Different Types of Pressure Treated Wood – 2022 Guide

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different types of pressure treated wood

Pressure treated wood has a lot of varieties, applications and capabilities.

You find it in almost every form of residential construction from support beams and ground-contact studding to decks and patios.

In this article we will take a close look at pressure treated lumber to find out what types are available and which is best for your project.

With the pressure treated wood review we will rate and compare all types of lumber.

Further, we will cover the different types and uses of the lumber and how to select the right options for you and your specific needs.

Best Uses for Pressure Treated Lumber

Pressure treated wood has a lot of uses. Here is where you will find the most applications of the lumber.

  • Decking. Almost all posts, framing and support beams for decks are mostly pressure treated woods.
  • Drywall framing. Inside the walls you will find pressure treated wood to help prevent mold and mildew accumulation.
  • Fencing. Pressure treated lumber is best served outdoors where moisture and ground contact are needed. Fencing is generally always pressure treated.
  • Awnings and overhangs. Eves and even shingles are made from pressure treated woods as well.
  • Docks and ramps. Because there are waterproof and resilient additives in the treatment process, pressure treated wood makes great use when building a boat ramp, dock or even pier.

History of Pressure Treated Wood

Pressure treating wood is a process and system that has been around for over a century. The process is fairly simple, vacuum pressure is applied to lumber and once the moisture and air is removed, chemicals are applied, which get into the wood due to the pressure build up.

Modern design uses large cylinders and can treat hundreds of pounds at a time.

Over time different chemicals have been used, including a form of arsenic. While more modern chemicals are less harmful, you still need to be careful with which chemical treatments 

Types of Pressure Treated Wood Compared

types of pressure treated wood compared

Primarily, you will find that pressure treated wood is categorized by install location. These are above ground applications, ground contact, in-ground and marine use. Let’s take a closer look at the various types and their specific uses.

Above Ground Woods

When looking at the end tag on your lumber choices you will notice a lot of different use category codes, among other identifiers. Putting all of these together will tell you the use expectancy of the wood.

For above ground applications you will use “Use Category 3” lumber. UC3 comes in 2 forms, A and B.

UC3A is lumber treated for and coated against rapid water runoff. This is ideal for uses in wetter climates or near water sources where you need the water to run off quickly. UC3B is above-ground grade but is non-coated and has poor water runoff. 

Ground Contact Woods

Ground contact lumber needs to be coated and treated to not rot or corrode away.

It is also important to note that ground contact lumber doesn’t actually have to touch the ground. It is considered ground contact from surface level to 6-inches off the ground.

The UC4 designation is designed for ground contact and comes in three varieties, A, B and C. As you may guess, A (UC4A) is made for general use and light contact. UC4B is for more heavy duty use with more of the lumber in close proximity and general contact with the ground.

That leaves UC4C which is made for extreme duty. These planks are also designed and rated for in-ground use, which we cover next.

In-Ground/Critical Use Woods

In-ground or critical use is a rare specialty that is needed in very few situations. When it is needed, though, UC4B and C are both optimal choices. The difference will be load, expected use and ground moisture content.

This is generally used for support posts that need to be buried in the ground and in ground contact areas with severe deterioration possibilities. These can also be used where groundwater exposure is expected.

Marine Grade Woods

Saltwater and brackish water will require special treatment. This is found in the UC5 category. Once again you will find three sub categories A, B and C. Each is designed for special types of water separated by region.

UC5A, for example, is used in cold water, docks, piers and the like north of Long Island and San Francisco. B grade is for central waters that are generally warmer year round and C for southern Gulf Coast waterways.

Types of Treatment Chemicals

types of treatment chemicals

Aside from the wood grades, you also need to be concerned with the chemicals the lumber is treated with. There are four major categories of chemicals 

Borate Treatments

Borate is made from sodium salts created with water-based pressure treatment solutions. These types of treatment won’t discolor or stain the wood.

These treatments also help protect the wood from mold and mildew as well as insect infestations.

The biggest downside here is that in wet climates where the wood stays damp. In these situations the chemicals can leach out making the wood more susceptible to the issues the chemicals are included to protect.

Micronized Copper Azole (MCA) Treatments

MCA is one of the newer chemical treatment options that is also a water-based treatment. It is also environmentally friendly, safe for animals and humans and is the biggest alternative to CCA (see below). Just ensure the wood does not contact food or animal feed.

Like Borates, MCA treatments protect against mold, mildew, fungus and rot. However, when using MCA the wood will be dyed a light brown. MCA treatments can be used on all grades of wood from above ground to fresh water contact.

Alkaline Copper Quaternary (ACQ)

ACQ is also environmentally safe preservative that is made from copper and ammonium compounds. Like MCA, it will dye the wood but here, it will be a darker brown tone. Much like the MCA, though, lumber treated with ACQ should not be in contact with human or animal food.

The big downside here is that the lumber will also corrode any unprotected iron, including nails, brackets, fasteners and supports. You will need to purchase ACQ rated nails and fasteners., These are generally dipped galvanized and stainless steel.

Chromated Copper Arsenate (CCA)

CCA is one of the original treatment options and has been discontinued in most instances. Because arsenate is an actual form of arsenic, it poses a risk and danger to humans and animals with long term exposure.

This treatment also turns the wood a green hue. Since 2003, though, an agreement has been made by all lumber industry mills and factories to stop the use of CCA in residential applications. However, CCA treatments are still used for industrial applications. 

Wood Grades of Pressure Treated Woods

Along with the chemicals and wood types, you also need to understand the wood grades. The various grades tell you the structural strength, quality and even the number of knots, splits and defects to expect.

  • Premium. Premium is a term to represent few to zero knots, splits or defects. However, there isn’t a structural grading and is only used by wholesalers and retailers for general acceptance.
  • Select Structural. Labeled as SS on the ID tag. Knot and split requirements are strict and are ideal for large gap spacing.
  • Grade #1. SS1, also #1 or #1 & BTR, is used in all aspects of construction. The knots must be tight enough to not fall out and the splits are smaller than SS grade.
  • Grade #2. Grade #2 is a fallback for any areas where appearance isn’t required. The appearance may be good enough for one side to show, but in general the ratings allow for larger splits and bigger knots.
  • Grade #3. Grade #3 Structural is, as you expect, is lower quality than grade #2 or #1 and is allowed at least 57% clearance of defects. In most cases the grade #3 lumber is only used where it won’t be seen.
  • Construction Grade. This is another name for the SS #1 grade and is used in construction retail stores. Construction grade is commonly used for structural and load bearing uses.
  • Standard Grade. Mostly found in trusses and load-bearing applications where it isn’t seen, Standard grade lumber must be at least 43% clear for the appearance grading system.
  • Utility Grade. Utility grade lumber is rarely ever seen and has many more defects, knots and splits than appearance grade woods. You will find these boards used in framing, bracing or even in the construction of crates and pallets.
  • Appearance Grade. Like Premium, Appearance grade ratings mean nothing to the structural capabilities of the wood. This is used in highly visible application areas such as railings, decking and visible structural uses.

Buyer’s Guide: What to Look For When Buying Pressure Treated Wood

buyers guide pressure treated wood

Now that we know more about lumber grades, treatments types and their uses, let’s find out what is best for you. The following factors should be considered before you make a final purchase to ensure you get the right materials for your project.

Wood Species

There are primary woods from North America that are used for pressure treating. You can identify the wood type on the ID tag if you know what to look for. SYP, for example, stands for Southern Yellow Pine.

This is the most common type of pressure treated lumber and offers unique grain patterns and a strong, dense board.

Douglas fir is the second most common with a stronger cell structure which offers less warp and twisting than pine. Old growth is better at accepting the chemical treatment but is difficult to find and much more expensive.

Expected Usage

The grade and type of lumber you choose will also depend on the expected usage. Structural beams, support posts and framing will require stronger woods treated with ground or in-ground level chemicals.

However, appearance grades are needed for the surface areas that will be seen the most. These have reduced knotting and splits and are generally drier to accept primers and paints more readily.

Wood Sizes

The size of the wood is also important. Along with the type of wood used, certain lengths will require specific treatments.

Pressure treated lumber comes in a few measurements with 1-inch and 5/4-inch being the most common. You will also find larger varieties from 2 to 6 inches thick and widths up to a foot.

Your standard posts are also available in pressure treated lumber including 4×4 and 8×8 inch varieties. When it comes to length you can expect to find a wide selection anywhere from 4 to 16 feet long.

Moisture Content

You will also find the moisture content on the ID tag. This is the amount of moisture in the wood at the time of milling. Green lumber or unseasoned wood has a moisture content higher than 19% and is more susceptible to warping and shrinking.

Look at the ID tag for identifying the moisture levels. GRN is green lumber and above 19%. AD for Air Dried or KD for Kiln Dried and DRY for surface dried which offer moisture content between 16 and 19%.

Stamps and Logos

All lumber must be identified by the agency accredited with the oversight. The largest group is identified as MLB, or Maritime Lumber Bureau based in Canada. Certification for the grade rules that the lumber mills must adhere to.

The ALSC (American Lumber Standard Committee) and the NLGA (National Lumber Grades Authority) are the two most common stamps you will find. Mill grades and logos will also be stamped. You may find a secondary stamp that further identifies the visual grade.

Cost

When it comes to cost, everything mentioned above will come into play. From the size, length and even type of wood used, the biggest cost will come  from planned usage. non-appearance grades will be less expensive because they can have more splits and knots per inch.

However, that doesn’t mean all non-appearance grades are priced equally. Douglas fir, for example, will be more expensive than pine, but is also stronger and more durable around water sources.

FAQs about Types of Pressure Treated Wood

faq different types of pressure treated wood

In this section we will answer the more commonly asked questions about pressure treated lumber. If you have other questions please feel free to use the comment section below the article.

Q. How do I know which pressure treated wood I need?

  1. With the new Use Category system in place you only really need to know if you are using it above ground (more than 6-inches off the surface) or in-ground (6-inches to below surface). UC1 and UC2 are for the former and UC3 and UC4 for the later. 

Q. Will all wood require special fasteners and brackets?

  1. The short answer is no, but unless you are positive which chemicals have been used in the lumber treatment it is best to err on the side of caution. Look for fastener and bracket packaging to have an “Approved for ACQ” logo. This will be safe for all pressure treated wood.

Q. I heard treated wood can corrode aluminum, is this true?

  1. Absolutely. Most treatment chemicals can and will corrode aluminum. Pressure treated wood should never come in direct contact with aluminum, including nails, flashing and trim. If contact is made, corrosion can complete and eat through the aluminum all the way in less than 12 months.

Q. Who makes the rules about treated wood and lumber?

  1. There are several groups that all work together to ensure North American lumber is handled properly. In Canada you will find the NLGA and MLB. They work with the ALSC in the US along with the Internal Building Code and American Wood Protection Association to enforce the Use Category and stamping procedures in all mills.

Q. Why is a lot of pressure treated lumber warped?

  1. Almost all lumber is kiln dried and the removal of moisture in this manner prevents warping. Pressure treated wood is not kiln dried and holds more moisture content. As that moisture evaporates the boards can warp.

Conclusion

Pressure treated wood has many different types. As we have shown you will grade it on appearance, size, wood type, grain and even the chemicals used to treat it.

While there are many options, your main concerns should be where it will be installed, above ground or in contact with the ground. If you are installing above ground UC1 or UC2 grade lumber should be used. UC3 and UC4 for ground contact installs are best.

Hopefully you have a better idea of how to identify your pressure treated wood and what to expect with your purchase. Knowing what you need and what to expect can save your project a lot of money.

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AUTHOR

Nora has more than 5 years experience in the floor covering industry, acquiring vast knowledge about installation and material selection. She now enjoys working as a writer and an interior decorator. Her work has been featured in The Spruce, Homes & Gardens, Southern Living and Real Homes. See full biography here.

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