Ground conditions
If you have replaced a fence in this valley and watched the new one start to lean the same way as the old one, the fence was probably not the problem.
San Jose's own General Plan describes much of the soil in the city as moderately to highly expansive. Expansive soil takes on water and swells, then dries out and shrinks. Over a wet winter and a dry summer that is a real amount of movement, applied to the bottom of every post you have in the ground.
Not one competitor site in this market mentions soil at all, which is strange, because it is the single biggest reason fences in this valley fail early.
A post set shallow, in a hole barely wider than the post, with concrete finished flush or below grade, sits in the zone where the soil moves most. Water collects on top of a flat concrete collar and soaks into the post at exactly the point where it is weakest. The post rots at grade and the fence starts leaning downhill of whichever way the ground moved.
| Depth | Deep enough to get below the layer doing most of the moving, not just deep enough to stand the post up. |
|---|---|
| Hole width | A hole meaningfully wider than the post so the concrete has a real bearing surface. |
| Gravel base | So the end grain of the post is not standing in a bucket of water. |
| Collar shape | Concrete brought above grade and sloped away, so water runs off rather than sitting against the wood. |
| Cure time | Rails hung after the concrete has set, not the same afternoon. |
None of this is exotic and none of it is expensive. It is just the part of the job that is buried by the time you see the finished fence, which is exactly why it is the part that gets skipped.
Sources: City of San Jose General Plan, geology and soils appendix
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