There are many different types of treated wood. The most common a homeowner will come across is pressure-treated for exterior application. But there are many types of treatment for wood.
Most pressure-treated wood is either treated with a water-based or oil-based treatment.
Most residential pressure-treated wood is copper-based waterborne treatment, including alkaline copper quaternary (ACQ), copper azole (CA) and micronized copper azole (MCA) the three most common. Prior to 2004, Chromated copper arsenate (CCA) was also popular but is now primarily reserved for industrial and harsh marine applications.
Oil-based treatments such as creosote, pentachlorphenol, and copper- and zinc-naphthenate are not as popular despite their effectiveness in preventing rot is limited in residential application. This is largely because of the lumbers oily and sticky appearance from the oil treatment. Limited is to industrial applications like railroad ties and utility poles.
Because my specialty is deck building, I will focus on types of pressure-treated wood used in exterior applications. Answering what type of pressure-treated wood you should use in your exterior backyard projects. From fences to decks and raised garden beds. The right pressure-treated wood will make all the difference.
Before getting too far, let’s define some terms.
What is Pressure-Treated Wood?
To make pressure-treated wood. Wood is placed in a chamber depressurized by vacuuming the air out and replacing it with treatment. This causes the treatment to penetrate the wood deeper, increasing the level and depth of protection.
This differs from other forms of treatment like brushing, spraying or dipping. With limited penetration, these methods should be saved to field application during construction.
For example, I always have end-treatment beside by saw with a brush to treat the lumber after every cut before installing the board. But more on that later.
But for any exterior project, you should buy pressure-treated wood.
Why Should You Use Pressure-Treated Lumber When Building Your Deck?
In a word, longevity. Pressure-treated wood will last longer than untreated wood.
There are 2 things for which exterior wood is treated for.
The treatment makes the wood inedible for insects and fungi, preventing damage. Keeping the strength of the wood for extended years.
Fungi on deck joists will rarely be moss or mushrooms eating up the wood but will be small micro-organisms eating away at the wood. Destroying the strength of the wood but softening in the wood. To the point where you can crumble the wood between your fingers.
Of course, this means that your deck can longer hold the weight it used to. Often the rot will start on the top of the joists under the decking. Unnoticeable from on the deck, but the rot and damage will be more apparent from under the deck.
Pressure-treated wood prevents this decay for decays and fences alike.
Two things that pressure-treated wood is not treated for are moisture and UV damage.
Wood decking still needs to be sealed to prevent the wood from being damaged by excessive water. Regular sealing of wood will seal out excess moisture limiting cell damage in the wood.
Just like your skin can be damaged with excess sun exposure, wood exposed to continuous sun will also experience UV damage. This is why regular staining pressure-treated wood is beneficial. Like sunblock for wood. The stain prevents UV damage to the wood, extending its lifespan.
Sealing or staining are two key ways to extend the life of pressure-treated decking, but there are other ways to prevent joists rot for deck joists. Click the link to learn how to build a better deck that will last longer. Longer than pressure-treated wood alone.
But the key benefit of pressure treating wood is extended life. Saving you time, money and reducing the environmental impact of premature wood-rotting..
What are the Types of Pressure-Treated Lumber?
Now there are many types of treatment for wood. Here is a quick rundown of some of the most popular ones.
Waterborne Wood Treatment
· Chromated Copper Arsenic (CCA). Before 2004 this was the most common treatment with its distinct green colour that many of us remember. Arsenic is what makes it so effective but also why it is no longer used in most residential projects because of its health and environmental impact. But for heavy-duty use and marine applications, it is still used.
- Alkaline copper quat (ACQ). This has largely replaced CCA in deck application. It protects wood both for above-ground and ground contact applications making it ideal for a backyard deck. Formulated of copper-ammonia or copper ethanolamne mixed with various types and percentages of chloride, making the A, B, C and D in labelling.
- Copper azole (CBA). A mix of amine copper and co-biocides. For hard to penetrate wood like Douglas Fir, ammonia may be added.
- MCA (micronized copper azole) and CA (copper azole) – Both copper-based but with a high level of penetration for above-ground, below-ground, and freshwater construction
- Ammoniacal copper zinc arsenate (ACZA). Effective in treating Douglas fir making it common in the Western United States.
- Borate. Is a salt based preservatives. Effective in interior application but will leach out of the wood when exposed to water, therefore is not suitable for deck construction. Leaching also makes it inappropriate for ground contact application.
Oil Based Wood Treatments
- Copper naphthenate
Because of their environmental and possible health effects, oil-based pressure-treated wood are rarely used for in-home projects.
If you want to learn more about water and oil-based pressure-treated wood, go to Preserved Wood which has a handy tool explaining the different treatment and wood uses.
Is Pressure-Treated Lumber as Strong as Untreated Wood?
The chemicals used to pressure-treated lumber do not impact the board’s strength. But the method used to treat the wood may.
Commonly especially with denser wood like Douglas fir the wood will be incised to increase penetration of the chemicals. Increasing the level of wood protection but reducing the board’s strength.
Canadian Wood Council reduces the joists span for incised wood compared to non-incised treated wood.
For example, a 2×10 S-P-F joist not incised can span 15’2″ (4.62m) when spaced 16″ (400mm) on centre. But the same wood, when incised to increase treatment, can only span 14′ (4.26m). A reduction of roughly 8% in span.
But this reduction in strength is not due to the pressure-treating but the mechanical damage done to with wood to increase treatment.
What Are the Grades of Pressure Treated Wood?
I am going to answer this in two different ways. Because I think two questions are being asked in this one question.
One is the levels of treatment, the other appearance of the wood.
Treatment Level of Pressure-Treated Wood
There are three broad levels of pressure-treated wood.
- Above Ground
- Ground Contact
- Marine Lumber
Which American Wood Protection Association further divides into 12 Use Categories.
Above Ground includes UC1 and UC2. By the way, UC in all these categories is an abbreviation for “Use Category.”
Category 1 and 2 are for interior use only. These often include Borate treated wood. They have their advantages for inside your house construction but should never be used outside. For deck building, these are a no, no. Inside only, never outside.
But above ground includes UC3, both A and B. These can be used for deck construction if raised more than 6″ (150mm). The difference between A and B is water runoff.
If your deck is sufficiently sloped, the use of UC3A is acceptable. If there will be poor runoff, use of UC3B is required. Let’s be clear this is poor runoff, not standing water. The water must not be standing on the wood. It is not sufficiently treated for that.
Ground Contact is one Use Category, ground contact but is divided into 3 levels.
- UC4A – General Use
- UC4B – Heavy Duty
- UC4C – Extreme Duty
Between A and B, ACQ treatment increases by 50%. Going from 0.40 ACQ and 0.60 ACQ. Which says a lot about use. If it needs extra protection against rot, UC4B is worth it.
Remember earlier how I said CCA was no longer used for general residential applications. Here is where we start to cross the line. UC4B may be treated with CCA for its high rot resistance properties. But of course, you can still use less harmful treatment for heavy-duty ground contact wood like fence posts.
For Extreme Duty, we are now almost exclusively CCA treated. But if it’s a harsh environment prone to rot and it is critical that the wood does not fail it’s worth the extra safety steps required during construction to prevent rotting and failure.
For a more in-depth conversation about What is the Best Pressure Treated Wood for Ground Contact? Click the link.
The next level up is Marine lumber UC5, again broken into A, B and C. The single biggest difference in use between UC4 and UC5 is salt. If there is salt in the water, you need to upgrade to UC5.
That is oversimplified as the level of salt must coincide with the level of treatment. UC5A is for Northern Water, UC5B Central and UC5C for southern.
But I am a landlock prairie boy, and I don’t build saltwater docks. For more info, you are better off going to Deck and Docks which has a lot more experience supply lumber for saltwater docks.
American Wood Protection Association lists doesn’t end there. It also includes UCFA and UCFB, but these are both about protection against fire. But for decks with the rare exception, water and rot are a bigger concern than fire, and these two categories are irrelevant.
But now for your other question.
Different Grades of Appearance of Pressure-Treated Wood
The first difference of note and structural is incised and non incised wood.
If you use higher wood with higher levels of treatment, it will often be incised. Which does not matter for joists under the deck but can make a world of difference on top of the deck. Incised wood can never be sanded smooth and should not be used for handrails or decking boards. They may last longer, but you will not enjoy the texture.
Also, incised wood is harder, if not impossible, to stain or paint consistently. Again, making it harder to have a good-looking deck.
For appearance parts of the deck, you are better installing non incised wood. It will look better and, with proper maintenance and drainage, will last for years.
But Pressure Treated Wood is also graded both for appearance and structural strength. There are 5 standard grades.
Most construction codes require #2 and Better (2BTR) wood. Which deletes #3 from deck construction. But this is a good thing both in the durability of the deck and appearance. #3 will not look good or last on your deck.
You are often better to use Select or Premium for your deck. They will look better, stain better and last longer than the lower grades of pressure-treated wood.
More Information on Types of Pressure Treated Wood
So, you still have more questions.
Here are a few links to some real in-depth papers about pressure-treated wood.