Cedar 4x4 Lumber

A cedar 4x4 is a 3-1/2” square post — never a true 4 inches — cut from Western Red Cedar, prized because its natural resistance to rot, decay, and pests makes it desirable as a building material for outdoor projects without any chemical treatment. For decks, fences, pergolas, and posts that take Montana weather head-on, cedar earns its keep. The catch: pick the wrong grade or bury it wrong, and that natural durability won’t save you.

Here’s the honest trade-off. Cedar carries a premium over pressure-treated pine. Cedar’s premium commands 20-50% more but pays in zero-maintenance beauty. That’s the deal — you pay more upfront and skip the maintenance grind down the road. Both cedar and pine are softwoods, but cedar is tougher and more resistant to outdoor elements like moisture and insects, and its natural oils and acids make it resistant to decay, giving it a longer lifespan than pine.

What makes cedar resist rot is in the wood itself. The rot resistance comes from thujaplicins, giving roughly 4x the untreated lifespan versus 10-15 years for treated pine. The field record backs that up — heartwood cedar resists decay 5x better than treated pine, and cedar’s natural oils repel insects and decay up to 25 years untreated.

It also handles easy. Cedar’s Janka hardness of 350 lbf resists dents yet yields to fasteners, while at 2.5 lbs/ft it’s easy handling for solo DIYers. Light, strong, planes clean.

Sizes, Grades, and Finishes

Start with the dimension that trips people up. A 4x4x8 is always 3-1/2” × 3-1/2” actual — not 4”. The “4x4” is nominal — the rough-sawn size before planing. Every face loses about a quarter inch in that process. The rule for dimensional lumber between 2” and 8” nominal: subtract a half inch to get actual. So 4.0 minus 0.5 equals 3.5 on each face.

Why does this matter on the jobsite? Post bases, caps, and brackets are all sized to 3-1/2”. 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.

For weight, an 8’ smooth cedar 4x4 with actual dimensions of 3-1/2” x 3-1/2” x 8’ weighs about 23 lbs. Western Building Center carries cedar posts in S4S/Appearance grade in both 8-foot and 10-foot lengths. Lengths from various mills run wider — smooth red cedar is offered in 8’, 10’, and 12’ lengths, and some suppliers cut from 4 ft up to 24 ft.

Grades come down to knots and heartwood. The range:

Grade	What You Get	Best For
Clear Heart / Clear Vertical Grain	No knots, 100% heartwood	Load-bearing, visible posts
Select Tight Knot (STK) / Appearance	Small, tight knots; minor checks	Framed porches, rustic visible work
#1 Common	Tight knots, straight enough for show	Framed porches — the balance pick
#2 Common	Larger knots; sound sapwood	Budget rail posts, with bracing

The smart money sits in the middle. #1 Common hits the sweet spot for most porch jobs, while Clear Heart gets reserved for critical load-bearing or premium finish where you can’t afford a twist. Field experience bears it out — swapping #2 for Clear Heart posts cuts twist risk by about 40%.

Finish is the surface treatment, separate from grade. S4S — surfaced four sides — features a clean, planed surface on all four sides, providing a straight, uniform profile with square edges. It’s a basic, no-frills finish for projects needing a clean-cut appearance. The other two: True Rough (TR) retains the original sawmill surface and full thickness, preserving rugged character, while Rough Sawn (RS) is a classic natural-textured finish created by re-sawing mill-planed timbers — works in both rustic and modern designs.

One more split before you buy: green versus dry. Posts at 19% moisture content (green) warp wildly; kiln-dried to 12-15% hold straight. Kiln-dried is more dimensionally stable and ready to finish. Given Montana’s seasonal moisture swings, dry and acclimated stock saves headaches.

Applications and Installation

Cedar 4x4s cover a lot of ground. Use them for exterior projects like railings, fences, pergolas, gazebos and more. Common roles run to deck and porch posts, fence and gate posts, pergola and arbor framing, columns, and outdoor furniture. Cedar is ideal for tables, seating, and benches due to its natural resistance to rot and decay, and it works indoors too — wardrobes, chests, and carving.

On spans, cedar gives you a little more reach than the alternatives. The numbers put Western Red Cedar at up to 10 ft max recommended unsupported span (3.5” square), versus 8 ft for pressure-treated pine and 7 ft for Douglas Fir. For pergolas and covered structures in Montana, that span figure matters — snow load is the dominant structural concern here, so plan post spacing against your site’s load and don’t push the limit.

Cedar takes finish beautifully because it’s pitch and resin-free, which means it’s ideal for accepting and holding a wide range of finishes including dark stains, bleaches, solid colors, and semi-transparents. It also stays straight, flat, and resists warping and cupping when properly installed and maintained.

Now the part that defeats good cedar: installation mistakes. Three rules.

Keep it out of the dirt. Never bury cedar directly in soil. Set posts on concrete piers or metal post bases so the end grain isn’t sitting in ground moisture. Freeze-thaw cycling at grade is brutal in Montana — spring snowmelt keeps the ground line wet for weeks, and that’s what kills posts faster than the calendar would suggest. Keep cedar off grass and dirt to prevent moisture absorption from the ground.

Use the right fasteners. Cedar’s tannins react with ordinary steel and stain the wood. Go stainless steel or hot-dipped galvanized. Required fasteners with this lumber are hot-dipped galvanized (ASTM A153) or stainless steel.

Let it acclimate. Target 12-15% moisture content for a stable install. If green, sticker-stack dry for 2-4 weeks before building. Building with green stock and skipping that step is how you end up with cupped, twisted posts after the first season.

After it’s up, protect it. Use a sealant or stain to guard against moisture and UV, apply immediately after construction, and reapply every few years depending on exposure. At elevation, UV and the temperature swings work paint and stain hard, so plan on recoating.

Frequently Asked Questions

Is a cedar 4x4 actually 4 inches? No. A 4x4 is always 3-1/2” × 3-1/2” actual, not 4”. The nominal name refers to the rough-sawn size before the mill planes it down. Size your post bases, caps, and brackets to 3.5”.

Can I set a cedar post directly in the ground? Don’t. Cedar resists rot, but end grain sitting in soil still wicks moisture and breaks down. Set it on a concrete pier or in a metal post base instead. Keeping cedar off grass and dirt prevents moisture absorption from the ground.

Which grade should I buy for a visible porch post? For most jobs, #1 Common hits the sweet spot, balancing appearance, cost, and durability. Save Clear Heart for critical load-bearing posts or premium finish work where you can’t risk a twist.

Why does cedar cost more than treated pine? You’re paying for natural durability and low maintenance. Cedar runs 20-50% more but pays back in zero-maintenance beauty, with heartwood that resists decay 5x better than treated pine.

What fasteners work with cedar? Hot-dipped galvanized or stainless steel. Ordinary steel reacts with cedar’s tannins and stains the wood at every fastener.

Will a cedar 4x4 warp? It can if it’s wet and not handled right. Green posts at 19% moisture warp wildly; kiln-dried to 12-15% hold straight. If you buy green, sticker-stack and let it dry 2-4 weeks before building, then seal it and keep it off the ground.

How far can I span between cedar posts? For shaded structures, Western Red Cedar goes up to about 10 ft unsupported on a 3.5” square post. But snow load drives the real number in Montana — check your site load and tighten post spacing for pergolas and anything covered.