Pressure treatment only penetrates the outer layer. Every cut exposes untreated wood. Here's the science, the history, and the fix every Southern Oregon homeowner should know about.
The Treated Lumber Myth
Most people hear "pressure-treated lumber" and assume the entire board is protected. Rot-proof. Bug-proof. Good to go anywhere.
It's not.
Pressure treatment forces chemical preservatives into the wood under high pressure in a sealed cylinder. The preservatives penetrate the outer layer, typically 0.25 to 0.5 inches deep, but the interior core never gets treated. AWPA standards specify retention rates for that outer zone, not full penetration. Every cut on site exposes raw, untreated wood at the cut face, and end grain is the most vulnerable part of any board. Wood grain runs lengthwise like a bundle of straws, and moisture travels along those fibers much faster than across them. A cut end acts like a sponge, pulling water deep into the untreated core.
Why Pressure Treatment Exists
Old growth Douglas fir used to grow a dozen feet across and live for hundreds of years. The tree rings were so tight that milled planks naturally resisted moisture and could sit out in the weather for 30 years without rotting.
Today's lumber comes from farmed forests, trees grown fast and harvested young. Wide growth rings mean the wood is less dense, absorbs moisture quickly, and rots far faster than the old growth material that built homes for generations.
That drop in natural durability is why pressure treatment was developed. In the 1980s, copper-based treatments were introduced to force preservatives into the wood and bring farmed lumber closer to the lifespan people expected from old growth. It works, but only on the outer shell. The core still behaves like fast-growth wood, and every cut end reopens that vulnerability.
What the Photo Shows
The photo is a cross-section of a board pulled from a structure less than 20 years old. The outer shell held up perfectly, resisting rot and insects exactly as designed. But the interior? Completely hollowed out. The untreated core rotted away while the outside stayed solid.
This is what happens when cut ends are never sealed. Moisture enters through the exposed end grain, and nothing protects that interior wood.
Why This Matters in Southern Oregon
Decks here go through months of wet winter weather followed by intense summer heat. That freeze-thaw and wet-dry cycling is brutal on outdoor framing. Every cut end, posts, beams, joists, stringers, is a potential entry point for moisture. And those cut ends often sit in contact with concrete footings or metal brackets where water pools without drying out.
The Fix
After every cut, we brush on a copper-based wood preservative to seal the exposed end grain. The copper ions bond with the wood fibers and create a toxic barrier against fungi and insects. It takes seconds per cut and costs pennies, but a lot of builders skip it. Either they don't know the treatment doesn't go all the way through, or they figure the board is "treated" so it's fine.
It's not fine. The photo is the proof.
Trust the Process
Building a deck that lasts decades means getting the details right that nobody will ever see. Treating cut ends. Butyl joist tape over beam tops. Post brackets that keep wood off wet concrete. Hurricane ties that lock the framing down. None of it is visible once the deck is finished, but every piece of it is the difference between a structure that lasts and one that fails.
When you hire a contractor, you're trusting them with your home, your family's safety, and your investment. You should feel confident that the people building your deck understand the science behind every material they use, even the parts buried underground. Ask whether they seal cut ends with a preservative. The answer tells you whether they're building something to last, or just building something to pass inspection.