What Pressure-Treated Lumber Actually Is

Walk into any lumber yard and the treated wood section smells different. That greenish-brown tint on the boards isn't paint. The weight feels wrong too, heavier than regular framing lumber of the same size. You're looking at wood that's been fundamentally altered at the cellular level, not just coated with something that sits on the surface.

The treatment process doesn't happen by accident and it's not a simple soaking operation. It's an industrial procedure that uses vacuum and pressure to force chemical preservatives into spaces inside the wood that would otherwise remain empty. What comes out the other end is materially different from what went in.

What Wood Actually Is at the Microscopic Level

A cubic meter of spruce contains somewhere between 350 and 500 billion cells. Each cell is a hollow tube with walls made of cellulose microfibrils embedded in a matrix of lignin and hemicellulose. The interior space is called the lumen. In living wood, these lumens transport water and nutrients. In lumber, they're mostly filled with air.

The cell walls themselves have layers. The middle lamella between adjacent cells is rich in lignin. The primary wall is thin with randomly oriented cellulose fibers. The secondary wall has three distinct layers where cellulose microfibrils run at different angles, providing strength in different directions. The thickest layer, called S2, makes up most of the cell wall and determines the mechanical properties of the wood.

Cellulose provides tensile strength. Lignin provides compression resistance and waterproofing. Hemicelluloses connect everything together. This composite structure is why wood works as a building material. It's strong along the grain where the cellulose fibers run parallel, and it can handle compression loads without crushing.

The spaces between and within these cell walls matter for pressure treatment. The lumens are large enough to see under a microscope. The spaces between cellulose microfibrils in the cell wall are measured in nanometers. Both need to be filled with preservative for the treatment to work properly.

The Chemistry That Makes Wood Rot

Wood rots because fungi and insects can digest cellulose and hemicellulose. These organisms produce enzymes that break down the long polymer chains into sugars they can metabolize. Lignin is harder to break down, which is why rot-resistant wood species like cedar have high lignin content.

Fungi need moisture to function. Below about 20 percent moisture content, fungal decay essentially stops. Above that threshold, spores germinate and fungal hyphae penetrate the wood structure. The fungi secrete enzymes that diffuse into the cell walls and start breaking down polysaccharides.

Insects like termites have different strategies. Some species have gut bacteria that produce cellulase enzymes. Others physically chew through wood and digest whatever they can extract. Either way, they're after the carbohydrates locked in the cell wall structure.

Pressure treatment works by making the wood toxic to these organisms. The preservative chemicals either kill fungi and insects on contact or make the wood unpalatable enough that they avoid it. The chemicals don't make wood waterproof or structurally stronger. They just prevent biological attack.

The Actual Treatment Process

The industrial procedure happens in a horizontal steel cylinder called a retort, typically seven feet in diameter and up to 150 feet long. Lumber is loaded on tram cars that roll into the cylinder. Once the door seals, the process becomes a precisely controlled sequence of vacuum and pressure cycles.

The first step is vacuum. Industrial pumps remove air from the cylinder and from the wood itself. This pulls air out of the cell lumens and, partially, out of the spaces in the cell walls. The initial vacuum typically runs for 30 to 60 minutes at significant negative pressure.

With air removed, the cylinder floods with preservative solution. This is where the chemistry varies by treatment type, but the mechanical process stays the same. The liquid fills the cylinder completely, surrounding all the lumber.

Pressure comes next. Pumps force the preservative into the wood structure under 140 to 150 PSI. This pressure drives the liquid deep into the cell lumens and forces it into the microscopic spaces within the cell walls themselves. The pressure hold lasts several hours, ensuring thorough penetration.

After pressure treatment, the cylinder drains and a final vacuum pulls excess preservative from the surface of the wood. This recovered liquid goes back into the storage tanks for reuse. The lumber comes out wet with treatment solution but not dripping.

The treated wood sits on a drip pad for 24 to 48 hours while excess liquid continues to drain. Some operations kiln-dry the lumber afterward. Others ship it wet and let it air-dry at the job site or lumber yard.

Full-Cell vs Empty-Cell Methods

Two different pressure treatment protocols exist, and they put different amounts of preservative into the wood.

The full-cell method, described above, maximizes preservative retention. The initial vacuum removes as much air as possible, so when pressure forces the liquid in, both the lumens and the cell walls end up saturated. This is the standard method for lumber that will be in ground contact or high-moisture environments.

The empty-cell method uses a different sequence. Instead of starting with vacuum, it starts with positive pressure. Compressed air is forced into the cylinder and into the wood before the preservative floods in. When treatment pressure is applied, this compressed air fights back against the liquid. After pressure release, the compressed air expands and pushes liquid back out of the lumens.

The result is lumber where the cell walls contain preservative but the lumens are mostly empty. This uses less chemical, costs less, and still provides protection because the cell walls are where fungal attack happens. The empty-cell method works for above-ground applications where maximum protection isn't critical.

The Chemical Systems

Until 2004, chromated copper arsenate dominated residential pressure treatment. CCA contained copper for fungicide action, arsenic for insecticide action, and chromium to help everything bind to the wood. The arsenic raised health concerns despite the relatively stable chemical form it took in treated wood.

The EPA convinced manufacturers to voluntarily stop using CCA for residential applications. What replaced it was a family of copper-based systems with different co-biocides.

Alkaline Copper Quaternary, or ACQ, became the first major CCA replacement. It uses copper as the primary biocide and quaternary ammonium compounds to handle copper-tolerant organisms. ACQ treatment contains significantly more copper than CCA did, which solved the arsenic problem but created the corrosion problem with metal fasteners that deck builders know about. The high copper content is also why ACQ-treated lumber is particularly aggressive toward saw blades and cutting tools.

Micronized Copper Azole, or MCA, takes a different approach. Instead of dissolving copper in solution, it suspends microscopic copper particles in water along with azole fungicides. The micronized copper is less corrosive to metal than dissolved copper compounds, and the azole component provides additional protection against specific fungal species.

Copper Azole uses dissolved copper with azole co-biocides. It requires less total copper than ACQ for equivalent protection, which reduces both material cost and metal corrosion issues.

Borate treatments use boron compounds dissolved in water. Borates are highly effective against insects and fungi, relatively low in toxicity to mammals, and non-corrosive to metal. The downside is that borates leach out of wood when exposed to water. Borate treatment works for interior applications or exterior locations protected from rain.

How the Preservative Actually Bonds

When preservative solution enters wood, it doesn't just sit in the cell lumens like water in a bucket. The chemicals interact with the wood polymers, particularly in the cell walls where the protection needs to happen.

Copper ions from ACQ or copper azole treatments bond with lignin and cellulose through coordination chemistry. The copper forms chemical bonds with oxygen and nitrogen atoms in the wood polymers. This bonding process is called fixation, and it takes time after the initial treatment. As the wood dries and air reaches the treated material, oxidation reactions help lock the copper into the wood structure.

Quaternary ammonium compounds and azoles distribute throughout the cell wall matrix. These molecules are larger than copper ions and penetrate differently. They provide a secondary line of defense against organisms that might tolerate copper exposure.

Borates behave differently. They don't form strong chemical bonds with wood. Instead, they occupy spaces in the cell wall structure and redistribute as moisture moves through the wood. This is why borates leach readily and why borate-treated wood can't be used in wet environments.

The fixation process matters because unfixed preservative can leach out of wood or migrate to surfaces where it creates handling concerns. Properly fixed preservative stays locked in the wood structure for decades.

Retention Levels and What They Mean

The amount of preservative in treated wood gets measured as retention level, expressed in pounds per cubic foot. A higher retention level means more chemical stayed in the wood after treatment.

The American Wood Protection Association sets minimum retention levels for different use categories. These aren't arbitrary numbers. They come from accelerated decay testing where treated wood samples are exposed to aggressive fungal and insect attack under controlled conditions. The retention level that prevents failure becomes the minimum standard for that use category.

UC3B designation means above-ground use with periodic wetting. The retention requirement is lower because the wood won't be continuously damp. Deck boards and railings typically carry UC3B treatment.

UC4A means ground contact for general use. Posts, beams, and joists that touch soil or rest on concrete need UC4A treatment. The retention level is higher because continuous moisture exposure and soil contact create ideal conditions for decay organisms. This is the standard rating for deck construction where structural members contact the ground.

UC4B and UC4C designations indicate severe ground contact and critical structural members. Utility poles, foundation timbers, and permanent wood foundations need these higher treatment levels. The retention requirements are substantially higher than standard ground contact ratings.

The stamp on the end of treated lumber shows which category applies. Using lumber with inadequate retention for the application means premature failure. Using lumber with excessive retention costs more without providing additional benefit.

What Changes in the Wood

Pressure treatment doesn't just add chemicals. It changes how the wood behaves physically.

The treatment solution adds significant weight. Treated lumber can weigh 50 percent more than untreated lumber of the same dimensions immediately after treatment. As the wood dries, much of that weight leaves as water evaporates, but the preservative chemicals remain.

Dimensional stability suffers because the lumber swells during treatment and then shrinks as it dries. A nominal 2x4 treated board might measure 1.625 by 3.625 inches while wet, larger than the standard 1.5 by 3.5 inch finished dimension. As it dries to equilibrium moisture content, it shrinks back down, but not always uniformly. This causes warping, twisting, and cupping that makes treated lumber frustrating for fine carpentry.

The wood surface becomes more difficult to paint. Preservative chemicals on the surface interfere with paint adhesion. The wood needs to dry thoroughly before painting, which can take months. Even then, paint performance on treated lumber is inferior to paint on untreated wood. Clear sealers and penetrating stains work better than film-forming finishes.

When installing fasteners with power tools, the moisture content and chemical treatment affect how the wood splits and how screws engage the fibers. Fresh treated lumber drives differently than dried material.

Cutting or drilling treated lumber exposes untreated wood at the cut end. The preservative didn't penetrate all the way to the core in most species. Cut ends need to be treated with a brush-on preservative to maintain protection. This matters most for ground contact applications where the cut end might be buried. When cutting treated lumber, this exposure of untreated core material also affects how cutting tools interact with the wood, requiring more frequent blade changes and maintenance.

Species Differences and Incising

Not all wood species accept pressure treatment equally. Southern yellow pine has an open cell structure that lets preservative penetrate easily. Douglas fir and other western species have denser, more resinous cell walls that resist penetration.

For resistant species, treatment plants use a process called incising. Before treatment, rotating wheels with protruding teeth punch thousands of small slits into the lumber surface. These incisions break through the surface barrier and create pathways for preservative to enter.

Incised lumber looks distinctive, with patterns of short knife cuts across all faces. The incisions compromise some strength and create more surface area for moisture movement, but they make adequate treatment possible in species that wouldn't penetrate otherwise.

The depth of preservative penetration varies by species and treatment method. Southern pine might show preservative color all the way through the board. Douglas fir might show treatment only in the outer quarter inch. Both can meet retention standards, but the distribution pattern differs.

What Happens Over Time

Treated lumber outdoors goes through ongoing chemical changes. The copper in ACQ or copper azole treatments can oxidize further, changing from the greenish color of fresh treatment to a darker brown. This oxidation is part of the fixation process and doesn't indicate treatment failure.

Some preservative does leach from the wood surface, particularly in the first few years after treatment. Rain washing over treated lumber carries trace amounts of copper and co-biocides into the soil or onto lower surfaces. The amounts are small compared to total retention, but they accumulate over time.

The wood itself continues to age. UV radiation breaks down lignin at the surface, turning the wood gray. Moisture cycling causes checking and splitting as the wood expands and contracts. The preservative protects against biological decay but doesn't prevent physical weathering.

Testing on treated lumber from older decks and structures shows that properly treated wood maintains adequate retention levels for decades. The preservative stays fixed in the cell walls where it's needed. Surface weathering happens, but the internal protection remains functional.

The Environmental Questions

Pressure treatment extends wood lifespan dramatically. A treated deck post might last 40 years where an untreated post would rot in five. This reduces the number of trees harvested for replacement lumber and keeps construction waste out of landfills. The environmental calculation has to weigh preservative toxicity against wood conservation.

The copper-based preservatives now in use are less acutely toxic than arsenic-based CCA, but copper itself is an environmental concern at high concentrations. Copper leaching from treated wood into soil can affect plant growth and accumulate in aquatic environments. The industry response has been developing lower-copper formulations and using copper more efficiently through microencapsulation.

End-of-life disposal creates challenges. Treated lumber can't be burned in ordinary fireplaces or fire pits because combustion releases preservative chemicals into the air. Many landfills accept treated wood, but some jurisdictions classify it as special waste requiring separate handling. Grinding treated lumber for mulch or compost is generally prohibited because the preservatives persist in the processed material.

The wood preservation industry continues developing new chemical systems with reduced environmental impact. Some use non-metallic biocides. Others use encapsulation technologies that keep preservatives locked in the wood more securely. The goal is maintaining rot resistance while minimizing environmental release.

What You're Actually Getting

When you buy pressure-treated lumber, you're buying wood that's been fundamentally altered at the cellular level. The process forced chemicals into spaces that were originally filled with air or water. Those chemicals bonded with the wood polymers and changed how the material interacts with moisture, organisms, and metal.

The treatment doesn't make wood waterproof or stronger. It makes it toxic to fungi and insects. The mechanical properties come from the wood structure, not from the preservatives. The rot resistance comes from making the wood uninhabitable for decay organisms.

The stamp on the end tells you which chemical system was used, what retention level the wood achieved, which use category it's rated for, and which treatment plant processed it. This information matters because using the wrong treatment category for your application means buying protection you don't need or, worse, getting inadequate protection for the conditions the wood will face.

The greenish or brownish tint fades over time but the treatment doesn't. The preservatives stay fixed in the cell walls, defending against rot and insect damage for decades. That's the actual product being sold, whether or not the lumber yard explains it that way.

This is treated lumber. It's not regular wood with a coating. It's wood that's been invaded at the microscopic level by chemicals designed to remain there permanently.