4x4 Lumber

A 4x4 isn’t 4 inches. It’s a 3½ × 3½ inch post — the standard size for residential decks, fences, pergolas, and sheds across North America, typically Southern Yellow Pine east of the Rockies and Douglas Fir-Larch west, graded #2 for structural use. Two decisions separate a post that lasts decades from one that rots at the base or twists into a pretzel: which treatment grade you need (ground contact and above-ground aren’t the same board), and whether a 4×4 is actually adequate for your post height and load.

Get those right and the rack inspection right, and you’ve covered most of what trips up buyers.

Reading the Size, the Stamp, and the Grade

Start with the dimension, because layout errors trace straight back to it. The “4×4” is a nominal size: the rough-sawn dimension before drying and planing remove the excess. Every face loses about ¼ inch in that process. The rule for dimensional lumber between 2” and 8” nominal is to subtract ½ inch to get actual. So 4.0 − 0.5 = 3.5 on each face.

That 3.5 number is what your hardware speaks. Post bases, post caps, and brackets are all sized to 3½”. When you’re calculating joist spans or footing spacing, use 3.5”, not 4. Use 4 in your math and your bracket won’t fit.

The grade stamp tells you the rest. Look for the grading agency (SPIB, WWPA, NLGA), the mill number, the species group (SYP, DF-L, HF, or SPF), the grade — Select Structural → #1 → #2 → #3, with most big-box 4×4s landing at #2 — and the moisture content: KD-19 (kiln-dried to ≤19%) or S-GRN (surfaced green, which will shrink further as it dries).

For most jobs, #2 is the IRC minimum. Select Structural or #1 gives you fewer knots and higher structural values — worth considering if posts are visible or if you’re working close to span table limits. A #1&Better grade is a step up from the #2 grade common at the big boxes, which matters when you’re loading posts under Montana snow.

One moisture note that bites people: S-GRN boards will shrink and may cup or twist as they dry. If you’re buying ahead of a project, let green boards sticker-stack for a few weeks first.

Spec	Detail
Nominal vs. actual	4×4 nominal = 3.5” × 3.5” actual
Hardware sizing	Bases, caps, brackets all sized to 3.5”
Common species	Southern Yellow Pine (east), Douglas Fir-Larch (west)
Standard grade	#2 (IRC minimum, most common at big-box)
Moisture marks	KD-19 (kiln-dried) or S-GRN (surfaced green)
Grades, best to least	Select Structural → #1 → #2 → #3

Treatment Level: The Mistake That Shows Up Years Later

This is where most buyers go wrong, and it’s the one worth slowing down for. Ground Contact and Above Ground boards look identical. The difference shows up a few years later when posts start rotting at the base.

The American Wood Protection Association defines treatment levels using “use categories” (UC). The UC is on the end tag — the paper or plastic stapled to the end of the board, or stamped directly on the face.

The two you’ll deal with on a 4×4:

Category	Condition	Use For
UC3B	Above ground, exterior, uncoated	Deck boards, railings, fence pickets, pergola beams not touching grade
UC4A	Ground contact or within 6” of grade	Deck posts, fence posts, structural members near grade

UC4A has more than twice the copper retention of UC3B for posts touching soil, concrete, or within 6 inches of grade. The decision rule is simple: post set in concrete, in a post base, or touching soil → UC4A; post elevated on a framing member or hardware, never touching soil → UC3B.

Here’s the part to commit to memory. Don’t trust the color. Green-colored PT lumber can be either level. Read the tag.

And lean toward UC4A as the default outdoors. Even elevated post bases wick moisture, and concrete transmits moisture — a post base doesn’t eliminate contact. That makes UC4A the safe call for deck, pergola, and shed posts. In a climate where spring snowmelt keeps the ground line wet for weeks, that copper retention earns its keep.

Whatever level you pick, the fasteners have to match. Use hot-dipped galvanized (ASTM A153) or stainless steel fasteners with PT lumber — the copper preservatives eat ordinary hardware.

Species, Treated vs. Untreated, and When 4×4 Isn’t Enough

Match the wood to the job. Pine/SPF boards are well suited for indoor projects and rustic-look furniture, and they’re a fine choice for garages and basements. For outdoor projects, though, pine/SPF isn’t a good choice — it has no resistance to decay and wicks up moisture readily, so it rots quickly even when stained or finished, if exposed to rain or high-moisture conditions. Cedar is naturally resistant to decay, so it lasts well outdoors — lightweight and easy to work — though it’s a lower-strength choice.

The treated-versus-untreated math is stark. While untreated pine may last only 2–5 years in outdoor conditions, pressure-treated lumber can endure 15 to 40 years depending on climate, usage, and maintenance. Among species, SYP has higher compressive strength than Douglas Fir-Larch, which is why it dominates PT lumber east of the Rockies. It takes preservative treatment well and holds up under post loads. Both species work for structural posts when properly graded.

Know where a 4×4 runs out of room. It does not meet minimum depth requirements for most residential floor joist or ceiling applications under the IRC. For posts, the limits run like this:

Use	Limit
Deck post height	Up to 6 ft unbraced (per IRC); taller decks may require 6×6
Fence post spacing	6–8 ft apart for stability; embed depth per height and local code
Short shed beam	~4 ft span depending on load

For heavier loads, larger beams like 6×6s may be required.

Inspecting the Rack: Don’t Grab a Peeler-Core Post

The right grade and treatment still warp if you pull a bad post. The culprit is the peeler core. Many posts, especially 8-footers, come from logs used as peeler cores for plywood veneer, which is peeled from the log like paper towels from a roll. That process leaves only the very center of the log.

That center is trouble. The center “pith” is the least stable part of the log and is very prone to twisting, splitting, and bowing. This is particularly true of treated 4x4s, because the treatment exaggerates the warping.

You spot it at the ends. Examine the stack and reject posts with end grain that looks like a bulls-eye. Choose the straightest posts you can find, with the least amount of center pith. You want posts without a “bulls-eye” and without growth rings running edge to edge. If growth rings start near the center and expand outward, it’s almost certainly a peeler core. If the rings are off-center, it’s not the log’s center.

A practical workaround: buy 10-ft posts instead of peeler-core 8-footers and trim them to fit. Perfect lumber is rare, so it’s worth taking a little extra time at the rack to find the best you can.

Two finishing habits before you cut. Treated boards often arrive heavy and wet — pressure-treated 2x4s and 4x4s are often wet due to the treating chemicals, and they tend to warp and twist as they dry. Sticker-stack green stock a few weeks first. Then square up your reference end: trim a small amount — just 1/8” or 1/4” — off one end of every board before cutting project parts, so you get a smooth end you know is square.

Frequently Asked Questions

What is the actual size of a 4×4? The actual size is 3.5 inches by 3.5 inches. The board is cut at full nominal dimensions, then planed smooth, removing about ½ inch from each side. Use 3.5” for layout and hardware, never 4”.

Do I need ground-contact lumber for my deck posts? Yes. A post set in concrete, in a post base, or touching soil should be UC4A. Even elevated post bases wick moisture, and concrete transmits moisture, so UC4A is the safe default for deck, pergola, and shed posts. Above-ground deck boards and railings can use UC3B.

How can I tell UC4A from UC3B? Read the end tag, not the color. The use category is on the end tag stapled to the board, or stamped on the face. Green-colored PT lumber can be either level — read the tag.

Can I use untreated pine outdoors if I seal it? No. Pine/SPF has no resistance to decay and wicks up moisture readily, so it rots quickly even when stained or finished if exposed to rain or high-moisture conditions. Use pressure-treated or a naturally rot-resistant species for outdoor work.

Is a 4×4 strong enough for a deck post? For shorter decks, yes. The IRC allows 4×4 posts for deck supports, but taller decks may require 6×6 posts. A 4×4 works up to about 6 feet of unbraced height.

What fasteners should I use with treated lumber? Hot-dipped galvanized (ASTM A153) or stainless steel. The copper-based preservatives corrode ordinary fasteners.

Why do my treated posts keep warping? Likely peeler-core stock. The center pith is the least stable part of the log and is prone to twisting, splitting, and bowing — and treatment exaggerates the warping. Inspect the ends and reject bulls-eye grain.