep the gaps between the decking clear of debris. Inspect the deck each fall and remove any accumulated debris that’s resting on top of all joists. Brush out the debris with a stiff brush and wash it with a stream of water to completely clean the wood.

Clean the exposed sides and bottom of deck joists as soon as you see any signs of mildew, mold, or algae. These organisms contribute to deck joist rot. Your goal is to have all exposed treated lumber looking like it’s brand new all the time.

(Video) How to repair rotten deck joist.

Avoid sealing or painting the underside of deck joists. Paint and film-forming sealers will slow or prevent moisture from escaping from the treated lumber. If the treated lumber wasn’t treated properly, this trapped moisture will lead to rot.

Install a Deck Drainage System

One of the best ways to avoid deck joist rot is to install an under-deck drainage system. The best deck drainage systems are installed above the joists. This will provide 100% protection of all the joists and beams from moisture penetration. This type of system also helps to hold the screws in place longer.

If you’re building the deck yourself, choose an over-the-joist system that’s easy to install. Use this handy chart for comparison. If you’re hiring a builder, ask if the drainage system being installed is above the joists.

If you want a finished look to the underside of a deck, you should add a waterproof material to the underside of the joists that are protected by the deck drainage system that’s on top of the joists. To add extra protection, especially in high humidity areas, add ventilation to your under-deck ceiling with strip vents. These vents release hot air and help mitigate temperature extremes.

Choose Pressure Treated Wood

Pressure treated wood rot can happen for a host of reasons. It’s a known fact that treated lumber can and does rot. One source of the problem is poor quality control at the treatment plant. Mistakes happen and you might get treated lumber that simply doesn’t contain enough preservative. Unfortunately, there’s no way for you to test to make sure your lumber is going to perform well before you install it.

There are DIY liquids you can apply if you feel you may be at risk or you’ve inherited a treated lumber deck that you didn’t install. You can brush on copper naphthenate solution to help prolong the life of the deck joists.

Properly Ventilate your Deck

The best way to ventilate a deck is to keep all treated lumber at least 18 inches away from the soil. The next thing to do is to resist putting a decorative ceiling under the deck. Even if ventilation slots are incorporated into the ceiling, the lack of moving air hitting the deck can contribute to deck joist rot. You want any water that gets onto the deck joists to dry as rapidly as possible.

How to Repair Deck Joist Rot

If you’re handy, you can likely repair or replace rotten joists yourself. You also need to liberate the joist from all the decking above, remove any joist hangers or tie-rod connectors, and disconnect it from an end band board. Whether it’s a small or a large repair, you should understand how to inspect a deck joist so you know when to replace deck joists.

(Video) (How to) Prevent Wood Rot on Your Deck - Renee Romeo

Step 1: Inspect the Deck and Determine What Needs to be Repaired

If you feel you need to repair rotting deck joists, do what insect exterminators do. Use an ice pick or long-shaft flat-head screwdriver with a fine point to test for rotted wood. If wood rot is present, slight or moderate pressure will cause an ice pick to travel into the wood with little difficulty. Test your ice pick on wood that has no rot to see how it takes extreme pressure to get it to travel into the wood even 1/4 inch. You’ll then know instantly when you encounter rotted wood.

The first place to check for rotted deck joists is at the top where they are in contact with the decking above. Test the joist on both sides along the entire distance of the joist.

Step 2: Remove Deck Boards to Expose Joists

You may wonder how to replace deck joists. The process begins by removing decking fasteners is not easy if the fasteners are nails and have been countersunk into wood decking. If you use a cat’s paw nail-removal tool, you’ll undoubtedly damage the top surface of the wood.

Your job will be easy if the decking is installed with concealed fasteners and plastic parts that fit in between each piece of decking. Use a drill/driver in reverse to remove all fasteners.

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

Understand that any filler you were thinking of using to replace the rotted wood offers no structural value to the deck joist. What’s more, filler often has a very hard time bonding to the treated lumber.

You’ll then discover it may be difficult to create a neat-looking repair that matches the look of the original floor joist using filler or epoxy. If you insist on using filler, be sure to brush on copper naphthenate to the top of the freshly cut or chiseled wood to help extend the joist’s useful life.

Step 5: Reinforce the Joist with Sistered Material

The best way to repair deck joist rot is to sister new treated lumber onto a deck joist that has minimal wood rot. If just the top 1 inch of the joist is rotted, you can repair the rotted deck joist by screwing on a treated 2x4 to the existing floor joist.

(Video) DIY Deck Part 8 - Preventing Joist Rot

The sistered 2x material must always be at least 3 times higher than the depth of the rot. This means that if just one inch of deck joist is rotted, a 3.5-inch 2x4 will be sufficient.

However, significant rot will require you to replace instead of repair the rotted deck joist.

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.

Get more detailed instructions on how to replace deck boards here.

When to Replace Rotten Deck Joists

You’ll know when to replace deck joists when at least 20 percent of its width is rotted. At this point, you need to know how to replace deck joists. With this much rot in place, the joist has lost a significant amount of its strength and that joist probably no longer meets the building code for its span.

Get Started with Preventing & Repairing Deck Joist Rot

The sooner you start to inspect your deck for deck joist rot, the better. Be sure to inspect all joists, decking, and railing too. You want a safe deck for yourself, your loved ones, and any guests that will spend time on the deck.

If you’re not confident you can assess deck joist rot or feel comfortable hiring a professional, find a contractor near you.


How do you stop decking joists from rotting? ›

Install a Deck Drainage System

One of the best ways to avoid deck joist rot is to install an under-deck drainage system. The best deck drainage systems are installed above the joists. This will provide 100% protection of all the joists and beams from moisture penetration.

How do I keep my wood deck from rotting? ›

Boric acid (borate) is one of the most effective fungicides for use in treating wood rot. It can be applied to wood during construction to prevent future rot, or as a treatment to stop an active decay fungus from growing.

Do I need to treat decking joists? ›

If the substructure and joists of your Decking project begin to deteriorate, then it is important to replace or treat the material. There are two forms of rot, dry and wet rot. Wet rot happens when your timber has been exposed to moisture in the air, penetrating the wood, causing it to break down and rot.

What can I use to protect my deck joists? ›

Deck joist flashing tape is used to protect your deck's substructure – commonly made with traditional lumber – from moisture. As an organic material, traditional wood is highly vulnerable to mold, mildew, and rot. An effective and affordable way to protect your deck's substructure is by using tape for your joists.

What causes deck joists to rot? ›

Protecting deck joists from moisture will preserve them better than any other activity. Moisture is the number one reason for joist rot and failure. Along with reducing moisture, removing the fungi's food, will stop the fungi.

How long do pressure-treated deck joists last? ›

If you maintain and seal your pressure-treated deck, it can last you around 50 years.

Will vinegar stop wood rot? ›

Vinegar can stop fungal spores from spreading, allowing you to contain the wood rot and stop it from causing further damage, but it's not the only treatment that can or needs to be applied.

How do you fix rotten wood without replacing it? ›

You can repair rotten wood by first removing the rotting from the original board or joists. You can then fill the surrounding area with a soft wood polyester filler or wood patch using a plastic putty knife. This material fills the area and cures to ensure strength and resilience.

Will bleach stop wood rot? ›

Yes, bleach will stop wood rot from spreading and recurring in the future. It is a cheap yet effective method of treating wood rot. Apply a minimum of two bleach coats to the affected wood and the surrounding parts. For better results, ensure the treated wood remains dry.

How do you waterproof deck joists? ›

With a quality butyl tape, you can add years to your deck's usefulness. Use it as a sealer for the tops of your deck joists, rim joists, beams, and ledger boards. The tape is applied directly to the top edge of the wood. The builder must do this during construction or when replacing the deck boards.

How much does it cost to replace deck joists? ›

Replacing decking joists costs $150 to $350 per joist. Joists are often made from wood, which may rot, warp, or wear, and they are essential for your deck's structure and solidity. They may need to be replaced if they get too weak or damaged.

How long will untreated lumber last on a deck? ›

A deck made of untreated wood can last anywhere from 10 to 30 years. Decks made of treated wood and composite materials can last as long as 50 years.

Does homeowners insurance cover rotting wood deck? ›

Most causes of wood rot are excluded from homeowners insurance coverage. Homeowners insurance may cover wood rot if the damage is caused by a covered peril, like sudden and accidental water damage from a burst pipe. If the wood rots over time or because of a lack of upkeep, homeowners insurance won't cover repairs.

How long do wood deck joists last? ›

Whether for framing only or the decking too, pressure treated lumber lasts a long time. Pressure treated lumber exposed to freeze-thaw and wet-dry cycles without care or maintenance will last about 9-years. With proper maintenance and treatment, the same deck will last 40 plus years.

Why did my deck rot so fast? ›

Moisture and pooling water on decks can lead to rot and decay. Because dry rot is a fungus, it can spread like a cancer throughout a deck. Dry rot is caused by microorganisms that eat the cellulose from the wood leaving it brittle.

Why does pressure treated wood still rot? ›

Any pressure treated wood rot is usually due to a fungal issue. The fungi that cause this are very small organisms that move into the wood and feed on it over time. This causes the pressure treated wood to decay and soften which then turns into rot. All kinds of fungal issues basically create the same problem: rot.

What lasts the longest on a deck? ›

Aluminum and Steel

Aluminum decking is the most expensive out of the deck material and will last the longest with a lifespan of 30+ years. Steel decking can have a lifespan of 60+ years when correctly installed by a professional builder.

How do I know if my deck joists need replacing? ›

Look at the joists

If these show rot, that's a bad sign. If the wood is soft, or if you're able to push a screwdriver into the wood, that's an equally bad sign. Joists are hard to replace without tearing up a portion of your deck, so if they show damage, it's probably time to replace and rebuild.

Will baking soda stop wood rot? ›

Fungicides to defeat brown rot include: baking soda, hydrogen peroxide, tea tree oil, boron solutions, ethylene glycol or propylene glycol, vinegar, etc.

Does borax stop wood rot? ›

Simply put borates or borax are naturally-occurring water-soluable salt-like acids. They are about as toxic as table salt to humans and pets but kill wood-consuming insects like termites, powder-post beetles, and old house borers. More importantly, it kills the wood destroying microorganisms that cause rot.

Can you stop wood rot once it starts? ›

You'll first have to dry the wood by repairing leaks and/or running a dehumidifier. Once the wood is dry, you can apply a wood preservative that contains copper or borate, such as Woodlife Copper Coat Wood Preservative (available on Amazon). Monitor the wood because it's still at an increased risk of future rot.

What is the best product to repair rotted wood? ›

Best Wood Hardener
  • Best environmentally-friendly – PC Products Water-Based Wood Hardener.
  • Best easy to use – Elmer's Rotted Wood Stabilizer.
  • Best in class – Minwax 41700000 High-Performance Wood Hardener.
  • Best in durability – J-B Weld 40001 Wood Restore Liquid Hardener.
  • Best fastest drying time – Bondo Rotted Wood Restorer.
11 Oct 2022

Is there a product to repair rotted wood? ›

If you catch rotted wood in time, you can successfully repair it with epoxy resin. Repairing rotted wood means you'll be able to stain or paint it again to blend in with the surrounding wood.

What is the best wood filler for rotted wood? ›

Find out the best wood repair epoxy. Epoxy wood fillers are fantastic materials for DIY projects. They are one of the best products for fixing rotting wood. Epoxy fillers are also excellent in restoring wood that would be too costly to replace or too challenging to remove.

How do you harden rotten wood? ›

If you are trying to harden rotted wood which is in very large sections (beams for instance), drilling holes and pouring CPES into a funnel placed in the hole is an easy way of getting rapid penetration. The CPES will travel quickly through rotted timber, especially along the grain.

Will wet rot dry out? ›

Wet rot growth will stop once the moisture, and the source of the moisture is removed – this is why it's essential to treat the wet rot and the cause of the wet rot, to both remove the infestation and eliminate the chances of it returning.

Is Vinegar a wood preservative? ›

Vinegar is an ideal wood preservative.

What is the best deck waterproofing? ›

Here are some of the best deck sealers on the market:
  • Thompsons Waterseal Advanced Natural Wood Protector.
  • ECO-SAFE Wood Treatment – Stain & Preservation by Tall Earth.
  • Rust-Oleum Coppercoat Wood Preservative.
  • CabotStain Australian Timber Oil Penetrating Oil.
  • Thompsons Waterseal – Transparent Stain.

Can I use Gorilla tape for deck joists? ›

Gorilla All Weather Tape is a permanent, waterproof tape that can be used on wood.

Is it cheaper to repair or replace a deck? ›

Fortunately, repairing a deck is much cheaper, but there are still ways to save on deck repair cost. Consider whether it's a simple task you can do on your own such as cleaning and staining or adding nails.

How much does it cost to replace a 10 by 20 deck? ›

Cost to Build a Deck by Size
SizePressure-treated wood Average costExotic hardwoods Average cost
3 more rows

How do I know if my deck joists are rotten? ›

You can also tell if the wood is rotten by checking for these likely problem areas: Anywhere wood contacts the ground. Long sheets of grayish mold or darker-looking wood. Pest damage, those spots could be a combination of insect damage and dry rot since dry rot attracts termites and other wood-eating pests.

Can rotting decking be saved? ›

It is much easier to prevent rot than it is to repair the damage once rot has set in. When it is caught early, it may be possible to repair the damage without having to replace your whole deck. Remove any rotted boards and replace them with new, pressure-treated ones.

What happens if wood is not pressure treated? ›

The Difference Between Pressure Treated and Untreated Lumber

In contrast, untreated wood is in its natural condition. Exposure to moisture or water will cause water to penetrate its pores and make it rot. It is also vulnerable to the sun's ultraviolet radiation, which causes wood to lose its color.

What is the longest lasting material to build a deck with? ›

IPE wood is the longest-lasting deck material. Composite and synthetic decking are low-maintenance options.

Should I seal the underside of my deck? ›

If you stain the underside of your deck you create a barrier to moisture; however, this can actually trap moisture into your boards and cause them to rot at an alarming rate. This is because when you stain the underside of your deck, water can still get in from the nail or screw heads on the surface.

Is Thompsons water Seal good? ›

Thompson's WaterSeal has clearly not been a consumer favorite with deck stains or sealers. Mainly as it does not offer any UV protection and it grays in a few months. The Transparent and Semi-Transparent versions are prone to premature peeling and wearing after about 1 year.

What is a good waterproof sealer for wood? ›

According to them, these are the nine most effective wood sealers:
  • Thompson's Water Seal. ...
  • Rainguard Premium Wood Sealer. ...
  • DEFY Crystal Clear Sealer. ...
  • Anchorseal 2. ...
  • Roxil Wood Protection Cream. ...
  • Eco-Advance Exterior Wood Waterproofer. ...
  • Ready Seal Stain and Sealer for Wood. ...
  • Pure Tung Oil Natural Wood Sealer.

How can I make my deck joists last longer? ›

How to Make Treated Deck Joists Last Longer
  1. Using pressure-treated lumber.
  2. Covering the joists with a moisture-resistant stain or sealer before installation.
  3. Using metal joist hangers and leaving at least 1/8-inch between the end of the lateral joists and sides of the bearing joists to allow for airflow.
19 Mar 2021

How do I keep my deck from dry rotting? ›

One way to prevent dry rot is to coat your deck with protective materials that help lock out moisture and prevent fungus from growing. Deck stain is a great rot stopper because it locks out moisture while also improving the look of older or water-damaged decks.

Why is my deck rotting so fast? ›

Moisture and pooling water on decks can lead to rot and decay. Because dry rot is a fungus, it can spread like a cancer throughout a deck. Dry rot is caused by microorganisms that eat the cellulose from the wood leaving it brittle.

How do you keep pressure-treated wood from rotting? ›

The best way to protect from pressure-treated wood rot is to apply a deck preservative. Like we mentioned earlier, stains, paints and sealants are the best choice to use. You can find these items easily at the local hardware store.

What to do when deck is rotting? ›

5 Steps to Repair Wood Rot
  1. Gather tools and materials.
  2. Remove rotten boards Use a 5-in-1 rasp, chisel, or claw hammer to remove the rotted areas of wood. ...
  3. Apply wood hardener, filler, or epoxy If minor damaged wood cannot be easily replaced, wood hardener, filler or epoxy may help strengthen the damaged area.
3 Aug 2021

Can a wooden deck last 50 years? ›

A deck made of untreated wood can last anywhere from 10 to 30 years. Decks made of treated wood and composite materials can last as long as 50 years. Many composite decks come with a 20-year warranty – and often a lifetime guarantee.

How many years will pressure treated wood last? ›

How Long Does Pressure-Treated Wood Last? It depends on the climate, the type of wood, its uses, and how well it's maintained. While pressure treated poles can stay up to 40 years without any signs of rot or decay, decks and flooring might only last around 10 years.

What should never be done with pressure treated wood? ›

Pressure-treated wood should not be burned under any circumstances. The fumes can be toxic and the ash is very toxic. Do not use pressure-treated wood for making cutting boards, or for any food preparation surface.

Will borax stop wood rot? ›

Simply put borates or borax are naturally-occurring water-soluable salt-like acids. They are about as toxic as table salt to humans and pets but kill wood-consuming insects like termites, powder-post beetles, and old house borers. More importantly, it kills the wood destroying microorganisms that cause rot.


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