If you’ve ever walked through a lumber yard and felt that specific, resinous scent of sawdust, you know exactly what a 4x4 looks like. It’s beefy. It’s square. It’s the literal backbone of every deck, fence, and pergola in your neighborhood. But here’s the thing—it’s not actually four inches by four inches. If you take a tape measure to a standard piece of 4x4 wood, you’re going to find it’s actually 3.5 inches by 3.5 inches. That half-inch disappears during the drying and planing process at the mill. It’s one of those industry quirks that confuses first-time DIYers every single year.
People love this dimension because it feels permanent. You hold a 4x4 and it has weight. It has gravity. It feels like it could hold up a house, though you should probably check your local building codes before trying that.
The Reality of Choosing 4x4 Wood for Your Project
Not all 4x4s are created equal. Far from it. If you walk into a big-box retailer like Home Depot or Lowe’s, you’re usually looking at three main options: pressure-treated, cedar, or Douglas fir. Each one behaves differently under pressure and, more importantly, under the sun.
Pressure-treated (PT) lumber is the workhorse. It’s usually Southern Yellow Pine infused with chemicals—typically Copper Azole (CA-C) or Micronized Copper Quaternary (MCQ)—to keep the bugs and rot away. It's green-ish, heavy as lead when it's "wet" from the treatment, and it's remarkably cheap. But it warps. Man, does it warp. Because these posts are saturated with liquid, they tend to twist like a pretzel as they dry out in the sun. If you’re building a fence, a twisted 4x4 wood post means your gate won't shut in six months.
Cedar is the "luxury" choice. It’s naturally rot-resistant because of the tannins and oils in the wood. It smells incredible. It stays straighter than PT lumber. But it’s soft. If you hit a cedar 4x4 with a lawnmower, it’s going to leave a massive dent. You’re also going to pay roughly double, sometimes triple, what you’d pay for treated pine. Is it worth it? Honestly, it depends on if you're looking at it every day. For a pergola where the wood is right at eye level, cedar is a no-brainer. For a fence post buried in three feet of concrete? That’s just burning money.
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Why Your Posts Keep Rotting at the Ground Line
This is the biggest misconception in landscaping. People think "ground contact" rated wood is invincible. It’s not. Most 4x4 wood failures happen exactly where the wood meets the soil or the top of the concrete pier.
Why? Because water pools there. Fungus loves that. Even the best chemical treatment eventually leaches out or fails to reach the very center (the heartwood) of the post. If you saw a 4x4 in half, you’ll often see a "pith"—that tiny circle in the middle. That's the center of the tree. It's the most unstable part. When moisture gets in there, the post starts rotting from the inside out.
To avoid this, savvy builders have moved away from burying posts directly in the dirt. They use galvanized steel stand-off bases. You bolt the base to a concrete footing, and the 4x4 wood sits an inch above the ground. It looks cleaner and adds a decade to the life of the structure. It’s a small detail, but it’s the difference between a 10-year deck and a 30-year deck.
Sorting Through the Grades: What "No. 2" Actually Means
When you see a stamp on a piece of 4x4 wood that says "No. 2 Grade," that’s the industry standard for structural use. It means it has some knots, maybe a little bit of "wane" (that’s when the edge is rounded because it was cut from the outer part of the log), but it’s solid.
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However, for furniture or high-end outdoor structures, you might want "Appearance Grade" or "Clear." These are hand-selected pieces. No huge knots that might fall out. No cracks. If you're building a farmhouse-style dining table, you’re looking for Douglas Fir in a higher grade. Doug Fir is prized for its strength-to-weight ratio and its beautiful reddish tint. It’s stiffer than pine, which makes it great for long horizontal spans, but it doesn't take "ground contact" as well as treated pine or cedar.
Then there's the "heartwood vs. sapwood" debate. Sapwood is the outer part of the tree that carries water; it’s basically candy for termites. Heartwood is the dead inner core; it’s dense and naturally resistant. If you can find 4x4 wood that is "all heart," you’ve found the holy grail of lumber. It’s rare in big-box stores but common at specialized architectural lumber yards.
Maintenance: The Part Everyone Skips
You cannot just buy 4x4 wood, build a fence, and forget it. Well, you can, but it’ll look like grey drift-wood in two years. UV rays break down lignin in the wood fibers. This causes that silver-grey weathering.
- Seal it early: If you bought "green" treated wood, wait for it to dry (usually 2-4 weeks until water beads off it) then hit it with a penetrating oil stain.
- End-grain treatment: The ends of a 4x4 act like straws. They suck up water. If you cut a post, you must brush some copper naphthenate or a similar preservative onto the raw cut.
- Cap it off: Never leave the top of a vertical 4x4 wood post exposed to the rain. Put a decorative metal cap on it or cut it at a 45-degree angle. This keeps water from sitting on the end grain and starting the rot process.
I’ve seen $20,000 decks ruined because the contractor didn't spend $50 on end-grain sealer. It’s a tragedy of the trades.
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Structural Limits and Safety
Can a 4x4 hold up a second-story deck? Generally, yes, but the height is the limiting factor. According to the American Wood Council's Span Tables, a 4x4 wood post’s load capacity drops significantly as it gets taller. This is called "slenderness ratio." Basically, the taller the post, the more likely it is to buckle under a vertical load.
In many modern building codes (like the IRC), 6x6 posts have replaced 4x4s for decks that are more than a few feet off the ground. A 6x6 is nearly three times as strong as a 4x4 because of the way the math works on sectional area. If you’re building something where humans will be standing on top, always overbuild. A 4x4 is great for a garden bench or a mailbox; it’s risky for a balcony.
Actionable Next Steps for Your Project
- Check the Stamp: Before buying, look for the ALSC (American Lumber Standard Committee) grade stamp. Ensure it says "Ground Contact" if it's touching the earth.
- The "Twist" Test: Sight down the length of the board like you’re aiming a rifle. If it curves more than half an inch over eight feet, put it back. You'll never get it straight.
- Weight Matters: If the 4x4 feels unnaturally heavy, it’s full of chemical water. It will shrink as it dries. Factor that into your joinery.
- Hardware: Only use hot-dipped galvanized or stainless steel screws and bolts. The chemicals in treated 4x4 wood will literally eat through standard zinc screws in a matter of months.
If you focus on moisture management and pick the right species for the right environment, 4x4 wood is one of the most versatile tools in your DIY arsenal. It’s accessible, relatively affordable, and—with a little bit of oil and a good cap—it’ll outlast the mortgage.