Ask ten people how far tree roots go and you will hear everything from “as deep as the tree is tall” to “only as wide as the drip line”. Both ideas are wrong in useful ways. Knowing where roots actually sit helps when you are laying a path, planning a sleepout or wondering why the lawn near a big oak never thrives.
Wide and shallow, not deep and narrow
The old picture of a tree with a mirror-image root system, a carrot-shaped taproot plunging straight down, is mostly a myth for mature trees. The majority of feeder roots sit in the top 30 to 60 centimetres of soil, where oxygen and moisture are most available. Even large trees rarely have significant roots below about one to two metres in typical New Zealand soils.
What roots lack in depth they make up in width. On open ground, root spread commonly reaches two to three times the radius of the canopy, and sometimes more. A tōtara with a canopy six metres across could have roots reaching ten metres or further from the trunk. Exact figures vary enormously with species, soil and water.
What shapes root spread on a real section
Roots go where the conditions suit them, so the pattern on a suburban section is rarely a neat circle.
- Soil type: Free-draining volcanic soils around the Bay of Plenty let roots travel easily. Heavy clays in parts of Auckland and Waikato push roots closer to the surface, where they find air.
- Water: Roots follow moisture. Leaking pipes, irrigated garden beds and low spots that hold water after rain all attract growth.
- Obstacles: Foundations, compacted driveways and retaining walls deflect roots. They tend to run along the edge of a barrier until they find a gap.
- Competition: Where several trees share a boundary, their root zones overlap and spread unevenly towards whichever patch is least crowded.
The root zone you should protect
Arborists usually talk about a protected root zone, often estimated by multiplying the trunk diameter measured at about 1.4 metres above ground by a factor of around 12. A trunk 40 centimetres across would suggest a protection radius of nearly five metres. It is a rule of thumb rather than a guarantee, but it gives homeowners a practical starting point.
Within this zone, avoid:
- Trenching for services or irrigation without hand digging first.
- Raising or lowering soil levels by more than a few centimetres.
- Parking vehicles, storing building materials or mixing concrete.
- Laying impermeable surfaces such as concrete or asphalt over large areas.
Damage to roots often shows up slowly. A tree may look fine for a year or two after earthworks, then start dying back from the top as the lost roots fail to supply the canopy.
Natives, exotics and a few surprises
Many native species have comparatively shallow, fibrous root systems that suit them to forest floors with thick leaf litter. Pōhutukawa can send roots surprisingly far towards water and will happily push into cliff faces and rock crevices. Kauri have a sensitive, shallow feeder system, which is part of why soil disturbance around them is taken so seriously in kauri dieback areas. The Department of Conservation has background on native species and protecting them in the wild.
Among exotics, willows and poplars are well known for chasing water into drains. Eucalypts can dry out clay soils over a wide radius. Fruit trees on dwarfing rootstocks are generally modest and easy to manage near paths.
Roots are only one part of how a big tree affects a house. Shade from a dense canopy can be just as influential, keeping cladding damp and encouraging growth on walls. That side of the story is covered in this piece on shade, mould and exterior paint on the North Shore.
Frequently asked questions
Can tree roots damage a house foundation? Roots rarely break sound concrete foundations directly. Problems are more often caused by roots drying out shrinkable clay soils, leading to movement, or by roots exploiting existing cracks in pipes and old footings.
Is it safe to cut a root that is lifting my path? Cutting one small root far from the trunk is usually tolerable. Cutting large roots close to the trunk can destabilise the tree or starve part of the canopy. Get an arborist to look before removing anything thicker than a few centimetres.
Do root barriers actually work? Properly installed vertical barriers can redirect roots away from paths, drains or foundations. They need to be deep enough, typically 600 millimetres or more, and installed without severing major roots in the process.