4x6 Lumber

A 4x6 is a structural timber that sits between dimensional 2x stock and heavy 6x6 posts. Order it by the nominal name, but build to the actual size. The nominal size is 4 inches by 6 inches, but after planing and drying, the actual size measures 3½ inches by 5½ inches. That gap throws off layout and joinery if a builder forgets it, so plan every cut to the real number.

The single most useful thing to know: a 4x6 is the right call when one solid piece of timber beats nailing up a built-up beam from 2x stock. A 4x8, for comparison, is a 1.5× cross-section of a 2x8 in width, so it carries roughly 1.5× the bending load — a single 4x8 carries about the same load as a doubled 2x8, useful when you want a single piece of timber rather than nailed-together construction. The same logic applies stepping down to the 4x6.

Size, Weight, and Lengths

Length runs true. Lumber comes in standard lengths from 6 to 24 feet in 2-foot increments — 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24 feet. Unlike width and thickness, length is actual: an 8-foot board is truly 96 inches.

Spec	Value
Nominal size	4 × 6 in
Actual size	3½ × 5½ in (89 × 140 mm)
Cross-section	~19.25 sq in
Weight (kiln-dried Doug Fir)	~3.85 lbs per linear foot
Board feet per foot	2.00
Standard lengths	6 to 24 ft, 2-ft increments

Those weight figures are based on kiln-dried Douglas Fir at roughly 30 lbs per cubic foot. One caution worth keeping in mind for Montana yards: green (wet) lumber can weigh nearly twice as much. Some 4x6 stock is sold wet rather than dried — wet, construction grade, surfaced four sides (S4S, all four sides planed smooth), commonly in 6-, 8-, and 12-foot lengths. Wet stock moves as it dries. In a climate that runs from damp framing season to bone-dry winters, kiln-dried timber that’s allowed to acclimate cups and checks less.

The 4x6 sizes Western Building Center carries are Douglas Fir, #1 grade, in 8-, 10-, and 12-foot lengths.

Grade, Species, and Treated vs. Untreated

Three decisions drive a 4x6 purchase: grade, species, and whether it needs to fight ground moisture.

Grade. Select Structural or No. 1 grades suit heavy loads and exposed work; No. 2 or Construction grade handles general framing. Watch defects — warp, checks (cracks), and knots can reduce structural strength by 20 to 50 percent.

Species. This matters more than most buyers think, and it matters most in snow country. Span tables often assume Spruce-Pine-Fir #2 as the conservative residential default. Douglas Fir-Larch and Southern Yellow Pine allow 5 to 15 percent longer spans at the same size; Hem-Fir is structurally similar to SPF. Stiffness backs that up — Douglas Fir-Larch runs a modulus of elasticity of 1.7 at No. 1 and 1.6 at No. 2 (×10⁶ psi), among the highest of common framing species. Where roof and beam members carry deep-snow loads, that extra reach from Douglas Fir-Larch is a real consideration, not a spec-sheet footnote. Douglas Fir is well-suited for load-bearing uses and offers excellent screw and nail holding capability.

Treated vs. untreated. This is the outdoor call, and it’s not optional. Pressure-treated lumber, often Southern Yellow Pine, is treated to protect against termites, fungal decay, and rot. Treated lumber is critical to the performance and safety of an entire system — deck joists, beams, and ledger boards — and is building code approved for above-ground or ground-contact applications. Use it for anything fighting Montana’s freeze-thaw and ground moisture: sill plates, decks, docks, ramps, and other outdoor applications.

Surface. Two finishes cover most needs. S4S is planed smooth on all four sides. Rough sawn keeps texture for exposed work — milled to retain a resawn surface on all four sides, suited to rustic and architectural applications.

Applications and Installation

Where a 4x6 earns its keep: post and beam construction — vertical supports for decks, pergolas, and small buildings; rafter and joist supports where extra strength is needed; and deck framing for beam supports and joists. Beyond structure, it serves raised garden beds, sturdy fence posts, custom furniture legs and frames, and heavy-duty shelving for garages and shops. For bigger loads, step up — for longer beam spans, increase to a larger size or switch to engineered lumber like LVL.

Orientation is the install detail people get wrong. A 4x6 is rectangular, so it has a strong axis and a weak one. Position the 5½-inch face to resist the primary load — laid flat, the same board sags far sooner.

Verify the span before committing. Span tables exist for pressure-treated and ground-contact applications, and a deck beam span calculator handles outdoor loading. Solid-sawn lumber spans are governed by IRC R502 for floor joists and R802 for rafters. Mind the deflection limits: typically L/360 for floor joists — about ½ inch over a 15-foot joist — and L/240 for rafters. Outdoor beams run shorter than indoor ones — deck beam spans are typically 5 to 10 percent shorter than equivalent indoor beam spans due to the higher dead load of pressure-treated lumber and outdoor loading patterns.

Inspect every stick before it leaves the yard. Sight down each board for bow, crook, and twist. Look for checks and oversized knots. Order by nominal size, but lay out and cut to actual — always order by nominal size, since that’s what stores, suppliers, and building plans use, but work with the actual dimensions when planning layout and cuts so everything fits correctly.

Frequently Asked Questions

What is the actual size of a 4x6? Due to planing and drying, the actual size of a 4x6 typically measures 3½ inches by 5½ inches — not the nominal 4 by 6 on the label.

Does a single 4x6 replace a doubled 2x member? A solid timber can stand in for nailed-up built-up construction at similar depth. A single 4x8 carries about the same load as a doubled 2x8 — useful when you want one piece of timber rather than nailed-together construction. The same one-piece advantage applies to a 4x6.

Which species gives the longest span? Douglas Fir-Larch and Southern Yellow Pine allow 5 to 15 percent longer spans at the same size, while Hem-Fir is structurally similar to SPF. For snow-loaded members, the stronger species buy real distance.

When do I need pressure-treated? Any time the timber touches the ground or lives outdoors. Pressure-treated 4x6 is ideal for sill plates, decks, docks, ramps, and other outdoor applications, and is approved for above-ground or ground-contact use.

How much does a 4x6 weigh? About 3.85 pounds per linear foot for kiln-dried Douglas Fir. Green or wet stock can weigh nearly twice as much.

Do I need to check span tables for a 4x6 beam? For any meaningful span or load, yes. Solid-sawn spans are governed by IRC R502 and R802, and the published values reflect both bending strength and deflection so the lumber neither fails nor sags enough to crack drywall.