15 Ways to Prevent Deck Joist from Rotting Plus My Top Picks


“So, in practice decay by dry rot tends to be restricted to damp, unventilated areas”

TimberWise

Wow, deck joists are in the exact environment, which is ideal for them to rot, damp and unventilated. No wonder so many joists fail from rot, the deck is stacked against them. All pun intended, but wood is an amazing material, economical, versatile, beautiful and durable. A great example of wood construction is Horyuji TempleOpens in a new tab. near Nara, Japan. Constructed of wood but lasting over 1400 years old. If your deck joist lasts half that time, that will be a huge win. Wood is durable, but it must be cared for.

Preventing deck joists from rotting is a matter of knowing the enemy, and how to stop Fungi growth on joists will damage the structural strength of wood.

“With wood-rotting fungi, the hyphae spread through the wood, disintegrating the cell walls and reducing their strength.”

M.C. BakerOpens in a new tab.

Stopping joist rot is simple. Remove what fungi need, prevent rot. Remove that, and your deck joist will last for years. Fungi is like you and me; it eats, breaths, drinks, reproduces and wants to be comfortable. Remove one of these from under your deck, and your joist will be fine.

In scientific terms, joist destroying fungi need five things.

  1. Moisture
  2. Suitable substrate (food)
  3. Source of infection
  4. Oxygen
  5. Suitable temperature

The 80/20 rule (Pareto principleOpens in a new tab.) is that 20% of our activities cause 80% of the results. The same applies to protecting deck joists. 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. Unfortunately, wood is the very food fungi craves. So if you can’t remove its food, wood, you can make it less desirable for fungi? Treating wood will slow and prevent fungi growth.

An additional step is to remove the source of infection, but the returns will be minimal compared to moisture and food. Still removing infection will help to keep your joist in good condition for many years.

Trying to limit or control oxygen and temperature around your joist is next to impossible. Making the returns on your effort minimal compared with moisture and food. The time and money spent trying to control oxygen or temperature levels inhibiting fungi are futile.

The good news for those of us living in Calgary, where I build decks, is that winter is too cold for fungi growth. So, for about half the year, regardless of what you do, joist rot is halted by the cold. On a more practical note, here are 15 ways to prevent deck joist rot. If you like jump ahead to the end for my top 3 picks, the ones which have the most impact on preventing your deck joist from rotting.

The war on rot is won or lost with water. If you keep your deck joist dry, they will last for years. Failing to control the moisture level of the deck joist will have significant consequences. The best way to prevent joist from rotting is by minimizing deck joist moisture. Reducing moisture is the most effective way to prevent joist rot.

15 ways to prevent deck joist rot

1. Ventilation Dries Deck Joist Extending their Life

You can’t stop the rain, nor should we, but allowing your joist to dry will make all the difference. Joists get wet every time it rains but allowing fresh air movement reduces harmful humidity levels allowing your joist to dry. Drying doesn’t even have to be an instant event; fungi spores need time to germinate.

“It is recorded in laboratory culture experiments that spores can germinate between 7-10 days following suitable wetting”

TimberWiseOpens in a new tab.

Venting under your joist, allowing them to dry within seven days after rain, will prevent rot. The key is air circulation. All that fungi growing moisture needs a place to go. Designing your deck with vents or ¾” gaps between skirting material to allow the moisture to escape. It doesn’t want to be trap under your deck either, but it needs a way out. Please, for the love of joist, help the water get out!

Deck skirting design is key to allowing your joist to dry. I have dealt at greater length about skirting here if you would like more information.

2. Seal the Joist with a Water Repellant Sealer

Most pressure-treated woods are treated for fungi and insects not water. Sealing joist with a water repellant sealer will increase resistance to rot. As moisture is repelled from the wood inhibiting rot with low moisture content. Wood needs to have a moisture content above 18% for fungi to grow.

Wood sealers like Seal-OnceOpens in a new tab. warranty their products to repel water for 6 to 10 years. Which is shorter then the life expectancy of your deck, meaning to truly protect the joist you would need to reapply every 6 to 10 years, which would could be impossible with a lower deck that needs it the most. But as I see it, sealing the joist during construction will add 6 to 10 years of life to the joist. As water is repelled for the additional 6-10 years before the sealant has deteriorated.  Meaning if the joist will not rot for ten years without sealing, then sealing will increase their life to 16-20 years. Significantly increase the life of your deck joist.

3. Allow Water to Escape Helps Preserve Deck Joist

Trapped water is damaging to deck joist. All water wants to do is get to the ground and back into the water cycle. Trapping water on joist, beams and ledgers making it mad, causing damage. Raining running over your deck is fine. It’s just trapped water, softening the wet wood become a receptacle to rot.

Two critical areas trap water on your deck, multiply plies of joist or beams and behind the deck ledger. Wherever you have material tightly squeezed together, prevent water from escaping. The solution, don’t squeeze material together!

“Wood rot is a form of decay triggered by the combination of moisture and fungi (microscopic organisms). In order for fungi to set up shop, the wood must be continuously damp; fungi will not grow on dry wood.”

By Glenda TaylorOpens in a new tab.

The most economical solution is small strips of treated ¼” plywood installed between joist plies allowing water to drain. The same can be done for deck ledgers, attach small standoff strips between the house and ledger wherever you will fasten the ledger to the house. Allowing the water to drain away, allowing the wood dry, protecting your house and ledger safe for years.

Sandwiching plywood strips between wood will allow most moisture to escape, but the plywood strips are still trapping moisture. Another solution is spacer designed to handle moisture while limiting their impact. Companies like Deck 2 WallOpens in a new tab. have designed spacer that allows water to escape, along with increased ease in installation. Click this linkOpens in a new tab. if you would like to look over some of their trapped water solutions.

Gapping beams and ledger are not always the best solution. Often I will choose to redirect water instead, which we will discuss later.

Not joist, but part an important part of a deck is the beam’s post. The post should be set in saddles, lifting them out of the water, preventing water from capillary up the post, saving the post from rot. Saddles should be done long with extending the concrete piles six inches above the ground.

Redirect Water Preventing Deck Joist Damage

Often the best solution is prevention. Constructing your deck in a way that moisture doesn’t get trapped, causing damage. Remember the second Karate Kid movie? The best defence is not to be there. Redirecting water away from critical areas on your deck will prevent many problems.

This simple process keeps your house dry. Your roof is not waterproof. Not a chance! But ever shingle redirects the water up the top of the next shingle carrying the water off your roof and away from your house. Each is working together to direct water to keep your home dry through the biggest rainstorms. Water is a powerful force; stopping it is next to impossible, redirecting it will make all the difference. 

4. Overhang your decking, direct water away from the joist.

Water will run to the edge of the decking and away from the deck and joist. Cutting decking flush to the edge forces rain unto the joist, soaking the joist, causing fungi to grow. Even worse is when builders cut decking flush and then install facia flush to the top of the deck. The water has no place to go but on top of the joist. Overhanging decking will save deck joists by allowing the water to run away from the joist. The decking becoming a rain leader away from the joist.

5. Slope Joist

Sloping Joist is more beneficial for the decking allowing water to drain off the decking boards but is also beneficial for joist. By adding a slight slope on the deck, water can run off preventing pooling and raising the moisture level of the wood. Fungi need moisture levels to be above 18% to grow, keeping joist below this level prevents rot.

The rate of joist slope can range from 1/8” over a foot to 1/32” the number is not critical, but having a slope is.  Water will run where its directed, a slope will allow water to move off the joist. The greater the slope the faster it will drain but remember you have seven days. If a lower slope works better for your deck design, it will still work to protect your joist from rot.

6. Flash the ledger and beam.

Flashing will guide water off your ledger and away from the house. The same can be done with beams. Doubling up “L” flashing with a bead of silicone between them will shelter the joints of the beams from water. Protecting the beams can also be done with membrane flashing. The membrane or flashing protecting the top of the beam, especially the small crevasses between plies that can trap water.

7. Remove Protruding Post Railing

I understand the concept of tying railing post two, the joist, surrounding them with blocks for increased strength, but water becomes trapped beside the post and blocks and joists. A better design is covering the deck entirely with decking and screw railing post on top of the deck into blocks below. Minimizing water penetrating points and trapping water between material.

8. Remove Organic Material from Deck Joist

Removing organic material requires forethought and maintenance. Organic material on your deck brings fungi unto your deck and acts as a sponge collecting water. Gapping decking large enough to be able to sweep out trapped material will go a long way to preventing rot. You will need roughly a ¼” between boards to clean between them. Any smaller and it becomes traps material and becomes harder to remove.

9. Edge Fastening of Decking

There are many ways of doing this from Camo to hidden fasteners, but the point is to hold down the edge of the decking. If decking can cup, it creates a nice little crevice for water to hide under rotting the joist.

Facing screwing of decking also creates little dimples on top of the decking for water to pool, rotting the decking and rusting the fasteners — drawing water into the joist, below its protecting coating.

10. Use Membrane Flashing on Joist

Particularly effect under composite and PVC deckingOpens in a new tab., as their synthetic design doesn’t absorb water or rot. Covering all the joists with a membrane before decking both acts as a shelter diverting water of the joist but also sealing the penetrating screw holes. Keeping the water both off and out of the joist. The top of the joist is the number one area of joist rot. With the top 1 ½” off the joist under the decking has rotted from moisture trapped under the decking and wicked into the joist.

I prefer larger 3″ (75mm) tape that wraps over the edges of the joist, ensuring full coverage. What I also like is an unmarked tape, without printing showing up between the decking, fortunately, Deck Wise has you covered.

Having dealt with water, helping our deck joist survive, we can move on to the next most effective element, preventing deck joists from becoming food for fungi. Simply, we want to make the joist undesirable for fungi.

 11. Under Deck Drainage Installed above Joist

Designed for higher decks with a useable area below but can be used to protect joist from rot also. Similar to membrane flashing on joist but taking it to the next level and protecting not only the top of the joist but the entire joist from moisture. This level of protection is achieved by draping rubberized membrane over the joist between each joist bay. Protecting both the top of the joist and sides by collecting rain and diverting away from the joist.

“The average deck around here lasts 16 to 22 years, but with this system a deck can last three to four decades because the framing never gets wet.”

Jerry Herbert, of Colorado Deck Drain ExpertsOpens in a new tab.

There are a number of systems available, but the two most popular are DEK DrainOpens in a new tab. and Trex’s RainEscapeOpens in a new tab.. Colorado Deck Drain Experts even manufacture their own drainage system with a lifetime warranty of water protection but you will need to live in Colorado to use their protection system.

The full value of this product is the useable space underneath, but its added benefit is joist protection. Joist will last years longer when protected entirely from rain. Moistures a killer, deck drainage protects them.

Making Joist Inedible to Fungi

Reducing moisture under your deck will greatly help to stop joist rot. Remove the water, save the wood. Another approach to reducing fungi growth on deck joists is by treating the wood. Remember that ad years ago about Cheese. Where we have grown kids who just won’t move out of home because there Mom keeps serving cheese. Well, fungi will move away from your deck joist if you stop feeding them.

12. Use Higher Level of wood Treatment for Joist.

Wood that is treated against fungi and insects will last longer than untreated wood, but not all “treated wood” is created equally. The most common wood treatment today is ACQ (alkaline copper quaternary), which inhibits fungi and insects. Which is good but more is better. Using higher levels of treated wood will increase the life expectancy of deck joists. Non-ground contact is treated with 0.25-PCF, ground contact 0.40-PCF and 0.60-PCF for pilings.

“Pressure treating does make wood rot resistant. But — it doesn’t make wood water resistant.”

by Paul Fisette – © 2005Opens in a new tab.

Using wood treated to 0.40-PCF and for stamped ground contact will increase the joist resistance to rot. Often to achieve this level of treatment will mean the wood will be incised, which will reduce acceptable span lengths a small price to pay for durability. Also, incision joists aren’t as pretty, which may be undesirable for higher decks with the exposed joist. Finally, additional treatment will cost more. All small prices to pay for increase deck life but still an additional cost.

13. Treat End Cuts of Joist

All treated wood needs to be end cut treated. Treatment is only absorbed on the wood’s edges when cut. It exposes untreated wood. Work with a brush or spray bottle of end treatment beside your saw and simply treat ever cut insuring that all wood is protected. A few extra minutes could add years to your joist life.

14. Increase Level of Joist Treatment

I first heard this from Paul JackOpens in a new tab. from Decks Go, treat your joist with anti-freeze. This is the cheapest and most effective way to stop fungi growth. Add a little warm water and boric acid solution to the anti-freeze and brush or roll it over your joist. Making the joist inedible for fungi.

If you don’t like the idea of anti-freeze, you can use end treatment or sealer, but it will cost more then anti-freeze. Either way, increase the level of resistance to fungi growth will increase joist life.

Moving further down on our list of fungi prevention is limiting fungi infection. Fungi is natural, helping our gardens and forest. Often naturally living in the topsoil, helping the soil break down organic material encouraging plant growth. Its all a beautiful and wonderful thing until it breaks down your deck. Knowing that fungi spores are in the soil, we need to limit contact to increase joist life.

15. Landscape Fabric Minimizing Contamination of Joist

Just to be clear, fungi is good in the soil, so we need to keep it there. Spreading landscape fabricOpens in a new tab. over the ground under the joist will prevent soil and fungi from splashing on a low joist in a heavy rainstorm. If the joists are kept clean, without fungi spores, they will not rot. Fungi are like any other plant; without seeds, there are no plants. Keeping the soil down and away from the joist will prevent fungi from growing on the wood.

Top 3 Picks for Stopping Joist Rot

Now, this is hard because I like to incorporate multiple methods to prevent deck joist from rotting but if I had to pick only three, these are my three.

  1. Ventilation
  2. Treat End Cuts of Joist
  3. Membrane Flashing on Joist

I really wish I could select more, but here is why I would pick these three. VentilationOpens in a new tab. is the easiest and natural way to preserve the wood. It is only when wood is in a moist, cool environment that it begins to rot. Adding venting allows moisture to escape allowing the joist to dry, preventing rot. Also, venting has no time limit of value, unlike sealer and preservative which will deteriorate over time. Install a vent today and 50 years from now it will still be drying out the joist.

Treating end cuts to me is like not reclipping the bread bag. If you leave one end open, the entire bag of bread will dry out. You pay for the wood to be treated, and then after cutting it, you have created a hole in the protection allowing fungi to start destroying the joist. By resealing this cut, you have now closed up the joist again against fungi.

Most of the decks that I have rebuilt, it is the top portion of the joist that is rotted. The top section of the joist is where water sits and rot develops. It is the Achilles heel of wood joist. By protecting the top of the joist with membrane you have extended the life of the joist. It is increasing the level of defence at the joist most venerable part.

Especially with composite and PVC deckingOpens in a new tab., this extra layer of protection is worth it. You are buying decking to last 25 or even 50 years if the joist below fails before the decking that would be a huge waste. Better to protect the joist to last as long as the decking if not more. 

Yes, I commonly do these three and more, but these will have the greatest impact on preventing your deck joist from rotting. Keeping water off, allowing the joist to dry, making the wood inedible, all are allowing you to enjoy your deck for many years to come.

Ryan Nickel

A Red Seal carpenter, passionate about building decks to be enjoyed.

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