What Is Treated Lumber Treated With? A Complete Guide
When you buy treated lumber, you are buying wood that has been infused with chemical preservatives to protect it from rot, insects, and moisture. The short answer to what is treated lumber treated with is that it is treated with a combination of water, a fungicide, and an insecticide, most commonly copper-based compounds like alkaline copper quaternary (ACQ) or copper azole (CA). These chemicals soak into the wood fibers under pressure, giving the lumber a greenish tint and a long service life outdoors.
The Two Main Types of Preservatives
Modern pressure-treated lumber relies on two primary families of preservatives: copper-based formulas for residential use and non-copper formulas for specific commercial or marine applications.
Copper-Based Preservatives
Copper is the active ingredient in the vast majority of residential treated lumber sold today. The copper acts as a fungicide, preventing mold, mildew, and wood-rotting fungi from colonizing the wood. Two common copper-based treatments are:
- Alkaline Copper Quaternary (ACQ). ACQ uses copper oxide as the fungicide and a quaternary ammonium compound as the insecticide. It is the most widely used treatment for above-ground and ground-contact lumber. ACQ replaced chromated copper arsenate (CCA) in the early 2000s for residential use because it does not contain arsenic.
- Copper Azole (CA). Copper azole uses copper and an organic triazole fungicide (tebuconazole) to protect against decay. It is slightly less corrosive to fasteners than ACQ and is often used for decking, fence boards, and sill plates.
Both ACQ and CA are applied using a pressure-treatment process. The wood is placed in a large cylinder, a vacuum pulls air out of the cells, and then the preservative solution is forced into the wood under high pressure. This leaves the chemicals deep inside the wood, not just on the surface.
Non-Copper Preservatives
For applications where copper cannot be used (for example, in contact with aluminum or certain marine environments), alternative preservatives are available:
- Borate-based treatments (e.g., SBX/DOT). These use disodium octaborate tetrahydrate, a boron salt. Borates are effective against fungi and insects but are water-soluble, so they are only used for above-ground, weather-protected applications like interior framing or sill plates.
- Creosote. A coal-tar distillate, creosote is a heavy, dark oil used for railroad ties, utility poles, and marine pilings. It is not sold for general residential use because it can ooze out and stain surfaces.
- Pentachlorophenol (Penta). An oil-borne preservative used for utility poles and cross-arms. It is restricted to industrial and commercial applications.
How the Treatment Affects the Wood
The preservative does more than just add color. It changes the wood’s moisture content, weight, and corrosion potential.
Moisture Content and Shrinkage
Treated lumber is usually wet when you buy it. The pressure-treatment process forces water and chemicals into the wood, raising the moisture content well above the fiber saturation point. As the wood dries, it shrinks. A typical 2x6 treated deck board may shrink by 1/8 to 1/4 inch in width as it seasons. This is why builders often leave a small gap between boards and use hot-dipped galvanized or stainless steel fasteners.
Corrosion of Fasteners
The copper in ACQ and CA is corrosive to standard steel and galvanized fasteners. The American Wood Protection Association provides guidelines for fastener compatibility. For ACQ and CA, you should use hot-dipped galvanized fasteners (meeting ASTM A153) or stainless steel (types 304 or 316). Using plain steel screws or nails will cause them to rust quickly, leading to structural failure.
Color Change Over Time
Freshly treated lumber has a green or brown tint from the copper solution. Over weeks to months, exposure to sunlight and rain will turn the wood a silvery-gray color. This is a natural weathering process and does not mean the preservative has washed out. The active chemicals remain inside the wood for decades.
Safety and Handling of Treated Lumber
The Environmental Protection Agency regulates the use of wood preservatives in the United States. Modern residential treatments (ACQ and CA) are considered safe for typical outdoor use when handled properly.
What You Should Do
- Wear gloves when handling treated lumber to avoid splinters and chemical residue.
- Wear a dust mask when sawing or sanding to avoid inhaling wood dust that contains preservative particles.
- Wash work clothes separately from household laundry.
- Do not burn treated lumber. Burning releases the copper and other chemicals into the air as toxic smoke and ash.
What You Should Not Do
- Do not use treated lumber for cutting boards, countertops, or any surface that contacts food.
- Do not use treated lumber for animal bedding or beehives. The copper can be toxic to livestock and bees.
- Do not use treated lumber for indoor furniture where skin contact is frequent unless the wood is sealed with a clear sealant.
The EPA does not consider modern treated lumber to be a health hazard for children playing on decks or for adults building garden beds. However, if you have concerns about gardening with treated lumber, you can line the inside of the bed with heavy plastic to create a barrier between the soil and the wood.
The Pressure-Treatment Process Step by Step
The treatment happens inside a large steel cylinder at a treatment plant, using a process called the Bethell (full-cell) method.
First, the lumber is loaded into the cylinder. The door is sealed, and a vacuum pump pulls air out of the wood cells. This vacuum lasts about 30 to 60 minutes, depending on the species and thickness of the wood. Removing the air creates empty space inside the cell walls.
Next, the preservative solution is pumped into the cylinder while the vacuum is still applied. Once the cylinder is full, pressure is applied, typically between 140 and 150 pounds per square inch. The pressure forces the liquid deep into the wood. For ground-contact lumber, the treatment must penetrate at least 0.40 inches into the side of the wood, according to American Wood Protection Association standards.
After the pressure cycle ends, the solution is drained, and a final vacuum removes excess liquid from the surface. The wood is then removed and stacked for shipping while still wet.
Retention Levels and What They Mean
The amount of preservative left in the wood after treatment is called retention, measured in pounds of chemical per cubic foot of wood. Different applications require different retention levels:
| Application | Typical Retention (ACQ) | Common Use |
|---|---|---|
| Above ground | 0.15 lb/ft³ | Deck boards, railings, fence pickets |
| Ground contact | 0.40 lb/ft³ | Posts, retaining walls, landscaping timbers |
| Heavy duty (saltwater) | 0.60 lb/ft³ | Dock pilings, seawalls |
A 4x4 post rated for ground contact will contain more than twice the chemical load of a deck board rated for above-ground use. This is why you should never use above-ground-rated lumber for posts set into concrete or soil. The wood will decay from the inside out because the preservative concentration is too low.
How to Identify the Treatment Type on the Lumber
Every piece of treated lumber sold in the United States must have an end tag or stamp that tells you what preservative was used and what the retention level is. The stamp is usually ink-stamped on the end of the board or attached as a plastic tag.
Look for the following information on the stamp:
- Preservative abbreviation. ACQ, CA, or SBX (borate). If you see CCA, the lumber is old stock and should not be used for residential projects.
- Retention number. For example, “0.40” means the wood has a ground-contact retention level.
- Standard reference. Look for AWPA (American Wood Protection Association) or a third-party inspection agency logo.
- Species or mill code. This tells you where the wood came from.
If the lumber is intended for use in direct contact with the ground, the stamp will say “Ground Contact” or “GC.” Above-ground-rated lumber will say “Above Ground” or “AG.” Using the wrong rating will void any warranty and lead to premature decay.
Key Takeaways
- The direct answer to what is treated lumber treated with is that it is treated with copper-based preservatives (ACQ or CA) for most residential uses, or with borates, creosote, or pentachlorophenol for specific commercial applications.
- The preservatives are forced into the wood under pressure, providing long-term protection against rot, fungi, and insects.
- Copper-based treatments are corrosive to standard steel fasteners; use hot-dipped galvanized or stainless steel hardware.
- Treated lumber is wet when purchased and will shrink as it dries. Plan for gaps and allow the wood to acclimate before final installation.
- Do not burn treated lumber, and wear gloves and a dust mask when cutting or sanding it.
