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Setting Fence Posts That Last

The Forest Products Laboratory had zero failures in 200 treated posts after 50 years. Extension publishes 10 to 25. The gap is the cut end.

The Forest Products Laboratory put 125 CCA-treated and 75 penta-treated round posts in the ground at a severe-decay site in Mississippi. After 50 years, none had failed. Estimated time to half of them failing runs 78 to 96 years. Meanwhile Penn State publishes 10 to 25 years for a treated post, Oklahoma State 20 to 25, and Kentucky 20 to 35.

Both numbers are honest. They measure different things, and the gap is the whole subject.

The extension numbers are counting posts that got cut

FPL’s figure is time to decay failure for an intact post that was correctly treated to the right retention and never sawn after treatment. The extension figures bundle in everything that actually pulls a post out of a fence line. Machinery, livestock, frost heave, a rebuild, and rot starting at a top somebody trimmed in the field.

FPL does not just report a higher number. It argues the low ones are wrong, and says the same of utility poles: 33 years perceived against more than 75 measured.

Here is the mechanism, with a figure attached. Wood absorbs water through cut ends about 11 times faster than through lateral surfaces. A sawn post also carries heartwood that treatment barely penetrates, so cutting to height exposes wood that was never protected. FPL’s instruction is specific. Brush or spray every cut, hole and injury that may have gone through the treated zone. Use copper naphthenate at 1 or 2 percent elemental copper, or an oilborne solution of at least 0.675 percent oxine copper. Apply it sparingly, wipe the excess, not in the rain.

Do not confuse that with treating a post. A brush or spray coat penetrates less than a tenth of an inch sideways, and FPL puts its ground-contact service life at one to five years. Kentucky says dipping is not recommended for ground contact. The same bottle serves both jobs and only one works.

Better not to cut at all. FPL says field treatment is best limited by prefabricating, so buy posts the length you need and set them to a uniform depth.

The number on the tag, and one fact sheet that has it wrong

AWPA puts a farm fence post in Use Category 4A, general ground contact. FPL states that directly. At UC4A the southern pine retentions are 0.40 pcf for ACQ, 0.21 for CA-B and 0.15 for CA-C or micronized copper. Those are the products on the shelf. CCA, creosote and penta remain legal for commercial treatment but are not what a homeowner buys.

Oklahoma State’s fact sheet gives CCA retention for ground contact or high-moisture use as 0.25 pcf. That figure is exactly FPL’s UC3B above-ground retention. The sheet took the above-ground number and labelled it ground contact. FPL’s own guidance says not to put above-ground-treated wood in soil. Follow it and you buy the wrong post.

FPL muddies one thing itself. Its own table lists fence and deck posts under UC4A and horticultural posts under UC4B. That is 0.60 pcf for ACQ rather than 0.40. Call a trellis post horticultural and you buy half again the retention of the reader who calls the same stick a fence post. A second FPL document settles it: UC4A for line and brace posts, UC4B when the post is structural or hard to replace.

Concrete, where NRCS disagrees with NRCS

In Nevada, NRCS says post holes shall be backfilled with tamped soil unless otherwise specified, and reserves concrete for metal brace posts. Texas fully specifies concrete for wood posts, in a hole at least 12 inches across. Maryland makes it mandatory where the specified depth cannot be reached and in extremely wet or very sandy soils. Minnesota says nothing about backfill at all.

One agency, four positions. Geography is the likeliest explanation rather than disagreement. Nevada’s dry rocky ground makes tamped earth adequate, while Maryland’s wet soils and Texas clays create uplift problems concrete solves. The agency never says so.

What matters more is what nobody claims. No acceptable source says concrete makes a post last longer. Every source that discusses it treats concrete as a fix for a mechanical problem: depth, rock, saturated or sandy soil, uplift. Nor does any source say concrete speeds up rot. Whether it shortens post life against tamped soil, all else equal, is simply unanswered.

Both folklores argue about the material and miss the thing the standards actually specify. Oregon State said in 1947 that setting posts in concrete gives no advantage if moisture collects where the wood meets the concrete. NRCS Maryland requires the concrete be sloped at the top for positive drainage away from the post. Texas says crowned at the post base. Nevada says the same for earth backfill. Four documents point at one detail. Crown the backfill so water runs away from the wood, whatever you filled the hole with.

Two classes of durable wood, not one

The common advice treats black locust, osage orange, cedar and white oak as equivalent. FPL publishes them in two classes and says what the classes mean.

Heartwood in the resistant category serves above ground. Only the very resistant category generally performs in ground contact. That category holds four species: black locust, red mulberry, osage orange and Pacific yew. Eastern and western redcedar, the white oaks, catalpa and mesquite all sit one class down.

NRCS Minnesota, Texas and Maryland permit cedar and white oak untreated anyway. Oregon State measured more than 20 years in the ground from western redcedar, western juniper and Oregon white oak heartwood. NRCS Nevada goes the other way and requires an EPA-registered preservative on everything in soil contact. Three things sit behind that spread. FPL notes old-growth wood of a resistant species often outlasts second growth, and the Oregon State results are mid-century timber. The classes cover heartwood only, and a farm cedar post includes sapwood, which is never durable. And NRCS is writing a farm practice where a 15-year post is acceptable, while FPL writes a durability standard.

One footnote if you planned to buy locust and treat it anyway. FPL lists black locust heartwood among the species very difficult to penetrate.

Depth, and five bases that do not convert

NRCS Texas and Nevada set line posts at 24 inches, Minnesota and Maryland at 30, and the NRCS fact sheet and Penn State at 36. Mississippi State gives a multiple instead: 10 times the post diameter in clay and 15 times in sandy soil. For a 4-inch post that means 40 and 60 inches.

Mississippi State is solving a different problem and says so. Those depths put the post where it breaks before it uproots, while the NRCS figures are cost-shared practice floors. Kentucky’s one-third-of-post-length rule applies only to gate posts, so do not generalise it. And NRCS Maryland requires posts set below the frost line without naming a depth, which no acceptable source supplies. That is a local-code question, and it decides whether your fence survives a winter of freeze and thaw. Corner and brace posts go deeper everywhere. Texas says three feet, Minnesota four.

Three pieces of folklore, weighed

Charring the post end is contradicted by the one source that tested it. Oregon State’s 1947 bulletin reports charring shortened a Douglas-fir post’s life by several months against uncharred posts. It reduces the cross-section, and checks that open after setting expose unburned wood. One species, one old test. That is all the evidence there is.

Setting the post butt end down is neither supported nor debunked. No acceptable source addresses post orientation at all. The capillary mechanism usually offered sits awkwardly against FPL’s end-grain figure. That figure is directional in size, not in direction. The one directional instruction anywhere points the other way, for an unrelated reason: Kentucky says drive posts small end first.

Burning the offcuts is advised against by EPA, CPSC and the Forest Service, all in plain language. None states a federal prohibition, and FPL notes state and local authorities regulate it. So do not burn treated scraps, and check whether your state bans it. Kentucky adds the same caution about paint, which traps water and can trigger rot on wet or exposed untreated wood. One number governs all of it, the same one behind the woodshed arithmetic. Decay needs wood above roughly 25 to 30 percent moisture content. FPL’s own documents put the safe-keeping target at 20.

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