Treated 4x4 Lumber

A treated 4x4 isn’t 4 inches square. It’s a 3½ × 3½ inch lumber post — the standard post size for residential decks, fences, pergolas, and sheds in North America, typically Southern Yellow Pine east of the Rockies and Douglas Fir-Larch west, graded #2 for structural use. West of the Rockies — which means most of Montana — Douglas Fir-Larch is what shows up at the yard.

Here’s the part that trips up most buyers. There are two treatment levels, and they look identical. Ground Contact and Above Ground boards are nearly impossible to tell apart by eye. The difference shows up a few years later when posts start rotting at the base. Get this one decision right and a post lasts decades. Get it wrong and you’re digging it out before the deck pays for itself.

Ground Contact vs. Above Ground — Read the End Tag

This is the whole ballgame. 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.

Two levels matter for a residential 4x4. UC3B protects above-grade exposure; UC4A has more than twice the copper retention for posts touching soil, concrete, or within 6 inches of grade.

The decision rule is dead simple. If posts touch soil, are in concrete, or are within 6 inches of grade, use UC4A. If elevated on framing members and never touching soil, UC3B is sufficient and often more economical.

What You’re Building	Treatment Level	Why
Deck posts (in concrete or post base)	UC4A	Even elevated post bases wick moisture; UC4A is the safe default
Fence posts set in ground or concrete	UC4A	In-ground always counts as ground contact
Pergola posts on concrete footings	UC4A	Concrete transmits moisture; a post base doesn’t eliminate contact
Deck boards and railings	UC3B	Any above-ground exposure qualifies
Pergola beams not touching grade	UC3B	Above-grade exposure only

Whatever you do, don’t go by color. Don’t rely on the color. Green-colored PT lumber can be either level. Read the tag. Brown or green tint comes from the treatment formula, not the use category. ACQ, copper azole, and similar treatments give posts their familiar brown or green tint.

Size, Species, and Grade

The actual dimension drives every piece of hardware you buy. Post bases, caps, and brackets are all sized to 3½ inches. When you’re calculating joist spans or footing spacing, use 3.5 inches, not 4. A lot of layout errors trace back to this one confusion.

On grade: For deck posts, shed framing, and fence posts, #2 is the IRC minimum and what most stores carry. 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.

The chemistry changed years back, and for the better. CCA was popular for a long time but has been phased out in residential settings. Alkaline Copper Quaternary (ACQ) replaced it for decks and fencing. Copper Azole contains copper and organic azole fungicides. Micronized Copper Quaternary (MCQ) is a newer residential preservative using finely ground copper particles. All copper-based. All effective against the rot and insects that go after buried wood.

In Montana ground, expect real-world numbers to swing with drainage. Pressure-treated fence posts can last 20 to 40 years. In dry, well-drained soils, a properly installed post can easily reach the upper end of that range; in wet, clay-heavy, or poorly drained sites, life expectancy is shorter. The Flathead’s lacustrine clay and the spring snowmelt that keeps the ground line wet for weeks both push toward the short end of that span.

When a 4x4 Isn’t Enough — and How to Set It Right

A 4x4 covers a lot of jobs. It’s the most common choice for residential privacy fences, garden fences, and decorative boundaries up to about 6 to 8 feet tall. Step up when the load or exposure climbs. 6x6 posts are better for heavy gates, taller fences, and stronger corner support — and stepping up in key locations like gate openings, corners, and long exposed runs improves stability, alignment, and long-term performance.

For decks, code draws a clear line. Deck posts over 6 feet require 6x6 or larger lumber, footings must extend below the frost line, post-to-beam connections must be mechanically fastened, and bracing is mandatory in wind-exposed areas. That frost line matters here — across most of western Montana you’re already digging 36 to 48 inches, so plan post depth accordingly.

Installation makes or breaks the board. Start with the fasteners. Always use corrosion-resistant fasteners such as galvanized or stainless steel to prevent premature deterioration caused by chemical reactions between the wood preservatives and regular nails or screws. The copper that protects the wood eats ordinary steel.

Two more things separate a post that lasts from one that doesn’t:

• Seal every cut. Seal or treat any cut ends. This keeps the integrity of the treated wood intact and gets you the longevity you want from it. The factory treatment doesn’t reach the fresh end grain you just exposed with a saw.
• Pick the right backfill. Concrete footings create a very rigid base for most residential fences. In heavy clay or poorly drained soils, a gravel backfill may drain better and reduce frost heave. In the Flathead’s clay, gravel often beats concrete at the ground line.

For burial depth, the old rule still holds. Plan to bury roughly one-third of the post length below ground — for example, a 9-foot post for a 6-foot fence — subject to your local building code and frost depth.

And cap the tops. Post caps or a sloped cut on the post top help shed water and reduce checking and splits.

Frequently Asked Questions

What’s the difference between UC3B and UC4A treated lumber? UC3B protects above-grade exposure, while UC4A has more than twice the copper retention for posts touching soil, concrete, or within 6 inches of grade. If moisture contact is possible — ground, concrete, or soil — choose UC4A. If the post is never touching soil and only sees above-ground exposure, UC3B is sufficient and often more economical.

Is a treated 4x4 really 4 inches square? No. The actual size of a 4x4 board, once surfaced on four sides, is 3.5 inches by 3.5 inches. The “4x4” is a nominal size — the rough-sawn dimension before drying and planing remove the excess. The rule for dimensional lumber between 2 and 8 inches nominal: subtract half an inch to get actual.

Can I tell the treatment level by the color of the wood? No. Green-colored PT lumber can be either level. Read the tag. The brown or green tint comes from the preservative formula — ACQ, copper azole, and similar treatments — not the use category.

When do I need a 6x6 instead of a 4x4? Deck posts over 6 feet require 6x6 or larger lumber. For fencing, a 4x4 handles standard residential fences up to about 6 to 8 feet, while 6x6 posts are recommended for taller fences, heavy gates, corner posts, and locations with high wind loads or sloping terrain.

What fasteners work with treated lumber? Hot-dipped galvanized (ASTM A153) or stainless steel. The copper preservative reacts with bare steel and corrodes it. Regular nails and screws deteriorate prematurely against the wood’s chemicals.

How long will a treated post last in the ground? With the right treatment, installation, and site conditions, pressure-treated posts last 20 to 40 years. Dry, well-drained soils reach the upper end; wet, clay-heavy, or poorly drained sites run shorter.

Do I have to seal the cut ends? Yes. Seal or treat any cut ends to keep the integrity of the treated wood intact and get the longevity you want. The pressure treatment doesn’t fully penetrate to the core, so a fresh saw cut exposes wood the preservative never reached.