In February 2025 I put a straight edge across a concrete driveway on Glandovey Road, Fendalton, and measured a 38-millimetre step where a root had lifted one slab against its neighbour. The tree was an English oak, trunk 920 millimetres across at chest height, planted by the look of it when the house was built in the 1920s. The driveway was younger than the owner’s car: poured in 2013 as part of the post-earthquake repair, on ground that had produced liquefaction silt two years earlier. The owner had three quotes on the kitchen table, one to cut the roots, one to replace the drive, one to fell the oak, and wanted a fourth opinion before spending anything.
What the roots are doing under there
Oak roots live in the top 300 to 600 millimetres of soil because that is where the oxygen is. A concrete slab does not stop them; it shades the ground beneath, keeps it moist, and gives them an easy horizontal run with no competition. A root that was 30 millimetres thick when the drive was poured in 2013 can be 100 millimetres twelve years later, and the slab above it goes up by the difference. On Fendalton’s silty loam, with the Waimairi Stream keeping the water table high, roots grow fast and shallow. The 2013 drive was 100 millimetres of unreinforced concrete on compacted gravel — standard for a residential drive and no match for a root with ninety years of tree behind it.
The step itself was the least of it. The slab had cracked across its width where the root passed under, and the crack was letting water down onto the root, which encouraged it further. Roots and concrete do not reach an equilibrium. One of them gives.
Cutting roots: the arithmetic nobody quotes
The cheapest quote was to saw through the offending root on both sides of the slab, at around NZ$600. The problem is what happens next: neighbouring roots take over the run, the drive lifts again in five to eight years, and the same quote cuts the next one. Do that three times and you have removed a meaningful share of the anchoring roots on the driveway side of a tree that stands upwind of the house in a nor’wester.
My rule is that no root over 50 millimetres is cut within a radius of five times the trunk diameter, which for this tree is 4.6 metres, and no more than a quarter of the roots on any one side in a decade. The lifted slab was 3.1 metres from the trunk. The cut was not an option I could put my name to.
Living with it, and when that is the right answer
A 38-millimetre step is a trip hazard and a tyre problem, but not a structural failure. Grinding the high edge to a ramp costs NZ$300 to NZ$700 and buys three to five years. If the drive is otherwise sound and the tree is scheduled, this is often where the sensible money goes, and I said so in the report. This drive was not otherwise sound; the crack had propagated through two slabs and there was no reinforcing mesh.
Re-laying over a root you intend to keep
The design I specified was a bridging slab. The two cracked panels were cut out and lifted, the roots were exposed by hand and photographed, and rather than excavating around them the new slab was poured at 150 millimetres thick with two layers of mesh over 100 millimetres of compacted gravel that let the roots breathe. The slab sits 60 millimetres higher than its neighbours at the tree end and ramps back to level over 2 metres. The roots have room to grow another 40 millimetres before they touch concrete again, which on this tree is a decade or more. A root barrier was not installed; barriers only work where roots have not yet arrived, and driving one 900 millimetres deep 3 metres from this trunk would have cut more root than the quote I had already rejected.
Cost for the 28 square metres came to NZ$6,800, within the NZ$180 to NZ$280 per square metre range for a reinforced residential drive in Christchurch with root protection, against NZ$9,500 for full replacement and a tree removal figure of NZ$8,000 to NZ$14,000 that would also have required consent because the oak is on the district plan schedule. The pour was done by a Christchurch concrete driveway contractor who agreed to the slower hand excavation around the roots and to a site visit from me before the mesh went in. The oak is still there. So is the car.
Frequently asked questions
Can tree roots crack a concrete driveway? Yes, and they do it by thickening rather than by pushing. A root growing under a 100-millimetre unreinforced slab lifts it by roughly the increase in its own diameter, which for a fast-growing tree can be 5 to 10 millimetres a year.
Can I cut the roots under my driveway? Small roots away from the trunk, yes. Roots over 50 millimetres within about five times the trunk diameter should not be cut without an arborist’s assessment, because repeated cuts on one side remove the anchoring the tree relies on in a nor’wester, and a scheduled tree needs consent for root work.
How much does it cost to fix a root-lifted driveway in Christchurch? Grinding the lip runs NZ$300 to NZ$700. Cutting out and re-laying the affected slabs as a thicker reinforced bridging section is NZ$180 to NZ$280 per square metre. Full replacement of a typical 40-square-metre drive is NZ$7,000 to NZ$12,000 before any root barrier.