4x4x8 Lumber

4x4x8 at a Glance

A 4×4×8 is 3½ × 3½ inches actual, 8 feet long. The two decisions that matter: which treatment level to buy (ground contact and above-ground aren't the same board), and whether a 4×4 post is adequate for your height and load. Get those right before you head to the lumberyard.

In this guide:

• Actual dimensions and how to read the grade stamp
• Which treatment level to buy for your application
• Post height limits and load capacity
• Where a 4×4 works and where it doesn't
• Fasteners, end-cut treatment, and installation rules

Actual Dimensions and What the Numbers Mean

A 4×4×8 is not 4 inches wide. It's 3½ inches wide.

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: subtract ½ inch to get actual. So 4.0 – 0.5 = 3.5 on each face.

The 8 in "4×4×8" is the length in feet. Per NIST Publication P20, actual length must equal or exceed nominal, so an 8-foot board is at minimum 8'0" and sometimes arrives a few inches longer.

Why this matters when you're building: 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. A lot of layout errors trace back to this one confusion.

Species: What You'll Find at the Lumberyard

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.

How to Read the Grade Stamp

Every structural 4×4 has a grade stamp inked on the board. Five elements:

1. Grading agency: SPIB, WWPA, NLGA — the organization that certified the grading
2. Mill number: The specific sawmill
3. Species group: SYP, DF-L, HF, or SPF
4. Grade: Select Structural → #1 → #2 → #3 (most big-box 4×4×8s are #2)
5. Moisture content: KD-19 (kiln-dried to ≤19%) or S-GRN (surfaced green, which will shrink further as it dries)

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.

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.

Which Treatment Level to Buy

This is where most buyers go wrong. Ground Contact and Above Ground boards look identical on the shelf. The difference shows up a few years later when posts start rotting at the base.

The American Wood Protection Association (AWPA) 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 decision rule for a 4×4×8:

• Post set in concrete, in a post base, or touching soil → UC4A
• Post elevated on a framing member or hardware, never touching soil → UC3B

Don't rely on the color. Green-colored PT lumber can be either level. Read the tag.

Application Decision Table

Modern Treatment Chemistry

CCA (chromated copper arsenate) was the standard until 2004, when EPA removed it from residential lumber because of the arsenic content. Today's treatments are copper-based:

ACQ (Alkaline Copper Quaternary): Most common at big-box stores. Copper kills decay fungi; a quaternary ammonium compound handles copper-tolerant organisms. The greenish tint on most PT lumber is copper.

CA or CA-C (Copper Azole): Copper plus an azole fungicide (tebuconazole or propiconazole). Common under the brand name Wolmanized.

MCA (Micronized Copper Azole): Same chemistry as CA but with micronized copper — particles small enough to absorb directly into wood cells. Brands include Preserve and ProWood. Often has a browner appearance than ACQ.

For deck and shed work, all three are equivalent at UC4A retention. The brand doesn't matter. The UC level does.

Treated vs. Untreated: How Long Before Rot?

Untreated pine outdoors without any protection: 5 to 10 years before significant decay. Untreated pine in direct ground contact: 2 to 4 years in most climates, faster in wet regions. Cedar untreated above grade lasts 15 or more years. Redwood untreated can last 50 years above grade.

Use untreated wood where it makes sense: cedar fence pickets, interior framing. Not where moisture contact is routine.

Post Height and Load Capacity

A 4×4 post carries load vertically through compression. Capacity drops with height because longer unsupported posts buckle more easily.

Vertical Load Capacity by Height

These figures are approximate for #2 Southern Yellow Pine under axial compression:

For comparison: a 6×6 post at 8 feet carries approximately 18,000 lb, nearly three times as much.

A standard 12×12 deck at 50 psf (40 psf live + 10 psf dead) puts about 7,200 lb total on the posts. Split across four posts, that's 1,800 lb per post. Well within a 4×4's capacity at 8 feet. Load capacity rarely limits 4×4 posts; height and lateral buckling do.

2021 IRC Height Limits (Table R507.4)

The 2021 IRC revised deck post sizing to account for both species group and tributary area. Previous editions applied a blanket 8-foot limit regardless of the load.

Under the updated table:

• For Southern Pine and Douglas Fir posts at typical residential loads, 4×4s are allowed up to 8 feet in most configurations
• For small tributary areas (stair landings, small platforms), the 2021 IRC permits 4×4 posts up to 14 feet
• For posts over 8 feet in standard residential deck framing: use 6×6

Check with your local building department. Some jurisdictions still enforce the older 8-foot blanket limit.

The Notching Problem

If you're planning to notch a post for a doubled 2× beam connection, a 4×4 won't work. After notching, there isn't enough material left. A 6×6 has room for the notch and remains structural.

If you're using 4×4 posts, connect beams with a post cap instead. Simpson BC40 is the standard hardware for this. It keeps the post intact and the connection code-compliant.

A 4x4 Is Not a Deck Beam

Standard beam span tables (IRC 2021) don't include 4×4 beam data. The smallest single-member beam listed for deck framing is a 4×6, and that's not a gap in the table.

4×6 beam maximum spans (40 psf live + 10 psf dead load, #2 Douglas Fir-Larch):

A 4×4 spanning 8 feet horizontally under real loads will deflect noticeably. For deck framing, use 4×6 minimum. Size up to 4×8 or doubled 2× lumber once joist spans or beam spacing increase beyond those limits.

Where a 4x4x8 Works and Where It Doesn't

For pergola crossbeams, a 4×4 handles light decorative loads at spans up to 10 feet. Add climbing vines, solid shade fabric, or significant snow accumulation and the effective span drops quickly. When in doubt, use 4×6. It costs a few dollars more per board and eliminates the deflection problem.

Fasteners and Installation Rules

Required Fasteners

ACQ and CA treatments corrode ordinary steel. Standard electro-galvanized fasteners fail in ACQ-treated wood within a few years. The IRC (R317.3.1) specifies exactly what's acceptable:

• Hot-dipped galvanized to ASTM A153: the standard for nails, lag screws, and bolts in PT lumber
• Stainless steel, Type 304 minimum: required for pool decks and salt-air environments
• Type 316 stainless: for direct saltwater exposure

For connectors (post bases, joist hangers, beam brackets): use hardware rated for ACQ or CA use. Current Simpson Strong-Tie and USP/MiTek connectors carry ACQ ratings. Check the packaging. Connectors need ASTM A653 G185 galvanizing at minimum.

Don't mix metals. Stainless screws in a galvanized connector cause galvanic corrosion at the contact point. Match metals throughout the connection.

End-Cut Treatment

Every cut exposes untreated wood. The preservative only penetrates so far from the surface, and end grain absorbs moisture readily.

AWPA Standard M4 requires field treatment of all cuts in structural applications. The standard product is copper naphthenate at 2% copper, sold as Tenino Copper Naphthenate, QNAP by Nisus, or similar at hardware stores.

Apply it with a brush. Multiple coats on end grain. Let it soak in before installing hardware or setting the post in concrete.

At the bottom of posts, this matters most. A post set in concrete without end-cut treatment rots at the base faster than one properly treated, and you won't see the damage until the post moves.

Skipping end-cut treatment voids most PT lumber warranties.

Storage Before Installation

Most big-box PT lumber is wet, not KDAT (kiln-dried after treatment). Painting or staining wet PT traps moisture and the finish peels within a season.

Before finishing:

• Stack boards flat with 1" stickers between each layer for air circulation
• Cover from rain but allow airflow; don't wrap in plastic
• Keep boards off bare concrete using stickers or scrap lumber
• Allow 2 to 6 weeks of drying before painting or staining

Safety when working with PT lumber: Wear gloves and wash hands before eating. Wear a dust mask when cutting — don't breathe PT sawdust. Don't burn scraps; the copper compounds produce toxic smoke. Keep sawdust away from edible gardens.

Pricing and What to Expect at the Lumberyard

Lumber prices shift with the market. These are reasonable benchmarks at major big-box retailers in 2025. For tips on buying lumber in general — reading board quality, big-box vs. lumber yard, rough vs. S4S — see the buying lumber guide.

The treatment premium is typically $3 to $6 per board. Worth every cent on a structure you're building to last 20 years.

Pricing runs 20 to 30% lower in late fall and winter than at peak spring construction season. Buying in November for a summer deck project saves real money.

What to check before buying each board:

• No splits running more than a few inches
• No loose knots (a finger poke should feel solid)
• No significant twist or cup
• Wane (missing edge material) is fine on the hidden face, not where it carries load

Common Mistakes

Buying Above-Ground PT for posts set in concrete. Ground contact and above-ground boards look the same. The end tag tells you which is which. Posts in concrete bases or touching grade need UC4A.

Using standard galvanized fasteners. Old nails from the garage drawer are electro-galvanized. ACQ treatment corrodes them. Buy fasteners that say hot-dipped galvanized, ASTM A153, or stainless steel.

Skipping end-cut treatment. Two minutes with a brush at the bottom of each post adds years of life. It's a condition of warranty coverage on most PT brands.

Using a 4×4 as a deck beam. It won't support the load. Use 4×6 minimum, sized per the IRC beam span table for your joist span and spacing.

Installing wet PT lumber and painting over it. The paint traps moisture and peels fast. Stack and sticker for a few weeks first, or buy KDAT boards.

Sources

• American Wood Protection Association (AWPA). Preservative Use Categories. preservedwood.org/the-story/preservative-use-categories/
• Southern Pine Inspection Bureau (SPIB). Grademarks. spib.org/wood-services/lumber-grade-marks
• Southern Forest Products Association. Choosing Pressure-Treated Lumber: Which Preservative Level Is Best? southernpine.com
• Preserved Wood. Fasteners Advisory. preservedwood.org/how-to/fasteners/
• Simpson Strong-Tie. Preservative-Treated Wood FAQ. strongtie.com
• American Wood Council. What Is the Best Way to Deal With Exposed Ends of Unpreserved Wood When Pressure Treated Wood Is Cut? awc.org
• Preserved Wood. Field Treating. preservedwood.org/how-to/field-treatment/
• Decks.com. Deck Beam & Header Span Chart. decks.com/resource-index/framing/beam-span-chart-table/
• Engineer Fix. Should I Use 4×4 or 6×6 Deck Posts? engineerfix.com
• International Residential Code (2021). Section R507: Exterior Decks.
• U.S. Environmental Protection Agency. Overview of Wood Preservative Chemicals. epa.gov
• National Pest Information Center. Treated Wood Fact Sheet. npic.orst.edu/factsheets/treatedwood.html
• Archtoolbox. Lumber Dimensions: Nominal vs. Actual. archtoolbox.com/lumber-dimensions/
• Truitt & White Lumber. A Builder's Guide to Framing Lumber Grades. truittandwhite.com
• Western Wood Products Association. Interpreting Grade Stamps. wwpa.org