Pressure-Treated Grades and Retention Levels Explained

There are five different grades assigned to pressure-treated lumber, dependent on the wood’s quality and appearance.

The nature of a construction or renovation project and your budget will dictate which grade of lumber you will use. So what are the different grades of pressure-treated lumber exactly? Here’s everything you need to know.

The 5 Pressure Treated Lumber Grades

Pressure-treated lumber comes in 5 different grades: Premium, Select, and Number 1, 2, and 3. The higher the grade, the fewer the knots, splits, and general defects. For constructing a backyard deck, Number 2 or higher grade boards are required by code and are preferred by experienced carpenters.

Select structural, or SS, is the highest grade based on durability and strength. Standard grade lumber can be used for joists and other load-bearing applications.

Here’s what to generally expect based on these pressure-treated wood grading standards.

Premium: The highest grade for decking with a ¾ radius edge.

Select: Select graded pressure treated lumber has a fantastic appearance, high consistency and very few defects. All knots must be sound encased, and it needs to meet a 1/12-grain slope at a minimum.

Number 1: It allows for one hole every 3 feet and has no splits that are larger than the board’s width. Knots should be smaller than 2 ¾ inches.

Number 2: Use at least Number 2 pressure-treated lumber for deck construction. One hole is allowed every 2 ft., knots cannot be larger than 3 ½ inches, and the lumber will not have splits larger than 1 ½ times the board’s width.

Number 3: The lowest quality of pressure-treated lumber available. It should not be used to deck projects.

What’s the best-treated wood to use for decks? You’ll be in good shape if you use select, Number 1 or Number 2 grade wood.

How Are Grades Determined?

Many different types of wood are used in lumber, so there is a variety of properties, strengths, and appearances. Different species are used in deck framing and structure framing, for example. 

Grades are used to describe the quality of a type of lumber accurately. Grading inspectors at a lumber mill review the different pieces of wood and separate them based on many different factors, including grain angle, decay, damage, warp, knots, and wane. The wane is when there is bark or a lack of wood fibre along a lumber piece’s edge.

Grades are issued and put on stamps affixed to the lumber with the goal of providing an accurate guide for how the lumber will perform structurally.

This process has been used since 1960 in North America by trained inspectors to identify any defects that can weaken a lumber’s strength, even pressure-treated lumber that is generally more durable than other types of wood.

Additionally, on a lumber stamp, some mills may include an additional letter grade — A, B, C, or D — which notes its overall appearance taking into account the extent of blemishes. The size and the location of knots and other defects, as well as the grain slope, are also taken into account.

AWPA Categories

There are other systems used to classify pressure-treated lumber. The American Wood Preservers’ Association utilizes a 12-tier system to guide wood use. For example, UC1 and UC2 mean a piece of lumber should be used for interior projects, while UC5 is wood that can be safely immersed in saltwater.

The categories are even subdivided to take into account environmental factors. For example, UC5C wood is best used for a Gulf Coat climate, while UC5A can be used in colder waters.

For building decks, UC3 and UC4  are preferred, treated for outdoor use, but with different levels of treatment according to demand.

In simple terms, UC3 are treated for above-ground use. UC4 has roughly twice the amount of treatment and are for ground contact or high moisture area. 

If you want to learn more about the difference between Above Ground and Ground Contact, you should read What is the Best Pressure Treated Wood for Ground Contact?Opens in a new tab. I go into much more detail about what you should look for wood projects in contact with the dirt. Especially low decks.

Retention Levels

In addition to the different grades and AWPA categories, pressure-treated wood retention levels are also used to differentiate types of lumber.

Using chemical preservatives for protection varies based on the different types of wood and how well they accept the pressure-treatment process, especially if the wood allows the chemical preservations to be deeply ingrained.

Retention levels refer to the different amounts of preservatives that are present in the wood after it has been treated. Each type of preservative has its own levels of retention as well. The level on the stamp is code used for the preservative name in addition to the AWPA numbers.

Ryan Nickel

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

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