Why Your Fence is Only as Good as What's Below the Ground
When homeowners compare fencing quotes in Nelson, they compare what they can see: the material, the height, the style, the price per metre. What they rarely compare is what happens underground, because underground is invisible and most quotes don’t explain it.
That’s a problem. In Nelson, where the soil on a Port Hills section is different from the soil on the Waimea Plains, and both are different from the clay in inland Stoke, what happens below the ground is where most fences are won or lost.
This post is about the part of fencing nobody explains clearly until the fence is leaning three years into a twenty year life.
Where most fences fail, and it isn’t the timber
A well-specified treated timber fence should last 15 to 20 years in Nelson conditions. Plenty of fences in this region fail in five to eight. The timber is usually fine. The post installation is not.
The post is the structural foundation of every fence. It transfers wind load from the panels into the ground. If it isn’t set deep enough, isn’t set in material that drains, or sits in soil that moves seasonally, the post will move. When the post moves, the fence goes with it.
Nelson’s ground conditions make this particularly relevant.
Clay soils in the Port Hills, Stoke foothills and inland Richmond expand when wet and contract when dry. A post in clay that wasn’t set deep enough heaves upward through wet Nelson winters and settles unevenly over summer. After several cycles the post is visibly out of position and the fence leans or pulls apart at the joins.
Stony ground on the Waimea Plains and around the Richmond fringe resists boring. A hand-operated post-hole digger struggles to reach depth in stony ground without serious effort, so the installer makes a judgement call and stops short. A post that stopped short in stony ground performs about as well as a post that stopped short in clay.
Fill ground in some of the newer Richmond subdivisions and reclaimed areas is variable and unpredictable. Ground conditions need assessing before the depth is specified, not after.
Post depth is not a detail that shows up in most quotes. It should be.
How deep does a fence post actually need to go?
For standard fencing work, Kiwi Excavations installs fence posts at around 600mm deep.
For gate posts, anchor posts, and any post carrying a heavier load, that increases to around 800mm, depending on the application and the ground conditions. Those posts do more work than a run-of-the-mill line post: a gate post carries a swinging weight and takes repeated shock loading every time the gate closes, and an anchor post takes the tension at the end of a run.
Some proprietary alloy perimeter and pool fencing systems specify shallower footings, sometimes in the 400 to 500mm range, because the system is engineered as a whole and the manufacturer’s specification governs. Where a system specifies its own footing depth, that’s what we build to.
Outside those cases, 600mm is the normal standard, with deeper footings where the ground or the load calls for it. That judgement is made on site, which is the whole reason a site visit matters more than a phone quote.
Post size matters as much as depth. The minimum post for a residential timber fence is 100mm x 100mm H4 treated timber. Undersized posts are a common shortcut that directly shortens fence life.
On treatment: H3.2 timber is rated for exterior use above ground and is what goes into rails and palings. It is not suitable for in-ground posts. H4 is the correct minimum for any post in ground contact. Using H3.2 in the ground is one of the most common shortcuts behind premature fence failure in Nelson’s wetter areas.
A note on longevity. Even correct materials, proper installation and good drainage have a lifespan. Weather, time and exposure all play their part regardless of how well a fence is built. The job of good installation is to maximise that lifespan, not to pretend a fence lasts forever.
Drainage around the post
Concrete does not cause rot in fence posts. That’s a common misconception worth clearing up. What causes rot is water sitting against the timber at ground level, and poorly finished concrete contributes to that by creating a low point where water pools at the post base.
The right approach is drainage aggregate around the post in the ground, so water drains away from the base rather than sitting against it, and a concrete collar finished slightly proud in the centre so water sheds away from the post rather than pooling at the interface. H4 treated timber is designed for ground contact, but it is not designed to sit in standing water. H5 is the specification for posts in or near water. For most Nelson residential work, H4 with good drainage is the right call.
The other thing worth understanding is what happens when posts start to fail. It’s rarely just the post. A failing post creates a cascade: rails buckle as the post leans, palings twist as the rails move. Bracing a new post against an existing failing fence can buy time, but how much depends entirely on the footing condition and the ground underneath. If the ground that failed the first post hasn’t changed, bracing is temporary. Matt can assess whether a repair will hold or whether replacement is the more economical answer.
Ground preparation before the fence
On flat, stable, accessible ground, fencing follows the land naturally. The fence goes where the boundary is, the posts go in, job done.
On sloped, unstable, or reshaped ground, the preparation comes first.
Levelling for stepped fencing. A fence on a slope steps in level runs rather than following the slope continuously. Each step needs precise layout and setting, which takes someone who can read a slope and plan the steps rather than just install panels.
Retaining structures before the fence. Where the slope is significant or unstable, a retaining wall goes in before the fence, and the two need designing together because the wall has to carry the load the fence will apply. Our post on fencing on a slope covers this in full.
Drainage on waterlogged sections. Some sections in lower-lying parts of Richmond and Nelson drain poorly. A fence on waterlogged ground without drainage provision fails sooner than the same fence on well-drained ground. Sorting drainage before the fence goes in is the right sequence.
A fencing contractor without excavation equipment can’t do this preparatory work. They either skip it, refer it out and add a coordination layer, or price it separately through a subcontractor. A contractor handling both the ground preparation and the fencing does the sequence properly, with one set of eyes on the whole job.
What mechanical boring means in practice
Kiwi Excavations uses its own excavation machinery for post-hole boring. For the homeowner that means three things.
Consistent depth regardless of ground conditions. Machinery reaches the specified depth in stony ground or clay without compromise. A hand borer often doesn’t.
Clean, consistent hole diameter. A mechanically bored hole has uniform walls, so concrete or aggregate fills consistently around the post. A hand-dug hole in difficult ground is irregular, which affects how the post is supported.
No subcontracting. The person who assessed the ground conditions is the person doing the boring and installing the fence. No handoff between trades, no risk that the specification agreed at assessment doesn’t survive to installation.
None of this is unique to Kiwi Excavations. Any contractor with excavation equipment can do it. What’s less common is a fencing contractor who owns that equipment and brings it to residential jobs as a matter of course.
What to ask when comparing quotes
With two or three quotes in front of you, three questions cut through to the quality of the job being priced.
How deep are the posts going? If the answer is “standard depth” with no millimetre figure, ask for the number, and ask whether gate and anchor posts go deeper than line posts. Then cross-reference against the ground on your section.
What goes around the post in the ground? Concrete alone, or drainage aggregate and concrete? How is the concrete finished at the surface?
Who does the boring, and with what? Hand-operated or mechanical, owned or subcontracted. The answers reveal how much thought is going into the underground work.
The above-ground fence is what you’ll look at every day. The below-ground installation decides whether it’s still standing in 15 years.
If you want a fence that lasts in Nelson conditions, the site assessment is where it starts. Matt will look at your ground, specify the post depth and installation method your section actually needs, and quote what the job requires rather than what’s quickest to install. Rates by style are in our fencing pricing guide.
Get in touch to arrange a free site visit.
Frequently Asked Questions
How deep should a fence post be?
For standard fencing work, Kiwi Excavations installs posts at around 600mm deep. Gate posts, anchor posts, and posts carrying heavier loads go to around 800mm depending on the application and ground conditions. Some proprietary alloy perimeter and pool fencing systems specify shallower footings, sometimes 400 to 500mm, in which case the manufacturer’s specification governs. Ground conditions can push any of these deeper, which is assessed on site.
How long should a treated timber fence last in Nelson?
A well-installed fence, meaning H4 treated posts set to the right depth for the soil, proper drainage around the posts, and H3.2 or better palings, should give 15 to 20 years with reasonable maintenance. Fences installed with shallow posts in clay soils or without drainage around the base commonly fail within five to eight years. Installation quality matters as much as the treatment rating.
What’s the difference between H3.2 and H4 treated timber for fence posts?
H3.2 treated timber is suitable for above-ground exterior use, which is why it goes into rails and palings. H4 is specified for ground contact, carrying a higher preservative loading that resists rot and insect attack when timber sits in soil or concrete. Fence posts should be H4 as a minimum in Nelson conditions. Using H3.2 in the ground is a common shortcut that significantly shortens post life.
Does concrete around a fence post cause rot?
It can, if the concrete is finished flat or below the soil surface. Water migrates down the post, collects at the concrete interface at ground level, and sits against the timber in the wet-dry transition zone, which is where rot enters treated timber. The fix is a slightly domed concrete collar that sheds water away from the post, combined with drainage aggregate below to stop water pooling at the base.
Can fence posts be replaced without replacing the whole fence?
Yes, in many cases. If the rails and palings are sound but the posts have failed, post replacement can extend the fence life considerably. It takes careful work to avoid damaging the rails during removal, and the new posts need setting correctly. The same installation standards apply to replacement posts as to a new fence.
Why is my fence leaning after only a few years?
The most common cause on a Nelson property is post movement in reactive soil. Clay expands and contracts seasonally, and posts that weren’t set deep enough, or that had no drainage provision around them, get gradually pushed out of position. The fix depends on how far they’ve moved. A minor lean can sometimes be corrected without full replacement, but posts that have shifted significantly usually need to come out and go back in properly.