Different Types of Pressure Treated Wood

Image of treated post cap, so what are the different types of pressure treated wood?Pressure treated wood is reputable in the woodworking industry. Well, it has been used over the years, thanks to its ability to repel insects. These insects play a massive role in the destruction of wood. In the last century or so, different chemicals have been used in the pressure treatment of wood, with the less toxic ones being used recently. So what are the different types of pressure treated wood?

There are many different types of pressure treated wood.

The pressure treated lumber boasts exceptional environmental qualities as far as carbon footprint, sustainability, and low-energy generation are concerned. Its life-cycle assessment is a testament to that. We have three major categories of treated lumber that are used in the construction industry. They are the Borate, Alkaline Copper Quat abbreviated as ACQ plus other treatments that resist rot, and finally the non-combustible (Non-Com). 

In the 20th century, the chemical Chromated Copper Arsenate (CCA) was used to treat wood. This compound was banned in the early 2000s following concerns that it poses devastating health risks, especially when inhaled. Less toxic compounds are in use today and are doing an excellent job offering protection to the wood.

Why Is the Type of Wood Treatment Important?

There are misconceptions all over regarding treated lumber; virtually everyone associates treated wood with toxicity. It is not necessarily true that treated woods are all dangerous, considering that each kind comes with its unique properties and applications. If you are looking to install a safe, durable, and resistant deck or dock, you must understand the differences in types of pressure-treated lumber. 

A practical example is your location. If you live in areas with salty waters, then using a treated wood meant for freshwater only would be a wrong move. Using a freshwater-only treated wood spells a catastrophe within a short duration, unlike when you use suitable lumber for such conditions.

As a homeowner, you need to stay up to speed with different categories of pressure treated lumber.

The Active Ingredients In Pressure Treated Wood

In almost all cases, there’s a use of acronym in the treated wood. They show the type of chemicals used in the treatment.

One way of protecting the woods in the harsh marine environments, the wood is pressure treated using a combination of water and preservatives that help keep the destructive pests away and reduce the speed of decay. In this section, I’m going to discuss some of the most common treatments for marine wood.

ACQ (alkaline copper quaternary) – This chemical has a quaternary compound that includes copper and ammonium. ACQ offers strong repellency against fungi, insects, and other marine animals that are out to damage the wood.

MCA (micronized copper azole) and CA (copper azole) are a Copper-based preservatives best for above and below-ground and used in freshwater conditions. The chemicals are good at penetrating deep into the wood, preventing rot due to water and pests in the process.

CCA (chromated copper arsenate) – This chemical combines chromium, copper, and arsenic that offers maximum protection. A lot of precautions must be in place when using woods treated with this chemical. The CCA is approved only for marine construction and must not be used for residential projects such as wood decks, fences, outdoor furniture, and a lot more.

Note: All of these chemical treatments of wood are water based. Water based treatments allow for complete penetration of the compounds into the wood. Therefore, you can apply paint, stains, and other finishes.

The Advantages of Using Pressure Treated Wood

There’s no denying how invaluable wood has been a building material over the past century and beyond. Unfortunately, wood is prone to decay, and this factor can limit its use. Much as there are other building materials such as concrete, aluminum, or steel, they tend to be expensive. You can limit the dangers of the destructive fungi by using pressure treated wood. Here are some of the advantages of using pressure treated lumber.

  • Resistant to Moisture

Wood in its natural state, when left in moist or wet environments, will soften very fast, which lets the fungus, bacteria, and other microorganisms invade it leading to wood’s decomposition. A pressure treated wood uses one of many compositions based on copper- a natural biocide that protects it from fungus and other microorganisms present in the moist areas.

Following pressure treatment, you can use your lumber in various applications, like decks that are constantly exposed to damaging elements.

  • Offers Resistance to Fungi

Most fungi are sensitive to moisture and would attach themselves to wood with the slightest chance. Once they embed into the wood, these creatures grow and multiply to promote decomposition. The more the wood continues to weaken, the more creatures prey on it, accelerating the decay. However, treated lumber has copper compounds like alkaline copper quaternary or copper azole that inhibit fungi growth. 

The ability of pressure treatment to protect your wood from fungi and such like organisms allows it to be used in applications where it will be constantly moist.

  • Repels Insects

As a wooden structure owner, you must have encountered problems with insects such as termites and carpenter ants; these creatures can destroy the wood in no time. These insects find it easy to bore through the lumber, causing structural complications in the process. In fact, the structure can crumble when exposed to moisture.

The copper compounds are available in pressure treated wood offer protection by repelling these insects. With the additional chemicals like borate, you can rest assured of total protection against these insects.

Sometimes, you may choose to mix both treated and untreated wood in the construction of a structure. If you can effectively lay out the treated wood, they can help protect the untreated woods, especially if the insects have to go through the treated lumber before accessing the untreated ones. The chemicals further scare away other insects such as spiders and cockroaches.

  • Ability to Resist Firepressure treated wood resists fire

Wood and fire are a friendly combination, and fire is arguably a wooden structure owner’s worst nightmare. During pressure treatment of wood, it is possible to add fire-retardant chemicals into the wood. This way, you will end up with a wooden structure that will resist fire damage. If you reside or looking to have a wooden structure in areas with increased fire risks, then having this kind of treated lumber would be your best shot.

  • Affordable in the Long Run

Pressure-treated wood is expensive compared to the natural and non treated ones. This aspect alone can put off many people, especially those looking to use wood in bulk for different construction projects. Even so, the treated timber stands a chance to last longer, especially in the areas prone to water, microorganisms, fungal growth, and damage by insects. 

If you compare pressure treated wood with other construction alternatives such as aluminum, steel, or concrete, you’ll find the use of treated lumber as a cheap yet reliable option. Additionally, you get so much flexibility with treated wood because you can use them for interior and exterior projects. Materials such as steel will likely corrode in the exterior environment unless offered enhanced protection through the coating.

  • Durability

If you put natural and pressure treated wood in the same environment with moisture and other destructive elements, you will realize that the treated lumber lasts longer. 

The ability of treated wood to resist insects and other fungi species outlast that of natural wood. Why is it so? Well, the treated wood is a little denser than its untreated counterpart.

Types of Pressure Treated Wood

Pressure treated wood is categorized into different types depending on their “level of retention,” in other words, the amount of preservative retained in the wood after treatment. This retention level is quantified in pounds of chemical per cubic foot of lumber(pcf). The longer the wood stays in the pressure chamber, the higher the preservatives’ content infused into and retained inside the wood.


Pressure treatment of wood does not make it stronger or weaker but more resistant to insect and moisture damage. The amount of chemicals retained by the wood determines how resistant the lumber will be.

Image of wolmanized wood, but Is Wolmanized Wood the Same as Pressure Treated?The American Wood Protection Association (AWPA) has the standards for retention levels needed for the various chemical preservative compositions for specific uses. 

The end-tag on lumber shows how well the wood will perform in different conditions. There are up to six distinct types of pressure treatment ratings. Each type identifies the quantity of chemical retention, determining how or where to best use the wood.

  • Above Ground

If you are working on the above-ground projects, then the best choice of pressure-treated wood is the UC3A. You must ensure that the wood is at least 6-inches above the ground; this allows for good air circulation and drainage and, above all, makes it easier to replace the components.

  • Ground Contact

Ground contact refers to anything that is in contact with the ground, also anything within 6-inches of the ground, grass, or wet areas. It is okay if it has poor drainage as well as air circulation or a sill plate. Usually, it is hard to repair or maintain. Pressure treated lumber used for ground contact usually has twice the chemical retention level above ground rated wood. UC3B or UC4A are perfect for ground contact applications.

  • In-Ground Contact for Critical Use

Pressure-treated lumber for in-ground or critical use comes with a greater retention level than the one in the above or ground contact. I advise that you use in-ground type lumber in areas with a greater risk of deterioration or critical structural uses, such as the foundation posts exposed to fresh or groundwater.

In the above case, the best pressure treated wood should be the UC4B and UC4C; UC4B comes with a higher retention level than the lower graded types. The UC4C has even more retention levels.

  • Marine Grade

Wood treated using water-based chemicals for saltwater immersion comes with significantly higher retention levels than the other categories. The UC5A contains almost double the weight of preservatives as the UC4C. The UC5B and UC5C have more than three times the amount present in the UC4C.

How Long Will Pressure Treated Lumber Last in the Ground?

According to the Forest Products Laboratory, pressure treated wood pieces placed inside the ground can last up to 40 years or more without rotting.

Causes of Rot In Pressure Treated Wood

Much as pressure treated wood provides resistance to many damaging elements, it can rot, especially if it is not cared for in the best ways possible. So what are the causes of rot in pressure treated wood?

  • Galvanic Rot

Eagle claw deck screw for pressure treated woodIn case you have noticed your deck boards rotting, then galvanic rot could be one of the reasons for this occurrence. The galvanic rock occurs when galvanized deck screws or nails are driven inside the pressure treated lumber.

The preservatives in the wood react with the nails, which results in oxidation. This oxidation reaction makes the wood corrode. You can, however, fix this corrosion issue through the removal and replacement of the affected deck boards, a move that helps prevent the spreading of the rot.

  • Water

Water could spell another reason for your pressure treated wood to rot. During the construction of wooden decks using pressure treated wood, a few homeowners or contractors pay close attention to the wood’s surface. You will likely realize that some sections appear bowed or as the center rises. It is also possible that some parts cup and sag. 

Failure to set all the woods similarly leads to the unevenness of the surface, an effect that usually goes unnoticeable and can be dangerous. Cupped surfaces of wood tend to crack; these cracks let water penetrate the wood, which results in rot from within.

  • Fungal Infestations

Infestation by fungi is another primary reason for the rotting of pressure treated wood. Fungi, tiny organisms, get into the wood and multiply; they feed on the wood and their continued presence results in the rotting.

How to Prevent Pressure Treated Wood from Rotting

We already know that when subjected to unfavorable conditions, even the much resistant pressure treated wood is not safe from rotting. So how can you prevent pressure treated wood from rotting?

Here are some of the best ways which you can use to prevent rotting of your pressure treated wood regardless of when it’s used.

  • Stain the Wood

Image of ready seal, Best Exterior Stain for Cedar SidingOver the years, staining has proven a reliable means to help prolong the life of pressure treated wood. Stains come with features that resist environmental aspects such as water, UV, and even fungi which are agents of deterioration of wood.

There are a lot of stains for pressure treated wood that you can use in the market. A perfect example is Ready Seal; this formula is more like a jackpot because it is both a stain and sealer in one.

Stains such as the ready seal are available in semi-transparent choice, which allows the wood grain to show hence protects and preserves the natural beauty of lumber.

  • Construct a Sloppy Surface

If you are constructing an outdoor structure with pressure treated wood, you should build it in a way that it can shed water. Porches and window sills, for instance, need to be sloped to shed water. If water collects in your structure and does not slide off, it will cause weakening of the surface and rotting in the long run.

  • Plug Your Fasteners

Leaving your deck screws and nails would lead to water collection. You must use stainless steel or galvanized nails and have them appropriately plugged. Proper fastening ensures that there is no room for rusting, which plays a role in the wood’s rotting.

  • Ensure Air Flow

With proper airflow around your structure created using pressure treated wood, you will reduce rotting chances. Airflow ensures that the surfaces are always dry, which reduces the chances of rotting.

Do You Need to Sand Pressure Treated Wood Before Staining?

Yes. All wood, including pressure treated wood, except the manufactured products like composite wood, need to be sanded before staining. 

There are different sanders that you can use to roughen your treated wood’s surface, depending on the amount of work it will involve. If you are working on larger surfaces, you need large sanders such as drum sanders. If you move from one job site to the next, then go for the cordless random orbital sander.


Pressure treated wood is a valuable construction component, and unlike their untreated counterparts, they offer more resistance to the damaging elements such as fungi and other insects. However, many people still find it hard to differentiate the types of pressure treated wood. So, 

What are the Different Types of Pressure Treated Wood?

Well, as we have stated before, pressure treated wood is grouped into different types depending on their “level of retention,” in other words, the amount of chemical preservative retained in the wood following its treatment.

Did you find this article helpful for your quest of understanding different types of pressure treated wood? Do you have a question that you want to add to this content? Any questions? Kindly let me know your thoughts in the comment section below.

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Tyron Otieno

Tyron is an avid woodworker and writer. He founded this website to help other woodworkers, whether hobbyists or professionals by sharing his knowledge and experiencie after a decade of woodworking.