Pressure-treated wood has become ubiquitous in construction, landscaping, and outdoor projects. But how exactly is wood pressure treated? This process forces preservative chemicals deep into the wood to protect it from rot, fungus, insects, and other environmental damage. Understanding how pressure treatment works and its applications and limitations allows for informed use of this popular building material.
What is Pressure-Treated Wood?
Pressure-treated wood refers to lumber that has been conditioned under extreme pressure to force chemical preservatives into the cellular structure of the wood. This pressure treatment deeply impregnates the wood with protective chemicals to enhance its weather resistance and longevity.
The chemicals used in the pressure treatment process vary, but commonly include pesticides, fungicides, insecticides, and water-repellants. These protective chemicals preserve the wood from mold, mildew, fungus, termites, and other pests that can cause decay.
Pressure-treated wood emerged in the 1930s as railroads began preserving railroad ties using creosote preservatives. The process expanded to residential construction in the 1950s as chromated copper arsenate (CCA) became the predominant treatment chemical.
Today, pressure-treated wood is ubiquitous in any application where long-lasting weather and pest resistance is desired. Decks, fences, retaining walls, utility poles, boat docks, and landscape edging commonly utilize pressure-treated lumber.
How is Pressure-Treated Wood Made?
The manufacture of pressure-treated wood is an industrial process utilizing heavy machinery. The goal is to permeate the wood’s cellular structure with protective chemicals under extreme pressure. Here are the basic steps:
1. Placing the Wood
The lumber is precisely stacked on special tram cars rolled into large steel cylinders, referred to as treatment cylinders or retorts. These towering horizontal cylinders can be over 150 feet long and 7 feet in diameter.
2. Creating a Vacuum
Once loaded, the cylinder’s door is sealed and powerful vacuum pumps remove the air. This vacuum helps permeate the wood with chemicals when pressure is later applied.
3. Adding Chemicals
With the vacuum created, the preservative chemicals are pumped into the bottom of the cylinder, submerging the wood under the liquid preservative solution.
4. Applying High Pressure
Hydraulic pumps pressurize the cylinder up to 150-160 psi, forcing the chemicals deep into the cellular structure of the wood.
5. Retention Time
The intense pressure is maintained for an extended time, usually around 4 hours, to retain the chemicals properly.
6. Drain and Dry
The pressure is released, the solution drained, and the wood is left to dry. Drying fixes the chemicals into the wood. The wood is now ready for treatment.
This intense process forces the protective chemicals much deeper into the wood than surface treatments like brushing or spraying. The result is lumber thoroughly saturated under high pressure to resist rot, pests, and water damage.
What Chemicals Are Used to Pressure Treat Wood?
The chemicals used to pressure treat wood have evolved over the decades. Here are some of the most common preservatives:
Chromated Copper Arsenate (CCA)
CCA contains chromium, copper, and arsenic and was the predominant pressure treatment chemical from the 1950s to the early 2000s. While effective, concerns over arsenic toxicity led to a phase-out in residential lumber.
Alkaline Copper Quaternary (ACQ)
ACQ contains copper and quaternary ammonium and is today’s most common arsenic-free pressure treatment. It protects against termites, rot, and fungal decay.
This preservative utilizes copper along with an organic azole co-biocide. It penetrates wood well for long-term protection from mold, fungus, and insects.
Copper combined with naphthenic acid makes this preservative effective against rot and moisture damage while limiting toxicity.
Boric acid or sodium borate create pressure treatments that are effective and non-toxic. However, leaching can occur over time.
A tar-like oil, creosote deeply penetrates and preserves wood from decay and pests. Its toxicity limits it to industrial applications like utility poles.
The specific pressure treatment chemical varies based on the desired application and treatment standards. But most modern solutions utilize copper and organic co-biocides to limit toxicity while preserving the wood.
Is Pressure-Treated Wood Toxic?
Pressure-treated wood does contain toxic chemicals, so caution is warranted when handling, cutting, or disposing of it. Here are some key toxicity concerns:
Historically, chromated copper arsenate (CCA) pressure treatments did contain arsenic, which can be highly toxic. This is far less common today but still an issue with older lumber.
Cutting or sanding pressure-treated wood produces dust containing concentrated levels of the chemicals. Proper safety gear like respirators and goggles should be used.
Care should be taken when disposing of old pressure-treated wood, which may contain high levels of chemicals. Burning is not recommended.
Soil and Water Contamination
There is some potential for the chemicals to leach out of pressure-treated wood into surrounding soil, plants, or water over time.
Proper handling and application is imperative when working with pressure-treated lumber. While the chemicals protect the wood, they can harm people and the environment. Always wear protective equipment during cutting, drilling, or sanding.
Common Applications of Pressure-Treated Wood
Here are some of the most popular uses for lumber that has undergone pressure treatment:
- Retaining walls
- Landscape beds
- Raised garden planters
- Boat docks
- Utility poles
Anywhere long-term contact with soil, water, or heavy foot traffic is expected, pressure-treated wood is ideal for enhanced longevity.
Pressure-Treated Wood vs Untreated Wood
So how does pressure-treated lumber compare to regular untreated wood? Here are the key differences:
The preservative chemicals extend the lifespan of pressure-treated wood to 15-30 years or longer. Untreated wood may only last 2-5 years outdoors.
Pressure-treated wood is more expensive than untreated wood, generally 20% to 50% more per board foot. This accounts for the treatment process.
The longer lifespan of pressure-treated lumber makes it more cost effective in the long run. Multiple replacements of untreated wood can add up.
Pressure treatment can slightly toughen the wood fibers, making the wood harder to cut and drill. Untreated wood is generally more workable.
The chemicals tend to give pressure-treated wood a greenish tint. Natural wood colors show through more on untreated boards.
There are trade-offs when choosing between pressure-treated and untreated lumber. Assess your budget, project needs, and longevity requirements when deciding.
Why is Pressure-Treated Wood Cheaper Over the Long Term?
While pressure-treated lumber carries a 20-50% premium upfront because of the intensive treatment process, this investment oftens pays dividends in enhanced lifespan and avoided replacement costs.
For example, an untreated pine deck may only last 2-5 years before significant rot and deterioration starts. Replacing the deck even just once in 20 years would likely exceed the upfront cost premium of pressure treatment. Avoiding multiple replacements makes pressure-treated the economical option.
Here are factors that contribute to the long-term cost effectiveness of pressure-treated lumber:
Avoided Replacement Labor Costs
Labor expenses to rebuild or replace untreated rotted components can be significant, especially for large projects. Eliminating frequent replacements with pressure-treated wood avoids recurring labor costs.
Consistent Quality and Appearance
Pressure-treated wood maintains strength, appearance, and dimensional stability longer. Untreated wood can warp, twist, and crack over time, necessitating early replacement.
Quality pressure-treatment reduces the need for frequent upkeep such as sealing, painting, or staining. Less required maintenance means lower lifetime costs.
For any project with over 5 years, the financial payoff of pressure-treated wood’s longevity outweighs the small premium.
How Long Does Pressure-Treated Wood Last?
There is no fixed lifespan for pressure-treated lumber. The longevity depends on factors like:
- Type of wood (softwood vs. hardwood)
- Specific pressure treatment chemical used
- Environment and exposure
- Quality of installation
Properly treated and maintained softwoods like pine, fir, and cedar have an expected 15-30 years lifespan. For hardwoods like oak, the lifespan may extend to 30-50 years.
Here are some guidelines for expected pressure-treated wood longevity in typical environments when properly installed:
- Decks: 15-25 years
- Fences: 15-30 years
- Retaining walls: 20-30 years
- Landscape beds: 20-25 years
- Freshwater docks: 10-20 years
- Saltwater docks: 10-15 years
- Raised garden beds: 20-30 years
Proper pre-drilling, fastening, sealing, and maintenance helps the wood last through its expected lifespan. But regardless, pressure treatment will far extend the wood’s resistance to rot and decay versus untreated lumber.
Using Pressure-Treated Wood in Vegetable Gardens
Pressure-treated lumber is often used to build raised garden beds. But is it safe for vegetable gardens? Here are some guidelines:
- Allow beds to weather 6-12 months before planting. This allows any surface chemicals to dissipate.
- Use only pressure-treated wood approved for residential use, avoiding industrial treatments like creosote.
- Line beds with plastic sheeting or landscape fabric as an extra barrier.
- Maintain a pH between 6.0-7.0, as more acidic soils can leach chemicals from the wood.
- Do not use pressure-treated wood for garden structures that have direct food contact, like boxes or tables.
With proper precautions, pressure-treated lumber can safely be used for raised garden beds. Always check the treatment certification and consult local regulations for additional guidance.
Is Arsenic Still Used to Treat Wood?
Arsenic was phased out of most residential pressure treatment chemicals in the early 2000s due to health concerns. Here are some key facts about arsenic in pressure-treated wood today:
- Chromated copper arsenate (CCA) and other arsenic-based preservatives are still used for industrial applications like utility poles and cross ties.
- Copper-based preservatives without arsenic are now predominant for residential use, like ACQ and copper azole.
- Wood pressure treated prior to 2004 has a high likelihood of containing arsenic. Exercise caution when handling.
- If wood has a greenish tint, it is likely CCA pressure treated with arsenic. Newer arsenic-free wood tends to lack green coloring.
- Always check certification stamps for “CCA” or the chemicals used to verify arsenic content and treatment standards.
While largely phased out, arsenic may still be present in older pressure-treated wood. Take safety precautions and properly dispose of any wood containing this toxic chemical.
How to Identify Pressure-Treated Wood
You can recognize lumber that has undergone pressure treatment by the following characteristics:
- Greenish tint – The chemicals often react with the wood to impart a faded green or brown-green coloring.
- Stamps – Look for ink stamps indicating the preservative used and certification agency. Common marks are ACQ, SmartSense, and Wolman.
- Oily feel – Many pressure treatment chemicals leave the wood feeling slightly tacky or oily to the touch.
- Price – The 20-50% price premium over untreated lumber is a giveaway.
- Vendor labeling – Reputable dealers will clearly label wood as “pressure-treated” along with the retention level.
When uncertain, ask your lumber supplier for documentation on whether wood has been pressure treated. The greenish coloring is your best indicator for older wood with no stamps.
Disadvantages of Pressure-Treated Wood
While pressure-treated lumber has many benefits, there are also some downsides to consider:
- Toxicity – Handling precautions are needed to avoid exposure to toxic treatment chemicals.
- Disposal issues – Offcuts and old wood may contain high chemical residue requiring special disposal.
- Leaching – Low risk of harmful chemicals leaching over time into surrounding soil and plants.
- Corrosion – Some treatment chemicals accelerate corrosion of fasteners and hardware. Stainless steel or hot-dipped galvanized fasteners are recommended.
- Workability – Dense pressure-treated wood is harder on saw and drill bits. Carbide-tipped bits are advisable.
- Appearance – The greenish tint may not be desirable for visible decorative projects. Stains can mask but not eliminate.
- Cost – Higher upfront cost than untreated wood. This is compensated long-term through avoided maintenance and replacements.
Weighing these downsides against the benefits will dictate whether pressure treatment suits a project.
Wood Pressure Treatment Process Machines
Specialized heavy-duty equipment is used in the pressure treatment process. Here are some key machines:
These massive horizontal steel cylinders up to 150 feet long apply extreme hydraulic pressure. They are also referred to as retorts or impregnation chambers.
Rolling tram cars ride on rails to transport stacked lumber into and out of the treatment cylinders. This facilitates large-scale treatment.
Powerful vacuum pumps remove air from the cylinders before pressurization. This allows deeper preservative penetration. 10,000+ cubic feet per minute capacity is typical.
These pumps pressurize the treatment cylinders to 150-175 psi, forcing chemicals into the wood’s cellular structure.
Process Control Systems
Automated computerized controls monitor and regulate the pressure treatment process for consistency.
Specialized equipment like this enables the efficient, high-volume commercial production of pressure-treated lumber.
Frequently Asked Questions
Here are answers to some common questions about wood pressure treatment:
What chemical is used for pressure-treated wood?
Popular options include ACQ, copper azole, and ammoniacal copper quat. These use copper paired with other pesticides and fungicides.
How toxic is pressure-treated wood?
It contains pesticides, so handle carefully and wear PPE during cutting or sanding. toxicity has reduced since arsenic was phased out from residential use in the early 2000s.
How to make wood pressure treated at home?
DIY home pressure treatment is challenging. Brushing on chemicals only penetrates the surface. Commercial pressure forces chemicals deep into the wood cells.
Does pressure-treated wood rot?
It resists rot far longer than untreated wood, but no wood lasts forever. Fifteen to thirty years is typical for pressure-treated lumber. Proper installation and maintenance extends lifespan.
Can pressure-treated wood be stained?
Yes, it takes stains well. An oil-based penetrating stain is best. Allow wood to dry fully after treatment before staining. Lightly sand first for best adhesion.
Is pressure-treated wood good for raised beds?
Generally yes, with some precautions. Allow beds to weather before planting. Line with plastic sheeting or landscape fabric. Maintain proper soil pH. Avoid types with creosote or excess arsenic.
Can pressure-treated wood be burned?
It’s not recommended. The chemicals release toxic smoke and fumes when burned. Follow local regulations for chemical residue disposal instead.
Pressure-treatment gives lumber long-lasting resistance to rot, insects, and fungal decay by deeply impregnating wood cells with protective chemicals. Understanding the pressure treatment process, preservatives used, benefits, and limitations allows for informed usage of this popular building material when enhanced weather-resistance is desired.
Despite some toxicity concerns, modern pressure-treated wood offers clear advantages for outdoor projects. While upfront costs are higher, avoiding frequent replacements down the road makes pressure-treated often the more economical choice. Just be sure to take safety precautions when handling, cutting, or disposing of treated lumber.