Yes, and retaining walls are one of the structures roots damage most reliably, because a retaining wall is already fighting the soil the roots are growing in.
A wall holds back a mass of earth that wants to move. Roots grow through that mass, thicken over years, and add outward pressure the wall was never designed for. They also change the drainage behind it, which is what actually causes most retaining wall failures.
Roots do not usually crack a sound wall by brute force alone. They exploit what is already there: a weep hole, a joint, a gap between sleepers, an existing crack, or a drainage layer that has silted up.
This page covers retaining walls specifically. For roots and pipes, driveways, slabs and foundations, see the full guide to tree roots and property damage.
How exactly do roots damage a wall?
Four mechanisms, usually working together.
Radial thickening. A root that grows behind or through a wall gets thicker every year. That expansion is slow, constant and extremely strong. It is the mechanism that lifts sandstone blocks, spreads timber sleepers and pushes segmental blocks out of line.
Drainage blockage. This is the one that does the most damage and gets blamed the least. Retaining walls rely on drainage: weep holes, an aggregate layer, an ag line behind the wall. Fine roots colonise that aggregate and that pipe because it is the wettest, best-aerated soil available. Once the drainage is blocked, water builds up behind the wall, and hydrostatic pressure on a saturated backfill is a far bigger load than the wall was designed to carry. Many walls “damaged by roots” actually failed from water the roots stopped draining.
Soil moisture change. A large tree draws a great deal of water out of the ground. On Sydney’s reactive clay soils, that drying causes shrinkage and settlement, and the wall and its footing move with it. Then the tree is removed, the soil rehydrates, and the ground heaves back. Both directions crack walls.
Leverage from the tree above. A tree rocking in a southerly transmits load through its root plate. If the root plate is immediately behind a wall, the wall is taking part of that load every time it blows.
What are the warning signs?
- Bulging, bowing or leaning in a section of wall rather than along its whole length
- Blocks, bricks or sleepers displaced out of alignment
- Stepped cracking in a masonry wall
- Weep holes that have stopped weeping after rain, or never run at all
- Soil, mulch or water escaping through joints
- Surface roots visible at the base of the wall or emerging through it
- The wall moving faster after a wet period
- Paving, steps or a path above the wall dropping or tilting
A wall that is moving is a safety issue before it is a repair issue. If it retains a driveway, a path, a play area or anything above head height, keep people away from both sides and get it looked at.
Which Sydney trees are the usual culprits?
Not all species behave the same way.
Ficus species, including Moreton Bay fig and Hill’s weeping fig, are the most aggressive root systems commonly planted in Sydney gardens. Hill’s weeping fig in particular was widely planted as a hedge and screen, and its roots travel a long way.
Liquidambar styraciflua is planted through the North Shore and is notorious for shallow, thick surface roots that lift everything nearby.
Camphor laurel is a large invasive tree with a vigorous root system and is a declared weed in NSW.
Eucalypts vary by species, but large specimens draw substantial moisture, which is the soil-shrinkage mechanism rather than the physical one.
Willows and poplars actively seek water and will find a blocked ag line from a long distance.
Melaleuca and Callistemon are moderate, but planted close to a wall they still cause problems.
Distance matters more than species. A modest tree planted a metre from a wall will cause more trouble than a large tree ten metres away.
Whose problem is it?
Three situations, three answers.
The tree is on your land. Yours, and so is the wall repair.
The tree is on the neighbour’s land. In NSW, disputes about damage caused by a neighbour’s tree are dealt with under the Trees (Disputes Between Neighbours) Act 2006 and are heard in the Land and Environment Court. The Court can order works to a tree where it is satisfied the tree has caused, is causing, or is likely in the near future to cause damage to property. Practically, an application needs evidence, which means an independent arborist report identifying the tree, the roots and the causal link. Get legal advice on your own circumstances; this page names the legislation, it does not advise on it.
Strata property. Retaining walls and landscaping are commonly common property, which puts the obligation on the owners corporation under the Strata Schemes Management Act 2015 rather than on an individual lot owner. Confirm the position for your scheme, because lot boundaries vary. Tree removal in strata properties covers how this usually plays out.
When do I need an arborist report?
An arborist report is the document that turns “I think the tree did it” into evidence. You need one when:
- You are in a dispute with a neighbour or a strata committee
- You are applying to council to remove a protected tree because it is damaging a structure
- An insurer has asked what caused the wall to fail
- An engineer is designing a replacement wall and needs the root zone defined
- A builder needs the tree protection zone set before excavation
A report from an AQF Level 5 arborist should identify the species, assess the tree’s health and structure, excavate or inspect the roots implicated, state whether the roots are causing the damage, and set out the options with their consequences. AS 4970 provides the framework for tree protection zones and structural root zones, which is what an engineer will want defined.
G McColl works at AQF Level 5, which is the qualification level that covers assessment and reporting rather than cutting alone (PLACEHOLDER: report fee and turnaround). How arborists assess trees sets out what that inspection involves, and the full range of arborist services covers the rest.
Can the tree be kept?
Often, yes, and it is worth asking before defaulting to removal.
Root pruning and a root barrier. Selected roots cut back and a physical barrier installed between the tree and the wall. This is not a homeowner job. Cut the wrong roots and you destabilise the tree, which turns a wall problem into a falling-tree problem. Structural roots close to the trunk should not be cut, and how much can safely be removed depends on the species, the tree’s condition and how much root there is elsewhere.
Rebuild the wall around the tree. Where the tree is worth keeping, an engineered wall designed with the root system in mind, including drainage that roots cannot block as easily, is often cheaper over twenty years than removing a mature tree and losing the amenity. Retaining structures are covered by AS 4678, and this is engineer territory, not arborist territory.
Crown management. Reducing the sail area with tree pruning reduces the wind load transmitted through the root plate. It does nothing about root thickening, so it is a partial measure, not a fix.
Should the tree come out?
Removal is the right answer when the tree is structurally poor anyway, when the roots are genuinely under a wall that has to be rebuilt, when root pruning would destabilise the tree, or when the species is one that will keep doing this regardless of what is done now.
Two things to sort out before booking tree removal:
Approval. Urban vegetation in NSW is regulated under Chapter 2 of the State Environmental Planning Policy (Biodiversity and Conservation) 2021, with the detail in each council’s development control plan. Damage to a structure is a recognised ground in many council policies, but it generally has to be evidenced, which brings you back to the report (PLACEHOLDER: confirm current clauses per council). See council approval for tree removal in Sydney.
Soil movement. Removing a large tree from reactive clay lets the soil rehydrate and heave. If the wall is being rebuilt, sequence the removal and the rebuild deliberately rather than rebuilding first.
What happens to the roots after removal?
They stay in the ground and decay slowly, which means the wall can keep moving as they rot and the soil settles into the voids.
Where the stump sits near or behind the wall, stump grinding removes the bulk of the root crown so the ground can be backfilled and compacted properly before the wall is rebuilt. Grinding close to a wall needs care about what the wall is founded on, so the wall builder and the grinder should talk to each other first.
If the wall is being rebuilt as part of a larger build, tree removal for construction covers sequencing with the other trades.
Frequently asked questions
How far from a retaining wall should a tree be planted?
Further than most people plant them, and it depends entirely on the species’ mature root spread. Ask before planting rather than after.
Can I cut roots on my side of the boundary?
In principle a landowner has some right to abate encroachment at the boundary, but cutting structural roots can destabilise a neighbour’s tree and create liability if it then fails. In NSW this is exactly the territory the Trees (Disputes Between Neighbours) Act 2006 covers. Get advice before cutting anything substantial.
Will a root barrier fix it?
It stops new root growth toward the wall. It does not remove roots already there, and it has to be installed deep enough and far enough to work.
Is the wall covered by insurance?
Gradual damage from tree roots is commonly excluded from home policies. Check your product disclosure statement, and expect the insurer to ask for a cause report.
How long before roots damage a new wall?
Years, usually, and the drainage blockage arrives long before the visible cracking does.
Do I need council approval to prune roots?
Root pruning of a protected tree can be treated the same as pruning the crown under some council controls. Confirm before starting.
Get the tree and the wall looked at together
A wall that is moving is a question with three parts: is the tree the cause, can the tree stay, and what has to happen first. An engineer answers the wall. An arborist answers the tree. The order they happen in decides how much the job costs.
G McColl works to AQF Level 5 across the North Shore and Northern Beaches, fully insured, and can inspect the tree, the root zone and the wall together and put it in writing.
Book an arborist inspection, or call 0403 134 299.