What is the Best Pressure Treated Wood for Ground Contact?

When every building a wood project like a deck that is close to or in contact with the ground using the right wood is essential to prevent premature rotting.

The best pressure-treated wood for ground contact or within 6″ (150mm) is wood classified as UC4A or UC4B. Often lumber companies will make this easy by tagging the wood as treated for ground contact. Treated at a level of protection to survive the increased fungi and moisture of contact with the ground.

That’s the simple answer, but why is looking for ground-contacting stamp of value when building a low ground level deck?

Why does the wood need to be treated for ground contact, and what other levels of treatment are available?

Following, I will answer precisely that and other wood questions related to building a low-level deck or wood joists in contact with the ground, along with the different levels of wood treatment.

Keep reading to learn more about the best pressure-treated wood for ground contact. 

What Is Pressure Treated Wood?

The term “pressure treated wood” simply refers to wood that has been chemical treatment to protect it against rot and pests. Reducing decay making your wood last longer outdoors and in contact with the ground.

The chemicals are the treatment. The pressure is the means.

Pressure-treated wood is treated under pressureOpens in a new tab. to increase the penetration and absorption of the chemicals.

Mechanical incisions can also be used to increase the amount and depth of chemical penetration. Incision is often used for ground contact wood but not exclusively. For denser wood, like hemlock incision is needed even for lower levels of treatment.

Regardless of the treatment, the more important labelling is level. Pressure-treated wood is classified for different applications.

Levels of Pressure Treated Wood  

The American Wood Protection AssociationOpens in a new tab. has standardized pressure-treated wood into 12 different classifications in the 1990s. They exist on a Use Category system, which is why the levels start with “UC.” The levels include:

  • UC1
  • UC2
  • UC3A
  • UC3B
  • UC4A
  • UC4B
  • UC4C
  • UC5A
  • UC5B
  • UC5C
  • UCFA
  • UCFB  

Use Categories 1 and 2 are for interior use only. Regardless of if you plan to paint or stain your deck, using this wood will prematurely rot. More on that later

Use Categories 3-4 are for exterior use. Good choices for deck building.

Number 3 is “appearance grade,” AKA can be used outside but with only minimal treatment. Not to be used in contact with the ground or even within 6″ (150mm), which is considered ground contact.

Number 4 is the one we are looking for here for low decks, high moisture areas or ground contact. If it touches the dirt, you want 4 or better stamped for ground contact.

Use Category 5, labelled for marine environment. With a very high level of chemical treatment. Wood for saltwater decks and other places where their high level of both moisture and corrosive elements.

A, B, and C are levels of the treatment within the use category.

Ground contact wood levels

  • UC4A = Ground Contact, General Use
  • UC4B = Ground Contact, Heavy Duty
  • UC4C = Ground Contact, Extreme Duty

Yes, not all ground contact is the same. Dry ground requires less treatment. But more importantly is how the wood will be use. The more extreme the use, “duty,” the higher the treatment level it should have.

Just in case you are curious like I was about the “F.” “F” is for fire treatment. Similar to rot treatment but not as important for most exterior projects.   

What is Ground Contact Wood Treated With?

There are three common types of chemical treatment for ground contact wood.

  • MCA (micronized copper azole)
  • CA (copper azole) 
  • ACQ (alkaline copper quaternary) 

Source: US Forest Products LaboratoryOpens in a new tab.

All three are copper-based. The biggest difference with ACQ is the addition of ammonium, making the wood less edible for fungi and insects, increasing its resistance to rot.

For wood needing protection against higher levels of moisture, fungi or insects, CCA (chromated copper arsenate) is used. Still copper-based but with the addition of arsenic. Yes, Arsenic, highly effective as wood treatment but with many adverse health effects like cancer; therefore is not approved or recommended for residential application. 

CCA is used in UC4B and UC4C ground contact wood. Which is for heavy and extreme duty, application. For UC4B the other two treatment are also used allowing for high level of protection without the negative consequences of arsenic.

Most ground contact wood will be treated with MCA (micronized copper azole) or CA (copper azole). Safe to work with but still protecting the wood.

Difference Between Ground Contact and Above Ground Pressure-Treated Wood

More treatment is the biggest difference. Both are treated with the same chemicals, but ground contact is treated more. Generally, twice as much as above-grade exterior wood.

Which makes sense. The most significant difference between being outside and being outside in the dirt is increased moisture levels, fungi and insects. These increased levels need added levels of protection.

“Ground contact shall be taken to mean permanent and direct contact with soil. Lumber treated for ground contact has a higher preservative retention level than above ground. This is to improve the performance of the treated wood when it comes in direct contact with the ground (soil), fresh water, high moisture areas, etc., where it is highly vulnerable to deterioration.”

MicroprosiennaOpens in a new tab.

This increase treatment can be achieved by increasing pressure or time being treated or by incising the wood for more treatment is absorbed. Incision also increases the depth of penetration.

Be careful always to check labeling if it is treated for ground contact. Not all wood that is incised is ground contact.

Wood Stamped for Ground Contact

I became aware of this at my Home Depot. All their 2×8 and 2×10 pressure-treated wood is incised, but when I inspected the label closer discovered that it was not treated for ground contact. It’s not the incision that makes it ground contact but the amount of treatment.

Is It Better to Use Ground Contact Wood for a Deck’s Joists?

Using ground contact pressure treated wood even when the deck’s substructure is not in contact with the ground can increase the life of the deck. The additional treatment increases the level of protection, minimizing the chance of the joists rotting. Increasing the deck’s life and decreasing the likelihood of needing to repair rotten joists.

Which is one of my recommendations for preventing deck joists from rottingOpens in a new tab., use joists rated for ground contact. Not my top pick, but it is a very cost-effective way to minimize rot. Regardless of the height of the deck.

Best Pressure Treated Ground Contact Wood for Backyard Deck Building

The best treated lumber levels are UC4A and UC4B since the AWPA designates them for ground contact service conditions. 

UC4A is typically used to construct fences and decks, making it an ideal treated wood for building a backyard deck that will contact the ground. UC4A works in all-weather environments, including freshwater, which is why I recommended it no matter where you live (so long as it won’t face extreme exposure). 

UC4B, on the other hand, is ideal for building permanent wood foundations. Again, I think this makes UC4B an excellent choice for building backyard decks. This wood level is slightly more durable than UC4A and can even endure saltwater in some cases.

UC4C has an even higher level of treatment but beyond what you will need in most backyard applications. Leave UC4C for docks and saltwater exposure.

Benefits of UC4A and UC4B Wood Treated for Ground Contact

I find that the benefits of these two lumber classes make them ideal for backyard conditions in most US and Canadian regions. As long as you are not by the ocean. UC4A and UC4B have the following perks:

  • Protection from wood rot
  • Protection from burrowing pests like insects
  • Protection from fungal build-up
  • Protection from decay processes
  • Increased longevity 
  • Less maintenance

Additionally, one thing I really like about these two treated wood types is that they are environmentally friendly. They’re designed to prevent their chemical treatment from leaking out into the surrounding ground and environment.

This has been a major change from the older versions of pressure-treated wood. Remember that green wood your Dad built with? Yes, it lasted a long time but not so good for the environment.

Decks built with UC4A and UC4B treated lumber can still be stained or painted. Allowing you to add your personal touch or shift the colour to compliment your house.

For decking, I would highly recommend that you do stain it. Pressure-treated wood, even ground contact wood, is treated against fungi and insects, not water. Sealing the wood along with protecting it against UV will maintain the decking’s integrity and appearance.

And don’t worry, the stain does not diminish the treatment it only adds to the level of protection. Plus, a little added colour.

Things to Note About Ground Contact Wood

There are a few things to keep in mind when working with this pressure-treated wood:

  • Never burn pressure-treated wood since it has a chemical treatment
  • Wear protective gloves, work glasses, and face masks when cutting and building with it
  • Always thoroughly clean your skin after working with ground contact wood

Conclusion About Best Wood For Low Wood Outdoor Projects

You may be wondering what the best pressure-treated wood for your outdoor DIY project is. Especially if it’s close to the ground. Look for wood label “GROUND CONTACT,” or it may be labelled asUC4A and UC4B levels of treated wood, as designated by the American Wood Protection Association. Then you will know that the wood will last. So regardless of if you are building a deck, a fence or a garden box, your hard work will last for years to come.

Ryan Nickel

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

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