Landscape and Fence Lumber

The single decision that determines whether a fence lasts five years or thirty-five: matching the wood’s treatment level to where it goes in the ground. Get that wrong and the posts rot at the soil line while the pickets above them still look fine. Get it right and the fence outlives the dog that runs alongside it.

Two paths get you there. Pressure-treated softwood, where chemicals do the rot-fighting, or naturally durable species like cedar that resist decay on their own. Untreated softwoods fail in 1-2 years outdoors; durable hardwoods or treated options last 20-50 years, saving you rebuild costs. That gap is the whole reason this guide exists.

Treated Versus Naturally Durable: The First Fork in the Road

Pressure-treated lumber is wood that’s been forced full of preservatives. Pressure treated lumber is natural wood that has been placed inside a pressurized tank and infused with protective preservatives. These chemicals penetrate deep into the wood fibers to help prevent decay and insect damage. Southern Yellow Pine is the workhorse here — cheap, strong, available everywhere, and it takes treatment well.

The chemistry matters. Pre-2004, most residential pressure treated lumber used Chromated Copper Arsenate (CCA). It was replaced by safer copper based alternatives like Alkaline Copper Quaternary (ACQ), Copper Azole (CA), and Micronized Copper Azole (MCA). Those copper-based formulas are what’s on the shelf today. Safer to handle, friendlier to the soil, and they do the job for posts and ground-contact work.

Then there’s the naturally durable camp. Cedar earns its reputation honestly. Its natural resistance to rot and insects makes it an ideal material for outdoor use. Cedar contains natural oils that act as preservatives, protecting the wood from decay and deterring pests. Western Red Cedar is the standout — a softwood with natural thujaplicins, oils that repel water and bugs, with decay-resistant heartwood that shrinks just 2.5% radially, staying flat outdoors. For Montana, that low shrinkage matters. Wood that stays put through freeze-thaw cycles is wood that doesn’t cup, twist, and pull fasteners.

Redwood plays a similar game with even tighter numbers. Redwood heartwood packs tannins for rot resistance — moderately hard, extremely stable with 2.2% shrinkage, ideal for horizontal surfaces. Douglas fir brings the strength but less natural rot protection. Douglas fir is renowned for its strength and moderate resistance to decay. Its natural durability makes it suitable for various climates, although it may not be as resistant to decay and insects as cedar or redwood. Spruce sits at the bottom for durability — a popular choice due to its affordability and availability, but it may not be as naturally durable as cedar or redwood and works best when properly treated and maintained.

Here’s the honest read: treated pine is the budget pick that performs. Above-ground decks need Class 2; ground-contact posts demand Class 1. Cedar is the no-chemical option that looks better and weathers gracefully. Match the wood to the job and the budget.

Read the Use Category Tag Before You Load the Cart

Every piece of treated lumber carries an end tag with an AWPA Use Category. That code tells you exactly where the board is rated to live.

Use Category	Exposure	Typical Use
UC3B	Above ground, uncoated or poor drainage	Decking boards, fence rails, pickets
UC4A	Ground contact, general use	Fence posts, deck posts, garden projects
UC4B	Ground contact, heavy duty	Retaining walls, structural supports

The American Wood Protection Association created the Use Category system to classify treated wood based on where and how it will be used. Each category represents a different level of exposure to moisture, soil, and insects.

The rule is simple. Rails and pickets that sit above the dirt? UC3B is fine. Anything that touches soil or stays wet? You want UC4A at minimum. Ground-contact treated lumber is treated to a higher level of protection and can be used for posts, beams, and lumber that touch soil or stay damp for long periods. For a retaining wall or structural support, step up to UC4B.

Why this matters in Montana specifically: ground-contact posts carry the load where soil stays damp through snowmelt and spring thaw. The ground line stays wet for weeks while everything above dries out. Put an above-ground-rated board in the dirt and it’ll fail at exactly the spot that’s hardest to inspect.

Check the end tag. Every time.

Grades, Sizes, and Inspecting Before You Buy

Fence lumber comes in a predictable set of dimensions. Cedar dog-ear pickets in the 1x6-6’ size are the standard for privacy and stockade runs. For heavy landscape and retaining work, treated railroad ties come in both #1 and #2 grades.

Know your nominal versus actual. A 4x4 isn’t four inches — it measures 3-1/2” x 3-1/2”. Common fence sizes run 1x6 and 1x8 pickets, 2x4 rails, 4x4 posts, 6x6 timbers and posts, and 6x8 landscape timbers.

Grade drives stability. Reach for #1 grade for fewer knots and straighter stock. On cedar, the grading runs Clear Heart for no knots and premium siding, and A&Better for fewer knots on fences.

Moisture is the silent killer of a fresh install. Buy air-dried to 19% MC max. Wetter than that and the boards shrink after they’re up — gaps open, fasteners loosen, rails bow. Kiln-dried under 19% minimizes the post-install surprise.

Then inspect the actual board. Sight down a 16-ft board and keep the bow under 1/4”. A moisture meter and your own eye save you from culling junk later. Two minutes at the rack beats a weekend of replacing twisted pickets.

Installation: Posts, Fasteners, and Finishing

The lumber’s only half the job. The other half is setting it right.

Posts go deep. Strong and rot-resistant treated lumber is perfect for structural posts and fencing that contact soil or concrete. Set them 2 to 3 feet deep in concrete footings to resist frost heave — and in Montana, frost heave is the whole game. Frost depth runs 36-48 inches across most of the western valleys, so a shallow-set post is a post that gets pushed out of the ground come spring.

Fasteners are non-negotiable. Treated-wood chemicals eat ordinary steel. Use code-approved hot-dipped galvanized or stainless steel corrosion-resistant fasteners and G185 hot-dipped galvanized connectors for exterior use. And don’t mix metals — if using hot-dipped galvanized fasteners, use hot-dipped galvanized connectors. If using stainless steel fasteners, use stainless steel connectors. Mixed metals corrode each other.

A few mechanics that pay off:

• Pre-drill holes at the ends of boards, no closer than 1/2” from board ends or sides, to help prevent splitting.
• Use screws to improve holding performance.
• Install the un-cut end of support posts in ground contact applications.
• Treat all field-cut ends and drilled holes with a brush-on wood preservative. Copper naphthenate formulations work; deck stains and sealers do not provide adequate protection.

That last one trips up a lot of folks. Cut a treated post to length and you’ve exposed untreated wood at the cut. Brush preservative on it or you’ve created a rot entry point.

Finishing waits. Treated wood holds moisture from the treatment process. Allow treated wood to dry prior to application. Test the wood with a few drops of water to see if the wood is dry enough to readily absorb water. Typically, treated wood will be dry and ready for finishing 60 days after installation. Stain too soon and it won’t take.

Thermally Modified Wood: The Chemical-Free Alternative

Worth knowing about for above-ground work. Thermally modified wood — thermowood — skips chemicals entirely. It uses a completely different approach with no chemicals at all. High quality lumber is heated in a controlled, low oxygen kiln environment, typically 300+°F, with steam. This permanently alters the wood’s internal structure: the sugars that fungi and insects love are broken down, and the wood’s ability to absorb moisture drops dramatically.

The payoff is stability and life. Dimensional stability improves by up to 60%, meaning far less shrinking, swelling, warping, or cracking. Typical lifespan runs 25-30+ years in above ground applications like decking, siding, and cladding, with some species rated for 40+ years with proper installation and maintenance.

The limits are real, though. It’s not ideal for direct ground contact or heavy structural load bearing — use PT or concrete for posts. And the wood is brittle after treatment, so special care is needed when fastening. Costs more upfront too. For fence boards, decorative screens, and rails, it’s a strong option. For the posts going in the dirt, you’re back to UC4A treated lumber.

Frequently Asked Questions

What’s the difference between above-ground and ground-contact treated lumber?

It’s the level of protection. Above-ground treated lumber is used for decks, railings, and other structures not in contact with soil. Ground-contact treated lumber is treated to a higher level of protection and can be used for posts, beams, and lumber that touch soil or stay damp for long periods. On the end tag, UC3B is above-ground and UC4A or UC4B is ground contact. Anything touching dirt needs the ground-contact rating.

How deep should fence posts go?

Two to three feet deep in concrete footings. In a freeze-thaw climate, that depth resists frost heave — the upward shove the ground gives a post every spring when the soil expands and contracts. Shallow posts get pushed out and the whole fence racks.

Why can’t I use regular screws on treated lumber?

The copper-based preservatives corrode standard steel. You need code-approved hot-dipped galvanized or stainless steel corrosion-resistant fasteners. And don’t mix metal types — galvanized with galvanized, stainless with stainless. Mixed metals trigger corrosion between the fastener and connector.

How long before I can stain a new treated fence?

Let it dry first. Test the wood with a few drops of water to see if it readily absorbs — typically treated wood is dry and ready for finishing about 60 days after installation. If water beads instead of soaking in, it’s still too wet for stain to grab.

Is cedar worth it over treated pine?

Depends on the job. Cedar grows in wet climates, so it’s pre-adapted, with Class 1 durability and no chemicals. It looks better, handles cleaner, and weathers to a gray patina if left alone. Treated pine costs less and is the better choice for ground-contact posts. Plenty of fences use cedar pickets on treated posts — best of both.

What moisture content should I look for when buying?

Air-dried to 19% moisture content max. Wetter boards shrink after install, opening gaps and loosening fasteners. Sight down each board and keep any bow under 1/4 inch.

Do I need to do anything to cut ends on treated posts?

Yes. A cut exposes untreated wood inside the board. Treat all field-cut ends and drilled holes with a brush-on wood preservative like copper naphthenate. Deck stains and sealers do not provide adequate protection. Skip this and you’ve built a rot entry point right where it’ll do the most damage.