Where this starts
Rot needs moisture, oxygen and a temperature range. Below ground there is moisture and much less oxygen; above ground there is oxygen and the timber dries between showers. At the line where the two meet there is enough of both all the time, which is why a post dug out after failure is usually sound at the bottom, sound at the top, and gone in a band a few inches either side of the surface.
SO PUSH IT. Lean on the post at shoulder height and watch the base rather than the top. A post rotted at the line pivots there, with the timber below staying put. A post that moves as a whole unit with the ground around it has a different problem — the concrete or the ground is moving, not the timber.
PROBE THE BAND. A screwdriver pushed into the post at ground level tells you in a second what is left. Sound timber resists; punky, fibrous or crumbling material means the section is already gone and the post is standing on whatever is left at the sides.
CONCRETE IS NOT THE PROTECTION PEOPLE THINK. A post set in a concrete collar that is domed and shed properly does well; one set in a collar with a flat or dished top holds a ring of water permanently against the timber at exactly the height rot wants. That is why posts concreted in fail at the line as reliably as posts that are not, and often faster — and why the modern alternatives are a post spur, a metal post support, or a post standing on a drained bed of aggregate rather than in a cup of concrete.
ONE PATTERN WORTH SPOTTING BEFORE ANY REPAIR: several posts along one run failing together. Timber posts from one batch, set on one day, at one treatment level, reach the end of their life together — so replacing one post at a time in a fence that is fifteen years old is a subscription, and the useful question becomes how many are close behind.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Rot at the ground line
Most commonA post that pivots at ground level, with soft fibrous timber when probed at the surface and sound timber both above and below it.
How to check: Push at shoulder height and watch the base. A pivot at the line is the classic and almost universal failure.
Water held against the post by its concrete collar
Most commonA concrete collar with a flat or dished top holding water, moss or debris against the timber, and rot concentrated exactly at the collar's surface.
How to check: Whether the concrete sheds water away from the post. A collar that holds a puddle is a rot incubator.
Inadequate treatment for ground contact
CommonFailure within a few years rather than a decade, sometimes on posts cut to length on site so the treated surface was cut through at the bottom.
How to check: Whether the post was cut after treatment. Surface-treated timber cut to length exposes untreated heartwood at the cut.
Not deep enough, or the ground has moved
CommonA post moving as a unit with its concrete rather than pivoting in it, often after wet weather, on a fence that has always felt loose.
How to check: Whether the post and its concrete move together. That is the ground rather than the timber.
Wind load on a solid fence
CommonA close-boarded or panel fence leaning consistently one way after gales, with posts intact but loosened, often on an exposed boundary.
How to check: Whether the whole run leans one way. A solid fence is a sail, and the posts carry all of it.
The batch reaching the end of its life together
Less commonSeveral posts failing within a season or two of each other along one run, all the same age and the same specification.
How to check: How many others are soft when probed. Posts installed together fail together, and one at a time is the wrong repair strategy.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
A pivot at the ground line is rot at the ground line, which is where almost every timber post fails; a post moving with its concrete is the ground instead.
- 2
It takes a second and tells you how much section is actually left, which the intact-looking timber above and below does not.
- 3
A flat or dished collar keeps a permanent ring of water at exactly the height rot needs, which is why concreted posts fail at the line too.
- 4
Posts set on one day from one batch fail together, and replacing them one at a time over several years costs far more than doing the run.
- 5
A close-boarded fence is a sail and puts far more load into its posts than a slatted one, which changes both the post size and the spacing that should have been used.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where the fence is a boundary structure and the ownership or the line is unclear, because replacing posts may move a boundary in fact if not in law.
- Where posts are failing along a retaining or a terraced section, since a fence doing any retaining is a different structure.
- Where the fence is close to a highway, a footpath or a neighbour's property and is leaning, which is a liability rather than a maintenance question.
- Where several posts are soft and the run is long, because the economics of doing the whole line at once are usually decisive and worth pricing properly.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.