Why Do Fence Posts Rot at the Base First?

The ground line is where moisture and air meet

A few inches above and below where a post enters the soil is the spot exposed to the most repeated wetting and drying. That constant cycle is far harder on wood than being either fully buried or fully above ground, which is why rot so often shows up right at that line first.

Beaverton's wet soil makes it worse

Washington County's rainy season keeps soil moisture high for months at a time. Wood posts set in that ground stay damp longer than posts in drier climates, which speeds up the rot cycle at the base compared to what the same post might see elsewhere.

Concrete footings can trap as much as they prevent

Concrete is meant to stabilize a post, but if it's not sloped to shed water away from the wood, it can actually hold moisture against the post at exactly the ground-line zone that's already most at risk.

Why the top of the post rarely has the same problem

The exposed top of a post dries out quickly after rain and gets more airflow, so it rarely rots at the same rate as the buried base — even though it takes just as much weather exposure over the life of the fence.

What actually slows this down

Pressure-treated lumber rated for ground contact, a gravel base beneath the post instead of setting it directly in soil, and a footing shaped to shed water away from the wood all reduce how fast base rot develops. None of these fully stop it — they extend the timeline before a post needs attention.

What to do if a post is already rotting

A single rotting post can often be replaced without rebuilding the whole fence section, especially if it's caught before the rot spreads to the boards it's supporting.