Cedar 5/4x6 Lumber

Western red cedar 5/4x6 is the board most folks reach for when they want a deck surface, fence, or railing that fights rot without chemical treatment. The name throws people. A 5/4 board isn’t 1-1/4 inches thick when you get it home, and the 6 isn’t a true 6 either. The actual size is 1 inch by 5-1/2 inches, surfaced two sides and two edges, with all corners eased with 1/8-inch round-overs. That round-over is what gives finished decking its softened, walk-on-it-barefoot edge.

The reason cedar earns its keep outdoors comes down to chemistry. It’s a western red cedar deck board containing natural oils and resins that make it resistant to rot, decay, and insects. No treatment needed. For Montana decks and fences that sit under snowmelt every spring, that natural decay resistance is the whole point. One caveat that matters in this climate: cedar decking is generally used for the surface portion and can be used over treated framing — it’s for above-ground applications only. Keep it off the dirt. Build your structural frame from treated lumber and let cedar do the show work on top.

Sizes and Grades

Start with the dimension confusion, because it costs people money. 5/4 refers to lumber that’s nominally 1-1/4 inches thick before surfacing. After planing smooth to four sides, it measures about 1 inch thick by whatever width you need. Cedar 5/4x6 sits in a sweet spot — thicker than standard 1x decking but slimmer than 2x stock. That extra thickness over a 1x board matters under load. The thickness provides enough depth for robust joinery without excessive weight, making it ideal for spans up to 8-10 feet in pergolas or tables.

These boards run in lengths ranging from 6 to 16 feet across the wider market.

Grade is where the price and the look diverge most. The two you’ll see on cedar decking are Clear and Select Tight Knot.

Grade	Knots	Best Use
Clear	None — knot-free	Fine, highly visible work
Select Tight Knot (STK)	Sound, tight knots up to 2-3/4”	Rustic decking, fencing, siding
#1 / #2 Common	Larger knots, some checks	Budget or hidden framing

Clear is the premium. Grade ranks quality — Clear for knot-free visible projects, #1/#2 with some knots for hidden frames. Expect a meaningful jump in cost for it. Select Tight Knot gives you a warmer, more natural look at a friendlier number — the STK appearance grade highlights select knotty characteristics for a rustic look while every knot stays sound and tight rather than loose and prone to falling out.

Before you load the cart, read the end stamp. A stamp like “WRC 1x6 S-DRY” means Western Red Cedar, 1x6 nominal, surfaced dry. Moisture reads as “S-GRN” (green, over 19%) or “S-DRY” (air-dried to 19% max).

Moisture and Inspecting at the Yard

This is the part Montana builders can’t skip. Cedar ships either green or dried, and the difference shows up in your finished deck. Green lumber was historically cut oversized to account for shrinkage; cedar is planed to standard actual dimensions after kiln-drying to 8-12% moisture content. Kiln-dried stock minimizes the movement that opens gaps and cups boards.

How much movement? Cedar loses 4-6% radially and 8% tangentially as it dries — and tangential shrinkage across the face is what cups a board. For outdoor work, target 12-16% moisture content at install, higher than interior’s 6-8% to match ambient humidity and prevent cupping. In a dry-winter, wet-spring climate, getting drier stock and acclimating it properly is cheap insurance against callbacks.

Inspect every board. Don’t let the yard hand you a bundle sight unseen. Sight down each piece for straightness, check for warp, and reject anything with oversized or loose knots. Verify actual sizes and measure twice — actual width often varies 1/16 inch due to mill tolerances.

Installation

Acclimate first. For proper installation, allow lumber to acclimate on site 1-2 weeks prior to installation. Stack it where it’ll see the same air your finished deck will, and let it settle to local conditions before you fasten a single board.

Cedar is soft. That’s a virtue for working it and a hazard at the fasteners. Drill pilot holes at edges and board ends — cedar splits there if you drive screws cold. You can run hidden fasteners or face-screw; both work on 5/4x6. Whichever you choose, use stainless or coated screws so the fastener doesn’t bleed black stains into the wood around every hole.

Finishing is what keeps cedar from going gray. Western red cedar is pitch and resin-free, which makes it ideal for accepting and holding a wide range of finishes including dark stains, solid colors, and semi-transparents. A UV sealant preserves the natural warm tone. Plan to wash the deck semi-annually and re-seal every other year or so. At Montana elevation, the sun is hard on unprotected wood — staying on a wash-and-reseal schedule is the difference between rich cedar tone and weathered silver.

Frequently Asked Questions

What are the actual dimensions of a 5/4x6 cedar board? The actual size is 1 inch by 5-1/2 inches, surfaced two sides and two edges, with corners eased with 1/8-inch round-overs. The nominal “5/4” and “6” are pre-milling labels, not finished measurements.

Can I use cedar 5/4x6 for the frame of my deck? No. Cedar decking is used for the surface portion and can be installed over treated framing — it’s for above-ground applications only. Build the structure from treated lumber and use cedar as the walking surface.

What’s the difference between Clear and Select Tight Knot cedar? Clear has no knots and runs a premium price for fine visible work. Select Tight Knot is a western red cedar board where all knots are sound and tight, with a maximum knot size of 2-3/4 inches — a rustic look at a lower cost.

Why does cedar resist rot without treatment? It contains natural oils and resins that make it resistant to rot, decay, and insects. Those compounds are built into the wood, so no chemical treatment is required for above-ground use.

Do I need to pre-drill before fastening cedar? Yes, especially near edges and ends. Cedar is soft and splits without pilot holes. Use stainless or coated screws to keep the fasteners from staining the wood.

How do I keep cedar from turning gray? Apply a UV sealant or semi-transparent stain, then wash the deck semi-annually and re-seal every other year or more. Because cedar is pitch and resin-free, it accepts and holds stains and semi-transparents well.

Should I buy green or kiln-dried cedar? Kiln-dried minimizes shrinkage and movement. Cedar loses 4-6% radially and 8% tangentially as it dries, so dried stock that’s properly acclimated gives you a flatter, gap-free deck — worth it in a freeze-thaw climate.