Key Takeaways
- Pressure treated lumber is infused with chemicals that make it resistant to decay, fungus and insects.
- Building codes determine the type of lumber and the treatment level necessary for many applications.
- Above-ground pressure treated lumber is for use at a minimum of 6 inches above the ground.
- Ground-contact pressure treated lumber is typically for applications touching or within 6 inches of the ground, structural uses or applications with frequent moisture exposure.
- Green lumber has a high moisture content, while kiln-dried lumber is less vulnerable to shrinking and warping.
- A lumber grade indicates the number of defects a board has.
- Use category ratings indicate the level of chemical retention, with higher ratings indicating more protection from decay, fungus, moisture and insects.
What Is Pressure-Treated Lumber?
Pressure-treated lumber is wood infused with chemicals that give it resistance to decay, fungus and insects. Treated wood is typically for outdoor projects only, where its properties make it a popular choice for building decks, fences, gazebos and other exterior structures.
Types of Chemical Treatments for Pressure Treated Wood
Pressure treated lumber for home use is typically infused with copper-based preservatives, such as:
- Micronized copper azole (MCA)
- Alkaline copper quaternary (ACQ)
- Copper azole (CA)
These compounds give treated lumber resistance to insects, fungus and decay, as well as a level of moisture resistance. However, it’s necessary to apply wood preservative to the cut ends of the lumber, and periodic staining and sealing are necessary to give treated lumber long-lasting protection against ultraviolet rays and water damage.
Pressure treated lumber requires fasteners, braces and other hardware that have corrosion-resistant finishes, such as hot-dipped galvanized finishes, or materials that are made of stainless steel. Make sure any screws, nails and hardware is labeled for use with pressure treated wood.
The Two Types of Pressure Treated Wood
There are two primary types of treated wood common for home use: above-ground pressure treated lumber and ground-contact pressure treated lumber. Each has specific uses, but your local building codes are the final authority on which lumber is acceptable for different applications. Follow the codes for your area.
Above-Ground Pressure Treated Lumber
Above-ground pressure treated lumber is for applications where the lumber won’t contact or be within 6 inches of the ground. For example, deck railings, decking and stair treads, as well as fence rails and some pickets can be above-ground lumber.
Ground-Contact Pressure Treated Lumber
Ground-contact pressure treated lumber contains a higher level of the same chemical preservatives used in above-ground lumber. The additional protection makes ground-contact treated lumber the right choice where:
- The lumber touches the ground or is within six inches of the ground.
- The lumber is a structural component and is difficult to replace.
- The lumber faces regular or extended contact with moisture.
For example, fence posts and the joists, beams and posts of a deck frame should be ground-contact pressure treated lumber.
Our guide on the difference between treated wood and untreated wood gives more details on treated lumber, its uses and when to choose it for your project.
Key Pressure Treated Lumber Terms
Understanding common treated lumber terms helps you shop confidently for your project materials.
Green or Kiln-Dried
- Green: Green treated lumber hasn’t been dried and retains lots of moisture. The boards are more flexible but may shrink or twist. It can take several weeks or months for green lumber to dry enough to accept paint or stain.
- Kiln-Dried: Kiln-dried treated lumber has dried in an oven or kiln after treatment. This lumber is less likely to shrink and requires less wait time before painting or staining.
Paintable or Stainable
These factors indicate the lumber can accept paint or stain once it’s fully dry. If the lumber absorbs water, it’s dry enough for paint or stain. If the water beads, it needs to dry longer.
Edge Profile
Edge profile describes the board edges. For example:
- Square: Boards with a square edge profile have all four edges cut to 90-degree angles. The four sides are relatively flat and smooth.
- Radius Edge: Boards with a radius edge profile have four rounded edges. Decking boards that form the floor of a deck typically have radius edges to reduce splintering.
Lumber Grade
Grade indicates the typical number of defects in a board. Common grades include:
- #2: Boards contain medium-sized knots and likely have a significant amount of wane — bark on the edges or material missing from the edges.
- #2 Prime: Boards are similar to #2 boards, but have very little wane and offer a better appearance.
- #1: Boards have fewer knots and less wane than #2 and #2 Prime boards, resulting in a better appearance and greater strength.
- Standard: The grade applies specifically to decking boards and indicates large knots, but little or no wane.
- Premium: A decking-specific grade for boards with fewer defects and a better appearance than standard-grade boards.
Use Category
The use category system helps you determine lumber’s suitability for different applications. For example, UC3B indicates suitability for exposed, above-ground use, such as deck stair treads, while UC4B indicates suitability for heavy-duty, ground-contact applications, such as deck frame posts. Look for the UC rating on the tag fastened to the end of the lumber.
Find the Type of Pressure-Treated Lumber You Need
Shop for pressure treated lumber on Lowes.com by type, size, grade and more. Plus you can find the corrosion-resistant fasteners and hardware you need to build everything from a planter box to a deck. Or visit your local Lowe’s and talk to a Red Vest associate about your building project. They’ll make sure you get the lumber and materials you need for a long-lasting structure.