Preserved wood shows up in nearly every outdoor project: deck frames, fence posts, retaining walls, and the sill plates under a house. The label means the lumber has been run through a pressure treatment process that pushes preservative deep into the fibers rather than brushing it on the surface. Trade magazines track how dealers and contractors adapt to changing demand, and the equipment rental industry has published its own version of those insights; buyers who follow them catch sourcing and price trends early.
This article explains what the treatment process does, how to read the ratings stamped on each board, which preservative systems are on the market, and which fasteners will not corrode.
What Preserved Wood Is and How the Pressure Process Works
Pressure treatment happens inside a large steel cylinder. Lumber is stacked on rail cars, rolled into the cylinder, and sealed. A vacuum pulls air from the wood cells, then the cylinder floods with a water-based preservative solution. Pressure forces the solution into the wood, and a final vacuum pulls the excess liquid back out.
- Stack and load the lumber into the treatment cylinder.
- Draw a vacuum to pull air out of the wood cells.
- Flood the cylinder with the preservative solution.
- Apply pressure, often 100 to 200 psi, for the time the species and retention require.
- Release the pressure, drain the cylinder, and pull a final vacuum to recover surplus solution.
How deep the chemical goes depends on species. Southern yellow pine and other pines accept deep penetration because their cells are open. Douglas fir and other western woods resist penetration, so producers incise the surface with small slits before treatment to give the preservative a path in. The result is a protective shell around the core.
Why penetration depth matters
Penetration is what separates treated lumber from a dipped or brushed board. A shallow shell can be broken by a saw cut or a drilled hole, so field cuts need a brush-on preservative. Deep penetration means the protection survives years of weather and the occasional gouge.
Where preserved wood is required
Model codes require preservative-treated or naturally durable wood for structural members in specific locations: sill plates in flood hazard areas, framing within 6 to 18 inches of grade in termite zones, and wood in contact with concrete or masonry in damp conditions.
Keeping treatment charts, end-tag samples, and project notes organized in the shop follows the same logic as good home storage organization: label each bin, store by category, and keep what you reach for most at eye level. A builder who can put a hand on the right spec sheet saves the same minutes a homeowner saves finding a spare part.
Preservative Systems on the Market Today
For decades, CCA (chromated copper arsenate) dominated the residential market. The EPA and the industry agreed to phase it out of most residential uses at the end of 2003, and a new generation of copper-based systems took over: ACQ (alkaline copper quaternary), copper azole, and micronized copper quat. Each protects against decay fungi and termites using copper as the active ingredient, with a second compound that stops copper-tolerant fungi.
| System | Active ingredients | Typical use | Notes |
|---|---|---|---|
| ACQ | Copper plus quaternary ammonium | Decks, fences, ground contact | Most common above-ground and ground-contact treatment since 2004 |
| Copper azole | Copper plus an azole fungicide | Decks, ground contact, marine splash | Reaches required retention at lower copper loadings than ACQ |
| Micronized copper quat | Microscopic copper particles plus quat | Decks, fences, interior framing | Copper particles stay suspended in the treating solution |
| Borate | Disodium octaborate tetrahydrate | Interior framing, sill plates, termite zones | Water-soluble; not for weather exposure or ground contact |
| CCA (legacy) | Chromated copper arsenate | Retired from residential use after 2003 | Still found in older structures; take care when cutting or burning |
Why copper-based systems replaced CCA
The switch was driven by arsenic concerns in residential settings. Copper systems deliver the same decay and termite protection without the chromium and arsenic, at the cost of being more corrosive to fasteners. That single trade-off drives most of the fastener guidance in this article.
Borate: protection that stays indoors
Borate-treated lumber is popular for framing in termite-prone regions because the chemical diffuses through the wood and stops termites that ingest it. It stays put only where it stays dry; rain, ground contact, or constant dampness leaches it out.
Cutting, Framing, and Installing Preserved Wood
Treated lumber is harder on tooling than untreated stock because the preservative is abrasive and the wood is often wet. Carbide-tipped blades and bits hold an edge longer than steel. Predrill near the ends of boards to keep fasteners from splitting them, and treat every field cut with a brush-on copper preservative so the exposed shell is resealed.
Cutting and layout tips
- Cut with carbide-tipped circular saw blades and drill bits.
- Predrill holes near board ends and corners.
- Re-treat all cut ends with copper naphthenate or a matching brush-on preservative.
- Keep cutoffs off the ground and out of garden beds; do not burn treated scraps.
- Sort and sticker the stock so it can dry flat before installation.
Field treatment of cut ends
Every cut exposes untreated wood. A brush-on preservative restores a working barrier in minutes, and the coating deserves a refresh on ground-contact posts every few years. Skipping this step starts rot at the exact place the structure depends on.
Opening bundles and trimming flashing calls for a sharp edge too, and an auto-loading utility knife with a blade magazine system keeps a fresh blade ready without anyone carrying loose spares in a pocket.
Fastener and Hardware Selection for Treated Lumber
Copper in modern preservatives corrodes ordinary steel and aluminum. Fasteners and connectors on treated lumber need hot-dipped galvanized coating meeting ASTM A153, or stainless steel in high-corrosion conditions. Aluminum flashing, vents, and rails must never touch treated wood; copper eats aluminum fast.
| Fastener or hardware | Material or coating | Where it fits | Notes |
|---|---|---|---|
| Nails | Hot-dipped galvanized (ASTM A153) or stainless | Framing, decking, fencing | Electroplated bright galvanized nails fail fast on treated wood |
| Screws | Hot-dipped galvanized or stainless | Decking, hangers, general | Rated screws for treated lumber carry a corrosion classification |
| Bolts and lag screws | Hot-dipped galvanized or stainless | Post connections, beams | Use washers sized for treated lumber |
| Structural connectors | G185 galvanized or stainless | Joist hangers, post bases, ties | Check the connector label for treated-wood approval |
| Aluminum items | Never in contact with treated wood | Flashings, trim, vents | Copper in the preservative corrodes aluminum |
Nails versus screws
Joist hangers and framing connections call for nails because they shear less than screws under lateral load. Screws win for deck boards and anything that may come apart later. Either way, coating matters more than geometry: a cheap electroplated nail fails in months where a hot-dipped galvanized nail lasts decades.
Connector ratings and treated wood
Look for the G185 or stainless marking on joist hangers and post bases. Standard G90 galvanized connectors are not rated for direct contact with ACQ or copper azole stock, and the corrosion happens invisibly inside the joint.
The tool on site shapes the choice too: cordless roofing nailers with fuel cell power and several magazine types now drive framing and decking nails that once required a compressor, so fastener selection starts with what the crew can actually set.
Retention Levels, End Tags, and Use Categories
Retention is the amount of preservative left in the wood after treatment, measured in pounds per cubic foot (pcf). It tells you whether a board is rated for a dry deck above ground or a fence post buried in wet soil. Above-ground ACQ stock typically carries retention around 0.15 to 0.25 pcf, while ground-contact grades run 0.40 pcf and higher. Copper azole reaches the same protection at lower loadings, often about half the ACQ figure.
- Find the end tag or brand stamped on the board.
- Check the treatment standard and the preservative name, such as ACQ or CA-B.
- Read the retention number and the use category, usually written as UC3A, UC3B, or UC4A.
- Look for the third-party inspection stamp and the treating plant ID.
- Match the use category to the exposure: UC3A for protected above-ground, UC3B for exposed above-ground, UC4A for ground contact.
Above ground versus ground contact
Above-ground lumber sheds water and dries out between rains, so it needs less protection. Ground-contact lumber sits against wet soil, mulch, or concrete, where moisture never leaves and decay fungi never stop working. Using above-ground stock in a ground-contact location is the most common cause of early rot.
- UC2: interior, protected from weather; light duty
- UC3A: above ground, protected by a covering
- UC3B: above ground, exposed to weather, not touching the ground
- UC4A: ground contact, general use such as fence posts and landscape timbers
- UC4B: ground contact, heavy duty, for retaining walls and structural posts
- UC5: marine exposure, for saltwater contact
KDAT and moisture content
Kiln-dried after treatment (KDAT) stock is re-dried at the plant so it arrives at a stable moisture content. Standard treated lumber can arrive wet and then shrink, cup, and twist as it dries on the job. For deck boards laid with tight gaps, KDAT stock is the difference between a flat surface and one that opens up within a season.
Safety, Handling, and Long-Term Care
Wear a dust mask when sawing, gloves when handling, and wash up before eating. Sawdust from treated wood should never go into compost or mulch, and treated scraps must not be burned; the smoke carries copper and, in older CCA stock, arsenic.
Bundle straps and wrapper film come off faster with a proper cutting tool, and auto-loading utility knives with magazine blade systems reduce the reach-for-a-loose-blade accidents that send crews to first aid.
Care that extends service life
Treated wood still needs water to drain. Keep grade and mulch below the bottom of siding and deck framing, space deck boards so water falls through, and keep sprinklers away from post bases. Ends cut in the field need the brush-on preservative refreshed every few years on ground-contact posts.
- Soft, spongy wood near the ground line that a screwdriver pushes into
- Cracked or split post tops holding water
- Loose, corroded, or missing fasteners in connectors
- Fungal growth or heavy checking in structural members
- Bowed or sagging beams or joists
Inspection routine
A quick annual check takes minutes: probe post bases, wiggle rail posts, and look at connector corrosion. Catching a soft spot while it is a repair, not a rebuild, is the cheapest maintenance on the site.
Before the build, look at how finished outdoor rooms are presented in current design coverage; home design magazines influence residential construction and renovation decisions, and the details they popularize, from stair proportions to railing profiles, often become the features buyers expect. A preserved wood structure planned with those expectations in mind sells itself.