The relationship between tree roots and building foundations is a significant concern for Australian homeowners. While trees add value to a property, their root systems can cause serious structural damage when planted too close to buildings. Understanding how this damage occurs helps you protect your home.
How Tree Roots Damage Foundations
Tree roots affect foundations through two primary mechanisms:
Direct Physical Pressure
Large roots growing against or under foundation walls, slabs, and footings can exert significant pressure as they expand over time. While roots generally follow the path of least resistance and will not penetrate sound concrete, they can exploit existing cracks, push through weakened mortar joints, and displace lighter structures like garden walls, paths, and lightly reinforced slabs.
Soil Moisture Changes (Subsidence and Heave)
This is the more common and often more damaging mechanism, particularly on reactive clay soils common throughout parts of Hobart and wider Tasmania. Large trees extract enormous volumes of water from the soil through transpiration. On clay soils, this moisture removal causes the clay to shrink, leading to ground subsidence beneath or near foundations. When the tree is removed or during wet seasons, the clay re-expands (heave), and this cycle of movement cracks and distorts structures.
Signs of Root-Related Foundation Damage
Look for these indicators that tree roots may be affecting your foundations:
- Cracks in internal or external walls, particularly diagonal cracks around windows and doors.
- Doors and windows that stick or no longer close properly.
- Uneven or sloping floors.
- Gaps between walls and ceilings or walls and floors.
- Visible root growth against or near foundation walls.
- Cracking in concrete slabs, particularly in a pattern radiating toward a large tree.
If you notice these signs and there is a large tree within roughly the tree’s height distance of the affected structure, root-related subsidence should be investigated.
Solutions for Root Damage to Foundations
Arborist Assessment
The first step is getting a professional assessment from a qualified arborist and, if significant structural cracking is present, a structural engineer. An arborist report can identify which tree is responsible, the extent of root influence, and the best management strategy.
Root Pruning
In some cases, strategic root pruning on the side of the tree facing the building can reduce moisture extraction from the critical zone without killing the tree. This must be done carefully by a qualified arborist, as cutting too many roots can destabilise the tree and create a falling hazard.
Root Barriers
Installing a physical root barrier between the tree and the structure can redirect root growth away from foundations. Barriers are typically made from high-density polyethylene and are installed in a trench to a depth of 600 mm to 1,200 mm.
Tree Removal
When root pruning and barriers are not sufficient, or the tree is too large and too close to the building, tree root removal and tree removal may be the only permanent solution. On reactive clay soils, removal must be managed carefully, as the sudden cessation of water uptake can cause clay heave.
Prevention Is Key
The best defence against root damage to foundations is thoughtful tree selection and placement. Research the mature size and root characteristics of any tree before planting near structures. Our guide on tree roots in pipes covers similar principles for protecting underground infrastructure, and the tree root removal guide outlines your options when prevention is no longer possible.
If you suspect tree roots are damaging your home’s foundations, early action can prevent the problem from worsening. Find a qualified arborist in your area to get an expert assessment and explore your options.