A Complete Guide For Treated Lumber

Let’s get educated and discuss pressure treated lumber! All treated lumber is either green, brown, or in some cases, yellow! When you buy treated lumber, it is typically stored inside but can also be stored outside, depending on where you purchase the materials. 

There are also several different grades of treated lumber, ranging from premium, synonymous with number one grade, to many different levels of treatment. 

By the time you finish reading this article, you should have a better general understanding of treated lumber, and who knows, you may even be able to understand what the end tag found on each piece of treated lumber means. 

However, having an in-depth discussion about all the different levels of treated lumber could become confusing and flat-out overwhelming, even for deck builders like us! So, we won’t cover every type and level of pressure-treated lumber here today! 

Possibly the most intriguing part is that we are going to unveil some of the most common misconceptions, miss usage, and slightly dig into some technical aspects of the treating process, chemical compositions, and applications. 

So, if you are here to understand treated lumber, let’s get going!

Pressure Treated Lumber Purchasing Basics

Let’s cover a few basics before we get into the real meat! First, it’s good to understand that most treated lumber is a softwood, typically either longleaf Southern Yellow Pine or Douglas Fir. This kind of wood requires treating to prevent premature rot. So, how well the wood is treated will directly affect how long it will last.  

Not to dismiss the importance of premium treated lumber, but ultimately, all exterior wood substrates require some form of maintenance if you want that deck or porch to last a long time. However, there are a few special ordered heavier-use products available that will last longer. But we will cover that little later within the article. 


Building decks or porches that sit less than 6 – 18 inches from the ground without proper ground contact materials will also cause damages. A deck that would typically last 20 – 30 years could be reduced to a 10 – 15-year total life span if the proper materials are not applied. 

When you buy lumber, it is also vital to select boards with the least wood knots. Also, aim for selecting relatively straight boards and are not cupped or split (also known as checking.) 

We are talking about wood to put things into proper perspective, so some checking and knots are expected. Just make sure you are buying the most handsome lumber on the self. 

Pressure Treated Lumber Dry Time

Adequate dry time for pressure treated lumber is one of the most debated topics among deck and porch builders. However, how long you should wait for treated lumber to cure before priming, painting, or staining is not as controversial as some may think. 

Most general contractors’ skills excel in the actual building process, and sometimes product knowledge is not equally as strong. Therefore, if you ask technical product-related questions, you may not always receive a verified or scientific answer. 

Treated lumber manufacturers generally agree on how long it takes the timber’s core to dry and for copper compounds on the surface of wood substrates to reduce enough for proper paint adhesion.

Furthermore, like most building-related answers, there is an exception to the rule. Kiln tried treated lumber is ready to paint immediately. In fact, since kiln-dried lumber is already cured, it is best to paint or stain it immediately after installation.

Over-saturating kiln-dried lumber before coating could cause the wood to swell, check, and even slit prematurely. So, in wet outdoor weather, kiln-dried lumber could be finicky to install. You may need to cover the structure at the end of the day in more temperate environments until it’s time to paint or stain.  

Remember, wood has memory! Meaning, dried wood wants to remain dry!

Coating Treated Lumber

With no hyperbole intended, this section may be the most important information you receive from this article. If you have a newly installed deck or porch containing non-kiln-dried lumber, it is critical to let the wood dry out for at least 2 – 4 months before coating. 

Dry times will vary based on the humidity of which is influenced by your location and the seasons. Here is a list of reasons why not to coat right away unless you have kiln-dried lumber. 

  • The wood is likely still wet or green. Therefore, the paint or stain will not properly take or absorb. Note: (Lumber stored outdoors before purchasing does not contain adequate exposure.)
  • The chemicals used to treat the lumber are still on the surface and can cause an adverse reaction to paint compounds.
  • (ACQ), copper azole (CA) or micronized copper azole (MCA) These copper compounds are used to treat most lumber and limit acrylic polymers (paint) adhesion. 
  • Coating too early will consequently result in premature blistering and peeling.

Simply put, freshly treated lumber does not like paint. So, if you want the paint or stain to last, wait a few months before your applications. 

Treated Lumber Stats & Technical Meanings

The letters on end tags of treated lumber signify how preserved the lumber is. Simple enough, there are two main categories of treated lumber, above-ground and ground contact. 

Furthermore, there are five main compounds used for lumber treatment, four containing some variation of copper. The firth treatment EL2 is solely used for above-ground applications. 


The descriptions “above and below” may sound a bit self-explanatory, but it gets tricky. Every lumber company or mill has slightly different processes. Thankfully we have the AWPA (American Wood Protection Association) to create standards to level the standards.

When searching for ultra-durable treated lumber, ground contact is the minimum standard. However, there are several grades or types of ground contact lumber, also referred to as UC (Usage Categories.)

UC4 is where the world or premium ground contact begins. Here is a list of ground contact usage categories.

  • UC4A: Capable of getting wet at or below ground levels but mostly stays dry. 
  • UC4B: Intended for heavy/ structural usage in an area subjected to light moisture that needs to last for an extended time. (Crawlspace supports, for example.)
  • UC4C: A premium heavy/ structural alternative with increased moisture resistance.
  • UC5(A, B, & C): Marine Grade (intended for specific constant water exposure and never needs to dry)

Most lumber yards are limited to carrying UC4A. Anything above 4A will likely require ordering. So, make sure you order materials weeks ahead of time. 


Well, there you go! You are now a pressure treated lumber expert! So, go off and get premium lumber where it counts! Or, contact us to do it for you! 

If you need deck building or repairs in Raleigh, NC, or surrounding areas when you are ready, we would love to improve your outdooring living space with you! 

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