How To Protect Deck Joists From Rot Caused By Water

Deck joists are the most common and essential parts of your deck’s substructure. 

Holding and supporting the decking of your outdoor oasis. Spaced every 12″ or 16″ and the rare exception 24″ running perpendicular to the decking.

How To Protect Deck Joists From Rot From Water

But the number one reason for failure isn’t incorrect installation, overloading your deck or other scary scenarios but rot.

And this terrifying rot is caused and can be prevented by one thing, water, moisture.

Protect your deck joists from water and excessive moisture, and they will not rot.

It’s that simple.

Everything else doesn’t matter until you solve the moisture problem.

But how to protect your deck joists from water is a little more complicated. One size does not fit all.

So let’s go through a few methods to minimize moisture and water on your deck joists.

Then you will know exactly how to build your deck so that its joist will last. Giving you years of deck pleasure.

But let’s start by explaining the cause of joist rots better.


What Causes Your Deck Joists To Rot?

Rot refers to the decay of matter by either bacteria or fungi.

For decks, the most common cause for rot and decay is fungi that feed on the nutrients in your deck’s materials, weakening the wood’s strength.

Rot-causing fungi need moisture, water to grow and thrive. Remove the presence of moisture, and the rot will stop.

Prevent the joists from getting wet, and the rot will never start.

But the challenge is under your deck moisture is trapped.

Plus, being outside, snow, rain and splashing of water on your deck is inevitable.

Leaving it up to you to build your deck in such a way that the natural water will not rot the joists.

How To Protect Your Deck Joists Water, And Moisture to Minimize Rot

There are a number of ways you can go about protecting your deck joists from water and moisture damage.

Provide Ventilation Under Your Deck

It’s impossible to completely keep your deck’s substructure dry. The rain will come.

But minimizing the amount of trapped moisture or water underneath the deck joist will slow the growth of rot on your deck joists.

A good way to tackle moisture under your deck’s substructure is with good ventilationOpens in a new tab..

How To Protect Deck Joists From Rot From Water

Simply by allowing fresh air to move freely underneath your deck, you can reduce the chances of rot destroying your joists as this will help any moisture or trapped water to dry up, giving rot-causing fungi less time to germinate. 

This can be a simple as leaving tiny gaps between the skirting and rim joists (these only need to be ¾ of an inch wide). 

Or include air vents (grills) in your skirting design. For every 150 square feet of deck, you need a minimum of 1 square foot vent. Ideally, one on each side to allow cross-wind air movement.

For many lower decks, less than 12″ above grade, no skirting may be a better idea. Unless rodents are a concern, than “open” skirting like lattice that allows moisture to escape is a good option.

Allow Water To Run Off Your Deck

Stagnant water allowed to collect on your deck is a major cause of rot for the joists underneath.

One solution is to slope the deck slightly so water naturally runs off the joists and your deck.

The idea of a sloped deck may be disturbing. But a deck grade of 1/8″ over a foot is barely noticeable on a deck but is enough to encourage water to run off.

Adding an overhang for your decking is also a great feature that can help protect your joists from water.

The water will run off the edge of your decking, falling to the ground away from the rim joist.

Decking cut flush increases the potential that water run down the edge of the deck, soaking into the wood as it runs down.

Add Landscape Fabric Underneath The Deck

In order for rot to occur, the rot-causing fungi need a good source of food and water in order to thrive. To help limit this fungi’s access to both, you should add a layer of landscape fabric underneath the deck. 

This fabric will help limit the number of plants that can grow underneath your deck and limit the sources of fungi that can get on the joist from the soil..

By limiting water and soil from splashing on the joists as it hits the ground

There are plenty of benefits to adding landscape fabric under your decking, and protecting your deck joists from rot is one of them. 

Note: Do not be tempted to use plastic instead of landscape fabric. Plastic will only help water to remain stagnant and stay trapped underneath your deck rather than escaping.

As this can actually increase, the chances of your deck joists rotting rather than protecting them. By water pooling on the plastic under the joists.

Use Joist Tape

Joist tape is similar to regular duct tape but is designed to act as a barrier between any water and moisture dripping from the deck boards above. Good quality joist membrane (tape) will also seal around the screws preventing water penetration into the joists.

Installation is simple.

Once your joists are installed, you stick down a long layer of joist tape over the top edge of your joists – acting like a mini roof for the joists shielding them from water.

We go more in-depth about the importance of joist tape in another article. Answering about how joist tape works and why it’s necessaryOpens in a new tab. for decks. Click the link to learn more.

Consider The Materials Of Your Deck Boards

What type of decking you install will have an impact on your joists. 

For example, if you opt for a traditional wooden deck, the wood will absorb moisture and water during rainfall or snow.

This water will eventually wick down to your deck joists and increase the chances of rot growing because of the trapped moisture

On the other hand, PVC and composite decking are resistant to water and moistureOpens in a new tab.. Thus limiting how much water is held on the joist by the decking.

Although water and moisture can still drain down to the joists through the gaps between the boards, this type of deck will provide more protection and cover for the joists underneath compared to a wooden deck. 

This doesn’t mean you should never go with traditional wooden boards for your deck – but it’s a factor you should consider before making a final decision. 

Scalloped Composite Deck Boards

Scalloping the underside of composite decking is often done to reduce costs. So this is not an official recommendation but something to be considered.

Scallop boards have less contact with the joists with smaller contact points. Which reduces the area under the decking where water can become trapped.

The small channels under the decking also allow air to dry out the top of the joists. Like ventilation for the underside of the decking.

Not official, but could it help protect your joists while saving you money on your decking?

Preventing Fungi from Destroying Wet Joists

Now we can’t always prevent water from getting on the joists. So additional steps to prevent rot may be required.

Use Treated Wood

Regardless of how much water your deck will be exposed to, using pressure-treated wood for deck joists is essentialOpens in a new tab..

Most treatments are not designed to protect the wood from moisture, but they do make the wood inedible to fungi and insects (termites). Preventing rot.

The level of treatment does vary. The higher level of treatment, the more resistant to rot. Often the easiest way to distinguish is if the wood is labelled.

  • Above Grade
  • Ground Contact

Ground contact will be more resistant to rot than above grade (sometimes labelled Appearance). I moisture is going to be a problem installing ground contact joists even if they are not close to the ground can be a good idea.

The decay fungi that cause rot cannot survive in the wood even when moisture is present. Improving the lifespan of your deck joists. 

Seal Joists With Water-Repellent Wood Sealer

While using treated wood will help prevent decay and rot-causing fungi from eating away at your deck joists, they won’t do much to actually prevent water and moisture from seeping into your deck joists. 

As a result, sealing your deck joists will repel water off the wood.

Most wood sealers offer protection against water and moisture for up to several years, which can be a huge help when trying to extend the life of your deck joists.

Not all wood sealer is created equal, look for sealer that is designed to last for years, including a warranty to back it up.

This is best done before you install the joists. Or at least before installing the decking and blocking. For you can coat the entire joist.

This will add an additional step to your deck-building process. But if it saves you from rebuilding your deck, it will be worth it. 

Add Treatment To The Cut Ends Of Your Joists 

Your joists will be cut while building your deck, but this creates a problem when it comes to rot and moisture.

Even when using treated lumber, cutting your joists down to size will expose untreated wood within the core of each joist – and these ends need to be treated to help protect your joists! 

So, when your joists are installed and cut to length, it’s important that you take a few minutes to just add some additional treatment to the exposed edges. 

Stopping rot from starting at the end of the joist.

Final Thoughts 

Nothing will last forever.

But the above will help to minimize rot and extend the life of your deck joists.

Hope this helps. Enjoy your deck.  

Ryan Nickel

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

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