A wood preservative is a chemical treatment that protects timber from rot, fungi, and insects, extending the life of outdoor lumber for decks, fences, and landscaping.
For the full breakdown, see our best Eco Wood Preservative guide.
These chemical treatments—legally classified as pesticides by the EPA—penetrate the wood’s surface to create a barrier against moisture and biological decay.
The key difference between a wood preservative and a surface stain or paint is that proper pressure treatment forces chemicals into the wood fibers, not just onto the surface. That deep, protective shell is what gives pressure-treated lumber its long service life.
Two Main Classes: Oil-Type and Waterborne Preservatives
Wood preservatives fall into two broad categories based on what carries the active chemicals into the wood.
Oil-Type Preservatives
These use organic solvents as carriers. Oil-type preservatives are powerful but carry restrictions on where they can be used.
- Creosote: A thick, black coal-tar distillate used for over 150 years. It’s highly effective against fungi but is restricted to commercial applications only—railroad ties, marine pilings, and utility poles. Never for residential use.
- Pentachlorophenol (Penta): Dissolved in solvents, effective against stain and mold fungi. Also restricted to industrial use.
- Copper Naphthenate: A common oil-type used for poles and timbers, available in green or brown formulations.
Waterborne Preservatives (Residential Market)
These leave a clean, paintable surface and are the standard for consumer lumber. The EPA currently registers these waterborne types for residential timber:
- Alkaline Copper Quaternary (ACQ): Contains copper oxide and a quaternary ammonium compound. Types include ACQ-A through ACQ-D.
- Copper Azole: A copper-based formulation effective against decay and insects.
- Polymeric Betaine: A newer registered preservative.
- Borate: Often used for interior applications or as a fire-retardant additive.
- Chromated Copper Arsenate (CCA): Once the dominant choice for residential lumber, but its voluntary phaseout in 2003 (largely complete by 2004) ended most consumer use due to arsenic concerns. CCA is still used for some industrial applications.
How Pressure Treatment Actually Works
The most effective method for getting preservatives deep into lumber is pressure treatment (PT). The process uses a large, airtight steel cylinder. The cylinder is filled with preservative solution, then vacuum and high pressure are applied to force the chemicals deep into the wood fibers. After the cycle, excess chemical is removed, leaving exactly the retention level needed for protection.
Paint or lacquer only covers the surface—it doesn’t penetrate to prevent internal decay.
What to Watch For With Treated Wood
- Corrosive fasteners: ACQ and Copper Azole are corrosive to standard steel fasteners. You must use hot-dipped galvanized, stainless steel, or specially rated connectors.
- Moisture content: PT lumber is often soaking wet when you buy it. Building immediately can trap moisture—let it season or dry first before finishing.
- Disposal caution: Never burn treated wood in open fires or stoves. The fumes from burning preservatives (especially creosote) are toxic. Check your local waste authority for proper disposal guidelines.
- Annual maintenance:
FAQs
Is a wood preservative the same as wood stain?
Can I buy creosote-treated wood for my garden?
No.
How do I know what preservative my lumber has?
References & Sources
- U.S. Environmental Protection Agency. “Overview of Wood Preservative Chemicals.” EPA’s official classification and registration details for wood preservatives.
- U.S. Environmental Protection Agency. “Wood Preservatives and Treated Wood.” Detailed EPA guidance on pressure treatment and consumer safety.
- NIOSH / CDC. “Wood Preservatives and Their Hazards.” Health and safety information for handling treated wood.