Can You Use Untreated Wood for Deck Joists?

One of the most critical aspects of building a deck is the materials you use. 

While a lot of people focus on design and planning, some overlook the quality of the lumber they use for their deck’s beams, joists, and frame – and this can lead to some serious problems later down the line. 

Can You Use Deck Joists On Untreated Wood?

Here, we will be looking at untreated wood and whether or not it can be used for deck joists and other important parts of your deck’s frame.

This way, you can work out which is the best type of lumber for your deck! 

What Is Treated And Untreated Wood? 

First, let’s discuss what ‘treated’ and ‘untreated’ wood actually means. 

‘Treated’ wood is the term used for wood that has been altered by undergoing treatments and processes which help increase its life.

As a result, it’s better for prolonged use in buildings because the treatments help preserve the quality of the wood in many different ways. 

There are lots of different types of treatments used to preserve wooden lumber, and each one has a different issue it tries to tackle.

  • Chromated Copper Arsenic (CCA).
  • Alkaline copper quat (ACQ).
  • Copper azole (CBA). 
  • MCA (micronized copper azole) and CA (copper azole) –
  • Ammoniacal copper zinc arsenate (ACZA). 
  • Borate.

To learn more about each wood treatment optionOpens in a new tab., click the link.

The most popular types of applications and preservatives aim to help protect the wood from issues related to: 

  • structural degradation and deterioration, 
  • rot and decay fungi, 
  • pests such as termites or marine organisms for underwater applications 

All threats to your outdoor deck.

‘Untreated’ wood is wood that has not undergone any applications or treatments with preservatives. Left in its natural form but lacking in protection against the elements.

As a result, most builds prefer to use treated wood as it means that their builds (in this case, decks) last for longer with much fewer issues. 

Can You Build A Deck With Untreated Wood? 

It is possible to build a deck entirely with untreated wood. This means that the beam, frame, joists, boards, and railing can all be made from untreated wood if that is what you desire.

Because untreated wood is so much more affordable than treated wood, a lot of people think this is a great way to save money – but in the long run, it’s definitely not. 

Untreated wood begins to rot within the first or second year of use, depending on different environmental factors.

On average, a deck made from untreated wood will only last 5 years before it will need rebuilding due to structural damage and rot – whereas a deck made from treated wood can last 25 years or more. 

The closer to the ground it is the faster it will rot.

So, in the long term, a deck made from treated wood is the more affordable option.

Although it may cost you more to build, you will spend way less on treatments and rebuilding the deck when it quickly begins to rot.  

There are other benefits to using treated wood as opposed to untreated wood.

For example, using treated wood results in lower demand for timber and lumber, so fewer trees are cut down to meet that demand. As a result, using treated wood is more environmentally friendly. 

This means that although it is possible to build deck joists using untreated wood, it’s generally recommended that you use treated wood instead. 

On the other hand, untreated wood does have its advantages. It is more affordable and easier to source than treated wood.

High demands and price increases are not uncommon for the chemicals used to treat wood, which can lead to shortages in supplies.

Plus, many people hoping to build their own deck on a budget will see untreated wood as the far more affordable option – but, as we mentioned earlier, the long-term cost of untreated wood is definitely more expensive than treated wood. 

Types Of Treated Lumber For Your Decking

Types Of Treated Lumber For Your Decking

So, most people use treated wood to build the deck joists for their decking (along with other important parts of the frame), but what type of treated wood should you use? 

The most common type of treated lumber for deck joists is pressure-treated lumber. This type of wood is treated with different chemicals while under pressure.

It’s the most affordable type of treated wood. Other types of treated wood include surface-coated wood, where the preservative chemicals have been either ‘painted’ or sprayed onto the wood. 

Treated Wood Categories For Decks

Each type of treated wood falls into a different category depending on the chemicals used to treat the wood.

These categories are used to help builders know what type of treated wood is best to use for different builds.

For example, treated wood that falls under the UC1 category is designed for interior dry use. So, what types of treated wood should you use for your decking? 

Most decks are made using either UC3 or UC4 treated wood. 

UC3 is classified as wood used for above-ground exterior construction. This makes it perfect for decks. The UC3 is split into two further groups, UC3A and UC3B.

UC3A is wood that is coated and best treated for rapid water runoff, and this makes it the most common type of treated wood for sheltered deck and porch flooring.

UC3B, on the other hand, is uncoated and thus is poor for water runoff. UC3B wood is mainly used for parts of your deck’s stairs, railing, and boards.

The UC4 group is also used to group treated wood that can be used for exterior construction but for ground contact with fresh water.

It’s also split into two subgroups, UC4A and UC4B. UC4A is for general use while the UC4B group is for heavy-duty use, such as permanent wood foundations for buildings. 

UC4A is the most common type of treated wood used for parts of your deck that come in contact with the ground, although you can use UC4B too.

So, UC4A is used for parts of your deck’s straits, the ledger, posts, and beams. 

So – what about the joists? 

Best Treated Wood For Deck Joists

The best type of treated wood for deck joists depends on the type of deck joist you have.

For low decks with flush beams where the joists come in contact with (or are very close to) the ground, you will need to use a category UC4A or higher type of treated wood.

This is to help protect the wood as it comes into contact with moisture from the ground.

For joists with a drop beam, you can get away with using a UC3B or higher category of treated wood. This is because your joists are well above the ground and so, don’t require that additional level of treatment. 

So, what type of treated wood you should use for your deck joists depends on how close they sit to the ground. 

Final Thoughts

So, can you use untreated wood to make your deck joists? 

Although it’s possible, it’s definitely not recommended – otherwise, you will have to replace them with entirely new ones in just a few years!

By using treated wood, your deck joists will be better protected against rot, pests, and more issues that can reduce the life cycle of your deck.

So, it’s the more affordable option in the long term and is the safer option too. 

So, use the guide above to find out what type of treated wood you need to use for your deck joists, and good luck! 

Ryan Nickel

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

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