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How Does Pressure Treated Wood Work?

Pressure treated wood is wood that has been infused with chemical preservatives under high pressure to protect it from rot, fungal decay, and insects. This process helps extend the lifespan of wood used in outdoor settings like decks, fences, and landscape timbers. Understanding how pressure treatment works provides insight into why it makes wood more durable.

The Pressure Treatment Process

Pressure treatment forces preservative chemicals deep into the wood. This is accomplished by placing wood in a sealed chamber and creating a vacuum to remove air from the wood’s cellular structure. Then a preservative is introduced under high pressure, forcing it to penetrate deeply and permeate the wood cells.

The pressure forces preservatives into the dense, heartwood interior of timber, instead of just the more porous outer sapwood layer. Treatment also coats interior cell walls, making the wood resistant from the inside out. Standard pressure treatment forces absorb around 6 to 12 pounds of preservative per cubic foot into the wood.

Preservatives Used

Common preservative chemicals used include:

Chromated Copper Arsenate (CCA) – This preservative contains chromium, copper, and arsenic as active ingredients. CCA-treated wood has been phased out for residential use since 2004 due to arsenic toxicity concerns, but is still used for industrial applications.

Alkaline Copper Quaternary (ACQ) – This preservative is made of copper, a quaternary ammonium compound, and carbonate. ACQ is currently the main pressure treatment chemical used for outdoor residential lumber.

Copper Azole – This preservative uses copper coupled with an organic azole compound. Azole aids in copper penetration while also having anti-fungal properties.

Micronized Copper – This preservative uses micronized copper particles suspended in liquid for deeper penetration into wood cells. Micronized copper pressure treatment is growing in popularity.

Light Organic Solvent Preservatives (LOSP) – LOSP treatments use organic solvents to carry water-repellent preservative chemicals into the wood. Common LOSP treatments include Wolmanized and Natural Select brands.

Benefits of Pressure Treated Wood

Pressure treatment provides several advantages over untreated wood:

  • Longer lifespan – Preservatives help prevent rot, fungal decay and damage from wood-boring insects like termites. Treated wood used outdoors can last for decades.
  • Added stability – Treatment chemicals help stabilize wood cells against moisture and swelling/shrinking. This results in stabilized wood that holds its form better.
  • No need to seal or stain – Preservatives make sealing or staining unnecessary to maintain outdoor wood’s lifespan. This saves time and upkeep costs.
  • Wider range of lumber options – Treatment lets builders use less-expensive timber species like southern pine for outdoor projects and still get decades of performance.
  • Retention levels – Higher chemical retention levels during pressure treatment provide longer-lasting protection. Common retention levels include .25, .4, and .6 pounds of preservative per cubic foot.

How Pressure Treated Wood Deters Decay

There are several ways that infusion with preservative chemicals deters decay processes like fungal rot and insect damage:

  • Toxicity – Preservative chemicals are directly toxic to wood-degrading fungi and insects. This toxicity prevents infestation and slows bacteria/fungi growth.
  • Dimensional stability – Treatment reduces swelling from moisture absorption. This added dimensional stability leaves less gaps for organisms to penetrate.
  • Disruption of enzymes – Preservatives oxidize and complex with proteins, disrupting enzyme systems in fungi. This hinders the ability to break down and metabolize wood cellulose.
  • Lower moisture content – Chemicals keep moisture content lower in wood cells. Drier wood cannot support microbial growth as well.

Cell wall reinforcement – Some treatments deposit chemicals within cell walls, reinforcing the cellulose structure against enzyme breakdown.

Myths About Pressure Treated Wood

There are some common myths about pressure treated lumber:

  • “Treated wood is unsafe” – Modern treated wood does not contain arsenic and is safe when installed properly. Handlers should wear gloves and wash hands after working with treated wood.
  • “Bugs/mold won’t eat treated wood” – Treatments deter most fungi and insects but don’t make wood 100% resistant if conditions are favorable enough for organisms. Proper installation is still important.
  • “Treated wood lasts forever” – While treated wood resists decay for decades, it does not last indefinitely. Periodic inspection and replacement of damaged boards may eventually be needed on decks, fences and other outdoor projects.
  • “Treated wood is maintenance-free” – While treated lumber does not need finishes to preserve it initially, it should still be maintained just like other building materials. Periodic cleaning, inspection for damage, and re-sealing cuts/holes will maximize a treated project’s lifespan.
  • “All treated wood is the same” – The level of durability depends on the chemical retention level during treatment. Higher retention levels provide better protection. Use grade stamps to choose lumber with adequate treatment for the project.

Treated Wood Grades and Markings

Shoppers should look for stamps and grade marks that indicate the treatment level when purchasing pressure treated lumber:

  • AWPA U1 Use Category System – This stamp denotes the intended use and treatment needed for different situations like ground contact, above ground, etc. to match the lumber’s preservative retention level.
  • Grade stamps – These show the preservative used, the treatment facility’s ID number, and the minimum amount of preservative absorbed during pressure treatment.
  • I.S.O. – This international quality control stamp ensures the treatment process met standardized procedures.
  • Dried After Treatment (DAT) – A DAT stamp means the wood was dried to below 19% moisture content after treatment for maximum stability.

Chemical Information Stamp – Required stamp indicates the chemicals used for preservative treatment.

Safety Precautions When Working With Treated Wood

While safe for approved residential uses, builders should still take precautions when handling, cutting or installing treated lumber:

  • Wear dust mask and goggles when cutting or sanding to avoid inhaling fine treated sawdust particles.
  • Wear gloves when working- gloves should be discarded after use if they may have come into contact with preservative chemicals.
  • Wash hands and arms thoroughly after handling treated wood and before eating/drinking.
  • Avoid prolonged skin contact with treated lumber.
  • Never burn pressure treated wood, as toxins can be released with the smoke and ash.
  • Wear eye protection when pressure washing treated decks/structures.
  • Avoid using treated wood near areas where food is grown.

Proper handling allows builders to safely take advantage of the benefits pressure treated wood provides for outdoor applications. Following manufacturer recommendations and grade stamp requirements ensures the material is ideally suited for the intended use. Understanding how wood preservatives work inside the wood’s cellular structure makes it clear why pressure treatment provides superior rot resistance compared to surface treatments. With reasonable precautions, pressure treated lumber allows durable, long-lasting outdoor structures to be constructed from sustainable and economical renewable wood resources.

Frequently Asked Questions About Pressure Treated Wood

What are some common uses for pressure treated wood?

Pressure treated lumber is commonly used for decks, fences, landscaping projects, retaining walls, pole barns, picnic tables, playground sets, and any other outdoor wood structures. It provides durable, long-lasting performance in wet, high-decay environments.

How long does treated wood last outside?

Treated wood can last for decades when properly installed and maintained. Typical lifespan estimates range from 15-30 years for decking boards, up to 50 years for structural framing, and 10-15 years for landscaping timbers. Higher treatment retention levels result in even greater rot resistance.

Does treated wood need to be sealed or stained?

Sealing and staining are optional for pressure treated projects. Preservatives already protect the interior wood cells from moisture and decay. Sealants mainly affect appearance and help minimize surface cracking and checking.

Can treated wood be used in ground contact?

Yes, lumber treated for ground contact can be buried or placed directly on soil. Choose grade stamps indicating use categories 4A or 4B for ground contact applications. An additional moisture barrier under the wood is still recommended.

Is treated wood safe for raised garden beds?

Lumber pressure treated with modern ACQ, micronized copper, or copper azole chemicals is considered safe for raised garden bed use. Be sure the grade stamp matches the intended use. Also, line beds with a weed barrier fabric before adding soil.

Can treated wood be used for interior projects?

Treated lumber is only recommended for certain approved exterior, wet applications. The chemicals are not intended for interior human living spaces. Untreated pine, cedar or composite lumber are better choices where appearance and fewer chemical concerns are priorities.

Does treated wood need to dry before using?

Allowing pressure treated wood to fully dry for 3-6 weeks is ideal, but lumber can still be worked when slightly wet if needed. Pre-drilling and using corrosion resistant screws are smart practices when fastening wet treated boards during construction.

Can treated lumber be painted or stained a different color?

Yes, paint, stains and clear finishes can be applied to pressure treated wood for aesthetic purposes after construction. Allow wood to dry first. Use finish products specifically formulated for exterior wood.

Are there disposal restrictions for treated wood?

Many municipalities prohibit or restrict placement of pressure treated wood scraps and used lumber in normal waste collection bins. Find out if your local waste authority allows treated wood disposal before beginning a project.

Conclusion

Pressure treated lumber allows outdoor structures to be built to last using economical wood materials. Understanding how forcing preservative chemicals deep into wood deters decay provides insight into why treatment makes wood more durable. When handled properly, pressure treated wood is a safe, sustainable choice for decks, fences, retaining walls and other outdoor projects that will stand up to the elements for decades. Using lumber with grade marks that match the intended use, allowing wood to dry after installation, and taking basic safety precautions enable homeowners and builders to confidently utilize pressure treated wood for projects that will endure outside.

Laura Kassovic

Laura Kassovic, a former engineer at Intel SOC, now dedicates her efforts to mentoring startups in the realms of Wearables and AI. As a co-founder of New Tech Brake, she spearheads a wireless sensing solution enterprise catering to diverse applications including product development, research, location tracking, and people monitoring, as well as asset and cargo supervision. The platform empowers developers to craft an array of innovations such as fitness trackers, temperature-monitored cargo systems, medical trial tools, smart running garments, or even straightforward transmission of unprocessed accelerometer data to cloud-based repositories.

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