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Many people assume that all treated lumber can be used interchangeably for various outdoor projects, but this is a misconception. Not all treated lumber is created equal—some types can withstand harsh conditions like constant ground contact or even being submerged in water, while others are far less durable in these environments. Understanding the different grades of treated lumber is crucial for building long-lasting structures, especially those exposed to moisture and the elements.
In this blog post, we’ll explore the two main categories of treated lumber—ground contact and above-ground use—and explain why it’s important to choose the right type for your project. You’ll also learn about the chemicals used in pressure-treated lumber and how they affect its durability.
Key Takeaways:
- Treated lumber falls into two categories: ground contact and above-ground use.
- Ground contact lumber can sit directly on or in the ground without rotting quickly, while above-ground lumber should remain at least 6 inches from the ground.
- The type of chemical treatment in lumber has changed over the years, affecting its durability and cost.
- Lumber is graded by how much chemical retention it has, from 1A to 5C, with different grades suited for specific applications.
- Always check the label on the lumber or consult a specialist to ensure you’re using the right type for your project.
The Shift in Treated Lumber Over Time
For many years, pressure-treated (PT) lumber was highly durable, thanks to the use of chromated copper arsenate (CCA), a chemical solution that contained arsenic. However, due to health concerns, the EPA pushed for the phase-out of CCA-treated lumber around 2009. Since then, most treated lumber for residential use has been treated with other chemicals, such as alkaline copper quaternary (ACQ) and micronized copper azole (MCA).
While these new solutions are safer, they’re also less effective at preserving wood, which means not all treated lumber is equally durable. The amount of treatment a piece of lumber retains after drying is called “chemical retention,” and it determines what the lumber is best suited for. Higher chemical retention makes the lumber more resistant to rot, making it ideal for ground contact or even underwater use.
Lumber Grades and Their Uses
Lumber is graded on a scale from 1A to 5C, and each grade corresponds to a specific use case:
- Category 2: Suitable for indoor use, such as sill plates resting on lightly damp masonry.
- Category 3A: Ideal for exterior structures that are mostly covered, like porch floors or vertical fence pickets.
- Category 3B: Used for outdoor, flat structures like deck boards and porch railings that dry more slowly.
- Category 4A and 4B: For ground contact use, such as fence posts and deck posts. Higher categories like 4B are often used in environments with high moisture or for structural components that are difficult to replace.
- Category 5A and above: Reserved for marine use, such as docks and pilings, especially in saltwater environments.
Simplifying Your Lumber Choices
Though the grading system may seem complex, most manufacturers label treated lumber clearly as “above ground” or “ground contact.” This labeling simplifies the decision-making process for most projects. However, in specialized situations, such as construction in tropical environments or near saltwater, it’s best to consult a local lumber supplier who can guide you to the right product.
Looking Ahead: The Future of Treated Lumber
Some major distributors, like Home Depot, are reportedly moving toward eliminating non-ground contact treated lumber from their inventory. This decision stems from the confusion and warranty issues that arise from using the wrong type of lumber. For now, however, both ground contact and non-ground contact treated lumber are still widely available, so it’s essential to know the difference and make informed choices.
By understanding the differences in treated lumber, builders can ensure they are using the right materials for their specific needs, ultimately creating stronger, longer-lasting structures.
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