The path in question was poured in Wadestown sometime in the 1970s, dead flat according to the owner, and it stayed that way for decades until one joint near an old puriri started rising by a few millimetres a year. By the time I looked at it, the middle slab had heaved almost forty millimetres, enough to catch a wheelbarrow wheel on every trip up from the section gate. Nobody had touched the tree in all that time. It had simply kept doing what a tree does, and the path had been sitting in the way of it for longer than either of them had any say in the matter.
How a root actually lifts concrete, rather than just pushing at it
A root does not lift a slab the way a jack does. What moves the concrete is secondary thickening: a root that started out a few millimetres wide keeps adding girth every growing season for as long as it lives, the same way the trunk does, and it does this along its entire length, not just near the tip. Where that root runs directly under a compacted base course, the compacted material resists sideways expansion far more than the loose soil above it does, so the growth takes the path of least resistance, which is upward against the underside of the slab. Give it fifteen or twenty years and a root that was twenty millimetres across when the path went in can be seventy or eighty millimetres across underneath it. Something has to move, and it is always the lighter, unreinforced thing on top.
Why the roots causing the damage are shallower than most people assume
There is no deep taproot doing this. On the great majority of trees found on New Zealand sections, including puriri, pōhutukawa, plane and most of the exotic shade trees planted through Wellington’s older suburbs, structural and feeder roots concentrate in the top 300 to 600 millimetres of soil, and they spread outward well beyond the edge of the canopy, often two to three times the canopy radius on a mature specimen. On a steep Wadestown section over weathered greywacke clay, roots also tend to track sideways along the more workable topsoil band rather than down through the harder, wetter clay beneath, which puts them exactly where paths, driveways and services are usually laid.
Why a root barrier so often disappoints
A vertical root barrier panel, installed to a metre or so depth along the edge of a path, sounds like a clean fix and sometimes buys a few years. In practice roots regularly find the bottom edge of the barrier and grow underneath it, or find the joint between panels, because a root under pressure from a barrier simply changes direction rather than stopping. On a shallow barrier the root can also surface again a metre or two further along, which moves the lifting problem rather than solving it. Barriers installed too close to the trunk carry a second risk: cutting off root growth on one side of a tree that is already leaning or is exposed to Wellington’s prevailing north-westerlies can unbalance the root plate that keeps it upright, which is not a trade worth making to save a footpath.
What tends to actually work
The more durable fix treats the root as a fact of the ground rather than a problem to remove. Flexible, sand-set unit pavers over a shaped aggregate base can be lifted, the base reshaped around the root, and relaid in an afternoon, which is a very different job from breaking out and repouring a slab. Suspended paving on a load-bearing cellular base, or a short timber boardwalk section that spans over the root zone instead of resting on it, keeps foot traffic level without the base course ever compacting against the root. Selective pruning of a root is sometimes reasonable, but only outside the structural root zone close to the trunk, and only after checking how much of the root plate that root actually represents; done inside that zone it can do more to the tree’s stability than the heaved path was ever doing to anyone’s ankle.
Working out where that line sits, and whether a lift is caused by one root or several converging ones, is the kind of assessment worth getting an opinion on before ordering pavers or booking a concrete cutter. I would rather have a Wellington arborist look at the root plate first than guess from the surface cracking alone.
Frequently asked questions
Can I just cut the root that is lifting my path? Sometimes, but only once you know how far that root extends back toward the trunk and how much of the tree’s anchorage depends on it; cutting a major structural root close to the base can destabilise the tree rather than just solving the path.
Will a root barrier stop the lifting for good? Usually not on its own. Roots commonly find their way under or around a barrier within a few years, so barriers work best as one part of a plan that also changes what sits on top of the root, such as flexible paving.
Does a lifted path mean the tree itself is unstable? Not necessarily. Lifting is a growth and pressure issue in the topsoil band, which is a separate question from whether the tree’s overall root plate and stem are sound; the two are worth assessing separately.