My brother bought a 1950s brick and tile house in Mornington in 2022, on one of those Dunedin streets where the section steps down the hill in two terraces and the driveway climbs from the road at about one in seven. The first winter he rang because the driveway had lifted: a slab about 2.4 metres wide had cracked clean across and the uphill edge sat 45 millimetres above the downhill one. Ten metres up the bank stood the culprit, a 14-metre sycamore nobody had planted and nobody had ever cut. I went down in August with a mattock and a chainsaw to find out how frost, loess and sycamore roots had worked together on that slab.
Sycamore: the Dunedin weed tree
Sycamore, Acer pseudoplatanus, seeds itself into every gully and bank in Dunedin. It grows fast, tolerates cold and wind, and puts out a wide shallow root system that hunts for moisture under anything that keeps the ground damp. A concrete driveway is exactly that: a lid that stops evaporation, so the soil under it stays wetter than the open bank beside it. On my brother’s place the biggest root under the slab was 90 millimetres across and had run along the underside of the concrete for over three metres, lifting as it thickened.
What loess does when it freezes
Much of Dunedin’s hill suburbs sit on loess, a wind-blown silt that drains poorly, holds water in its pores, and shrinks or swells with moisture. A wet loess subgrade under a slab is already marginal. Add a Mornington winter with frosts below minus three or four and the water in that silt freezes and expands. Frost heave can lift a driveway several millimetres on a bad night and drop it again as the ground thaws. Repeat that thirty times over a winter, with a root as a fixed high point under one edge, and the slab is rocking over a fulcrum. The crack was not the root alone and not the frost alone; it was the root giving the frost something to lever against.
The other loess habit is slumping. Where the root had drawn water out of the silt through summer, the soil shrank and the slab was bridging a void; come winter, wet, it swelled again. A slab that alternately bridges and floats does not last.
Deciding whether the tree stays
Could the sycamore stay and the driveway be repaired around it? I look at three things:
- Distance. The trunk was 10 metres from the slab. Root pruning is safe when the cut is at least five times the trunk diameter from the trunk, better at eight to ten. This trunk was about 550 millimetres through, so a cut at 10 metres was well outside the zone that would destabilise it.
- Species. Sycamore resprouts from cut roots and will come straight back under the new slab unless something stops it.
- Value. It was a self-sown sycamore on a bank, not a protected tree and not one anybody loved.
He kept it for the shelter it gives from the southerly. The compromise was a root prune along the uphill edge of the driveway at about 600 millimetres depth, roots cut clean with a saw rather than torn, and a rigid root barrier set vertically in the trench to stop the sycamore coming back under the new concrete. That is tree work and a day’s digging: roughly NZ$700 to NZ$1,400 on a Dunedin section depending on how much bank has to be opened.
Repairing the slab so it does not happen again
The old slab had to go. Once a driveway has lifted 45 millimetres there is no lowering it, and grinding the high edge thins the concrete where it is already weakest. The subgrade was wet loess with a root channel through it, so the useful part of the repair was below the concrete: pulling the root, compacting the silt, and getting a proper depth of free-draining base under the new pour so frost has less water to work with. The new slab needed to be thicker than the original 75 millimetres and needed control joints so future movement cracks where it is meant to. All of that is concrete work, not climbing, and once the roots were cut and the barrier was in it was handed to a Dunedin concrete driveway contractor to take out the old slab and pour the new one. The re-pour on a 2.4 by 9 metre sloping driveway came in around NZ$6,500.
Frequently asked questions
Can tree roots lift a concrete driveway? Yes. Roots grow toward the damp soil under a slab, thicken by 10 to 20 millimetres a year in a vigorous tree, and lift the concrete as they go. In Dunedin frost heave in wet loess makes it worse by rocking the slab over the root.
How far from the trunk is it safe to cut roots? At least five times the trunk diameter, preferably eight to ten. For a 500-millimetre trunk that is 2.5 metres as a bare minimum and 4 to 5 metres to be comfortable. Closer risks destabilising the tree.
Should I remove the tree or repair around it? Repair around it if the trunk is far enough away for a safe root prune and you can install a root barrier. Remove it if it is within a couple of metres of the slab or is a fast-growing weed species you do not value.