Most fences fail at the posts, not the pickets. A rotted cedar fence post snapping at the ground line is the single most common reason homeowners tear out a fence years before its time. Cedar earns its reputation because the wood contains natural oils that repel insects and slow decay, but the way you size, set, and seal that post matters just as much as the species. Get those three things right and a cedar post line can hold a fence upright for 20 to 30 years.
Why Cedar Holds Up Where Other Woods Fail
Western red cedar and its cousin, Northern white cedar, contain thujaplicins and other extractives that act as a built-in preservative. That is why cedar shows up in decking, siding, and shingles as well as fencing. Unlike a green pine post that starts wicking moisture the day it goes in the ground, cedar resists the fungal decay that eats structural fibers.
The trade-off is heartwood versus sapwood. Only the reddish-brown heartwood carries meaningful rot resistance. The pale, creamy sapwood on the outer edge of the log has almost none. When you buy posts, look for pieces that are predominantly heartwood, and be honest that even the best cedar is not immortal underground.
Choosing the Right Post Size and Grade
For a standard 6-foot privacy fence, a 4×4 post works, but a 6×6 gives you noticeably more stiffness and far more wood to lose before failure matters. Gate posts and corner posts should always step up to 6×6 because they carry the most load.
- Line posts: 4×4 or 6×6, spaced 6 to 8 feet on center
- Corner and end posts: 6×6 minimum, since they anchor the run
- Gate posts: 6×6, set deeper than line posts to resist sagging
- Length: buy posts a full 3 feet longer than the fence height so you can bury a third of the post
Grade matters too. A “select” or “architectural” grade post has tighter grain and fewer knots than a construction-grade one, and tight grain sheds water better. Expect to pay roughly $18 to $40 for a 4x4x8 cedar post and $45 to $90 for a 6x6x8, with prices swinging by region and lumber market.
The Bury-One-Third Rule
Depth is where most DIY fences go wrong. The reliable guideline is to set at least one-third of the total post length below grade, and never less than the local frost line. In a northern state where frost drives 42 inches down, a shallow 24-inch hole will heave every winter until the fence leans. Aim for holes 24 to 42 inches deep depending on your climate, and always dig below your frost depth so the post foots in stable ground.
Make the hole roughly three times the post width. For a 4×4, that means a 10 to 12-inch diameter hole. Wider at the bottom than the top, a slightly belled hole, locks the footing against frost heave.
Concrete, Gravel, or Both
This is the debate that never dies at the hardware store, so here is the practical version. Concrete gives the most lateral rigidity, which matters for gate and corner posts. But concrete also forms a bowl that traps water against the wood if you do not detail it correctly, and that trapped moisture is what rots posts from the outside in.
The method that lasts longest
- Pour 4 to 6 inches of crushed gravel in the bottom of the hole so the post base drains instead of sitting in a puddle.
- Set the post on the gravel, plumb it in two directions, and brace it.
- Pour concrete around the post, stopping a couple inches below grade.
- Slope the top of the concrete away from the wood so rain runs off, then cap the remaining gap with soil or gravel.
For pure line posts in well-draining soil, many pros now skip concrete entirely and tamp compacted gravel around the post in 4-inch lifts. Gravel-set posts drain freely, resist frost heave, and are far easier to replace down the road.
Sealing and Protecting the Post
Even rot-resistant cedar benefits from a barrier at the most vulnerable zone, the few inches right at the soil line where wet meets air. Before you set the post, brush a copper-based wood preservative or a bituminous fence-post sealer on the bottom third. Let it soak in overnight.
Above ground, a penetrating oil-based stain with UV inhibitors keeps cedar from silvering and cracking. Reapply every 2 to 4 years. Skip film-forming paints on posts; when they peel, water gets trapped underneath and accelerates the very decay you were trying to prevent.
Common Mistakes That Shorten Post Life
- Setting sapwood-heavy posts: pale posts rot years faster than heartwood ones.
- No gravel drainage: a concrete collar without a drain layer becomes a water trap.
- Cutting the top flat: a flat post top absorbs rain; add a post cap or bevel the cut to shed water.
- Backfilling with topsoil only: loose organic soil holds moisture against the wood.
- Skipping the frost-line depth: shallow posts heave and lean within a season or two.
How Long a Cedar Fence Post Really Lasts
Set correctly, a heartwood cedar fence post commonly lasts 15 to 30 years, and posts set in gravel with a sealed base can push toward the top of that range. Compare that to an untreated pine post, which often fails in 5 to 7 years, and the cedar premium pays for itself. If you want maximum longevity and do not mind mixing materials, some homeowners use pressure-treated posts for structure and cedar for the visible rails and pickets, getting rot resistance below grade and cedar’s warm look above it.
Whatever you choose, remember the fence stands or falls on the post. Spend your extra dollars and your extra care below the ground line, and the beautiful part above it will reward you for decades.
