When the old paling fence between my place and the neighbour’s in Sydenham finally gave up in 2022, the problem was not the fence. It was the 25-metre macrocarpa standing 1.2 metres inside my side of the line, with a trunk over a metre through and roots you could trip on for 8 metres in every direction. Six of the twelve post holes had roots in them. Around the same time a workmate in Halswell was fencing a section that had kept a 60-year-old English oak from the farm shelterbelt the subdivision replaced. Between us we learnt more about fence posts and roots than we wanted to.
Where the roots actually are
Most of a mature tree’s roots are in the top 600 mm of soil, and on Canterbury’s shallow silt over shingle they are often in the top 400. They spread well past the dripline, commonly to one and a half or two times the crown radius. The roots that matter for stability are the big buttress roots within roughly six to eight times the trunk diameter; for my macrocarpa that is 6 to 8 metres, for the Halswell oak with its 700 mm trunk, about 5 metres. Cut a root over 50 mm inside that zone and you have damaged the tree’s anchorage; cut two or three on the same side and you may decide which way it falls in the next nor’wester.
Standard fence posts go 600 to 800 mm into the ground, and a post hole borer does not care what it hits. On my boundary, 3 metres from the trunk, the old posts had been augered through roots 80 to 120 mm across when the fence was built in the 1980s. The tree survived, but the roots died back on that side and the nearby posts heaved as the stumps decayed.
Moving the posts, not the roots
The approach at both places was the same: find the roots before deciding where the posts go, then move the posts. We scraped topsoil along the fence line by hand to about 150 mm and exposed the roots crossing it. Anything under 30 mm we cut cleanly and left. Anything bigger got a post moved. Paling fences usually run posts at 2.4 metre centres, but going to 2.0 metres in one bay and 2.8 in the next cleared every significant root on my boundary without a cut over 40 mm.
Where a root sits exactly where a post has to be, the options are to hand dig a shallower hole beside the root and bring the post up on a bracket, or span a longer panel between two posts either side. At the Halswell oak, one 250 mm buttress root ran along the boundary for nearly 3 metres, so two posts were set on brackets over a shallow pad poured on top of the root zone. It has not moved in three years.
A hole full of concrete where a root wanted to grow is a permanent obstacle, and alkaline runoff while it cures harms fine roots nearby. Where posts were near roots we used the minimum concrete needed to set the post and backfilled the rest with compacted soil.
What it does to the cost
A standard 1.8 metre paling fence in Christchurch runs around NZ$180 to NZ$300 a metre installed. Working through a root zone added roughly 20 to 30 percent along the affected section: hand exposure of the roots, irregular post spacing, and extra labour. On my 14 metre boundary that was about NZ$700 extra. The Halswell job with brackets and pad was NZ$1,100 over standard for the 6 metres nearest the oak. Both of us paid for an arborist to look at the exposed roots before the posts went in, NZ$200 to NZ$350 for a site visit. Under the Fencing Act the extra cost from one owner’s tree is a reasonable thing for that owner to carry, and I did.
Fences that have already done the damage
If a fence is already through a root zone and the tree is showing thinning on that side, dieback above the cut roots, or a lean developing away from them, the fence is not the fix; the tree needs assessing. Root damage often takes three to five years to show in the crown.
The fencer matters as much as the arborist. Someone used to boring holes at fixed spacing on a rural line will not hand dig and improvise; someone who has fenced around trees treats exposed roots as a design input. When my workmate was choosing a Christchurch fencing contractor, the one question that sorted the quotes was what they would do when a post hole hit a root. The answers ranged from move the post to cut it out, and only one of those keeps the tree.
Frequently asked questions
How close to a tree can you put a fence post? Outside a radius of six to eight times the trunk diameter, standard post holes are usually fine. Inside that zone, expose the roots first and move the posts to avoid cutting anything over 50 mm in diameter.
Will cutting roots for a fence kill the tree? Cutting many small roots rarely does. Cutting one or more structural roots over 50 to 80 mm inside the buttress zone can weaken anchorage and cause dieback that shows up three to five years later.
How much extra does fencing through a root zone cost? Typically 20 to 30 percent more along the affected length, or NZ$500 to NZ$1,500 on a suburban boundary, covering hand digging, irregular post spacing, brackets or pads and an arborist’s site visit.