In February 2023 I spent three weeks in Auckland on storm response after Cyclone Gabrielle, working out of a yard in New Lynn. One job was a Mount Albert section off Carrington Road: a 1.8-metre timber pole retaining wall holding up the back lawn, and above it two silver birches and a big old totara. The wall had bellied out about 150 millimetres in the middle, the birches were leaning downhill, and the owner wanted to know whether the trees had pushed the wall over or the wall had let the trees down. The honest answer was both.
What Auckland clay does when it is saturated
Most of the Auckland isthmus sits on weathered volcanic and Waitematā clays that hold water like a sponge and let it go slowly. Wet, they lose much of their strength and get heavy: a cubic metre of saturated clay weighs close to two tonnes. In the Gabrielle fortnight every clay bank in the city was at its heaviest and weakest at the same moment.
A retaining wall is designed to hold a wedge of soil. It is not usually designed to hold a wedge of soil plus a tree.
The tree as a load the wall was never told about
A 15-metre silver birch with a 400-millimetre trunk weighs 1.5 to 2.5 tonnes above ground, and the root plate under it, a disc of soil and roots two to three metres across, adds a similar mass. Put that within a metre or two of the top of a wall and you have added several tonnes of surcharge to the backfill, right where the wall is working hardest.
Then the wind comes. A tree in a gale is a lever: the crown catches the load, the trunk transmits it, and the root plate rocks in the soil. Every rock toward the wall is a push. Dry ground resists it; soaked Auckland clay moves with it, and the wall takes the push. Those two birches rocked against the back of that pole wall for two days straight. That is where the 150-millimetre belly came from.
Roots through the face and roots in the drainage
Behind the wall I found the trees had been working on it for years before the storm. The totara had sent two roots as thick as my forearm through the gap between the timber poles, and where they exited on the downhill face they had wedged the boards apart by about 40 millimetres. Roots grow toward air and moisture, and the drainage layer behind a retaining wall is exactly that: free-draining scoria or gravel with water moving through it. Fine roots colonise it, thicken, then block it. Once the drainage is choked, water that was meant to run out the bottom sits in the clay behind the wall, and the wall is holding wet soil it was designed to hold dry.
The pattern is the same on nearly every wall I see with trees above it:
- Roots matted in the drainage gravel when the outlet pipe is pulled.
- Roots through joints in the face, wedging timber or displacing blocks.
- A wet stain low on the wall that never dries.
- Soil heaving or cracking at the top of the wall around the trunk.
What was worth saving and what was not
The totara stayed. It was about six metres back from the wall; its roots through the face were pruned flush and a root barrier was trenched along the upper side of the wall to about 600 millimetres depth to stop them re-entering. A totara takes that kind of root pruning at that distance without much drama.
The birches came out. They were 1.2 metres from the top of the wall, already leaning, and any wall rebuilt behind them would have been fighting the same load again within a few winters. Removing two 15-metre birches on a sloping section with a house below cost about NZ$3,800 including stump grinding, toward the upper end of Auckland pricing because everything had to be lowered on ropes rather than dropped. The root barrier and pruning around the totara added roughly NZ$900.
The wall itself was beyond a patch. It needed to come out and be rebuilt with drainage that would survive, and that is construction work rather than climbing. The sequence matters: the trees come out first, the stumps are ground or left low, and then an Auckland retaining wall builder takes over with a clear bank to work on and a wall that will not have birch roots in its gravel by year three.
Frequently asked questions
How far should a tree be from a retaining wall? As a working rule, keep the trunk at least as far back from the top of the wall as the wall is high, and double that for large or fast-growing species. A 1.8-metre wall wants trees no closer than 1.8 metres, and 3.5 metres or more for birch, gum or poplar.
Can tree roots push over a retaining wall? Yes, in two ways: directly, by thickening inside joints and behind the face, and indirectly, by blocking the drainage so the wall ends up holding saturated soil.
What does it cost to remove a tree above a retaining wall in Auckland? Roughly NZ$1,500 to NZ$4,500 per tree for mid-sized specimens where everything has to be rigged down rather than felled, plus NZ$200 to NZ$500 per stump for grinding.