Rot at the Base of a Wood Fence Post
Decay happens at the line, not underground
Posts rarely rot from the buried end upward. Damage concentrates in a band an inch or two either side of grade, where the timber gets moisture and air together. Dig down beside a failed post and the wood eight inches under is often still firm while the section at ground level crumbles between finger and thumb.
What keeps that band wet
Clay holds water against timber long after the surface looks dry. Add a concrete collar that has cracked, or one troweled flat instead of sloped, and rainfall collects in a ring at exactly the vulnerable point. Sprinkler heads clipping the fence line every morning do the same job more reliably than weather ever manages. Turning a head a few degrees away, or dropping its arc, removes a daily soaking that no amount of treatment will out last.
Probing rather than guessing
Push an awl or a long screwdriver into the timber at grade on all four faces. Sound cedar resists and the point stops within an eighth of an inch. Decayed wood swallows the tool half an inch or more under light pressure. Test every post in the run, not only the one leaning, because a leaning post is usually the second or third to go rather than the first.
Repair options
- A steel spur or post mender driven alongside the stub and bolted through will carry a post whose above ground length is still solid
- Sistering a treated timber beside it works where the fence has a short life ahead of it anyway
- Full replacement means breaking out the old concrete, which is where the real labour sits
A spur has a fair record on one bad post in an otherwise healthy fence. Used on five posts in a row, it is a way of paying for the same job twice.
Setting a replacement so it lasts
Six inches of compacted gravel beneath the post lets water leave rather than pooling against end grain. Crown the concrete so it sheds outward on every face. Keep soil, mulch and bed edging an inch or two clear of the timber, because anything banked against wood traps damp at the exact band that fails.
Timing the work
A post that fails the probe but still stands is a job for a dry autumn week, when clay digs cleanly and no storm sits in the forecast. Waiting until spring wind lays it over converts a controlled swap into an emergency with three broken boards attached to it.