Most lumber sold as “treated” at hardware stores today is pressure-treated with copper-based preservatives, primarily alkaline copper quaternary (ACQ) or copper azole. These chemicals protect wood from fungal decay and insect damage, and they replaced an older arsenic-based formula that was phased out of residential use in 2004. But the specific treatment depends on where the wood will be used, whether that’s a backyard deck, an interior wall, or a railroad tie.
Copper-Based Treatments for Residential Use
The green-tinted lumber you see stacked at home improvement stores is almost always treated with one of two copper-based preservatives. ACQ is a combination of copper oxide and quaternary ammonium compounds. Copper azole pairs copper with a different type of organic fungicide. Both are water-based, meaning the wood dries to a clean, paintable surface. They work by making the wood toxic to the fungi that cause rot and to wood-boring insects like termites.
These treatments are forced deep into the wood fibers using a pressure process. Lumber is placed inside a sealed cylinder, and preservative solution is driven in under high pressure so it penetrates well beyond the surface. This is why you’ll sometimes see a color difference between the outer treated zone and the inner core when you cut a board. The depth of penetration matters: wood that will sit in the ground needs more chemical driven deeper than wood used on an above-ground railing.
Borate Treatments for Interior Wood
For wood that stays dry and indoors, borate-based preservatives offer a different approach. The active ingredient is a boron compound (disodium octoborate tetrahydrate) that protects against termites, carpenter ants, and fungal decay. Borate treatments have been used on construction elements like sill plates and wall sheathing for over 70 years.
The catch is that borates are water-soluble. Rain or prolonged moisture will leach the preservative out of the wood, which is why borate-treated lumber is restricted to protected interior applications. If you’re building something that will get wet, copper-based treatments are the standard choice instead.
The Arsenic Formula That Was Phased Out
Before 2004, the dominant residential treatment was chromated copper arsenate, commonly known as CCA. It contained chromium, copper, and arsenic. CCA was effective and inexpensive, but concerns about arsenic leaching from decks, playsets, and other surfaces people regularly touched led manufacturers to voluntarily stop producing CCA-treated wood for homeowner use in December 2003.
CCA hasn’t disappeared entirely. It’s still permitted for commercial and industrial products like utility poles, foundation pilings, and fence posts in agricultural settings. If your home was built before 2004, there’s a reasonable chance the deck or outdoor structures use CCA-treated wood. The EPA did not require removal of existing CCA structures, so plenty of it remains in service.
Industrial Preservatives for Heavy Infrastructure
Railroad ties, utility poles, and marine pilings face harsher conditions than a backyard deck, and they get treated with heavier-duty chemicals not available to consumers. Creosote, a dark, oily substance distilled from coal tar, is one of the oldest and most widely used. Its composition is roughly 85% polycyclic aromatic hydrocarbons, 10% phenolic compounds, and 5% other organic chemicals. The strong smell and dark color of railroad ties come from creosote.
Pentachlorophenol (often called “penta”) is another industrial preservative used on utility poles and crossarms. Both creosote and penta are restricted to commercial and industrial applications because of their toxicity. They’re not something you’d encounter at a lumber yard, but they’re the reason infrastructure wood can last decades in soil and water.
How Use Categories Determine Treatment
The American Wood Protection Association classifies treated lumber into use categories (UC1 through UC5) based on exposure conditions. This system determines how much preservative the wood receives and which chemicals are appropriate:
- UC1 and UC2: Interior wood, either dry or occasionally damp. Insect protection is the main goal. Borate treatments typically fit here.
- UC3A and UC3B: Exterior wood above ground, either partially sheltered or fully exposed to weather. Deck boards and fence pickets fall in this range.
- UC4A through UC4C: Ground-contact wood, ranging from general use (fence posts, landscaping timbers) to heavy-duty structural applications. Higher retention levels of copper-based preservatives are required as severity increases.
- UC5A through UC5C: Saltwater immersion, from cooler northern waters to tropical marine environments. These applications typically require creosote or heavy copper-based treatments.
When you buy treated lumber, the end tag stapled to each board lists its use category and the preservative used. A board rated UC4A is safe for ground contact. One rated UC3B is fine for a deck surface but shouldn’t be buried in soil. Matching the category to your project matters because under-treated wood will rot prematurely in conditions it wasn’t designed for.
Why Fastener Choice Matters With Treated Wood
The copper in modern treated lumber creates a corrosion problem for metal fasteners. Copper is more chemically “noble” than steel or zinc, meaning it causes those metals to corrode faster through an electrochemical reaction. This issue became noticeably worse after the industry switched from CCA to ACQ and copper azole, because the newer formulas contain higher concentrations of copper.
Standard untreated nails and screws will corrode rapidly in contact with copper-treated wood, sometimes failing within a few years. Hot-dip galvanized fasteners (coated with a thick zinc layer) are the minimum recommendation. The zinc coating sacrifices itself to protect the steel underneath, buying time before corrosion reaches the fastener’s core. For the longest service life, stainless steel fasteners resist copper-driven corrosion far better than galvanized ones because stainless steel is chemically noble to copper rather than reactive with it.
This applies to joist hangers, brackets, and structural connectors too, not just nails and screws. If you’re building with treated lumber, check that all your hardware is rated for contact with preservative-treated wood. Using the wrong fasteners can compromise the structural integrity of a deck or fence long before the wood itself shows any sign of decay.