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Tree influence zone

The tree influence zone is 1 to 1.5x mature tree height around a tree. Roots inside this zone can shift differential ground movement under footings. AS 2870 Appendix C.

By Chalkline. Last updated 14 May 2026. Verified 14 May 2026.

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A tree influence zone is the horizontal distance around a tree, typically 1 to 1.5 times the mature canopy height, within which the tree’s root system can remove enough soil moisture to affect ground-movement behaviour under a slab or footing. Trees act as soil-moisture pumps: they draw water out of the soil they root in, drying it (and shrinking reactive clay) during the tree’s life, and releasing the draw when the tree is removed (allowing the soil to rewet and swell).

AS 2870:2011 Appendix C provides guidance on tree influence and the geotech’s soil report must call out trees within the influence zone of the proposed footings.

The 1× to 1.5× rule of thumb. For a 10 m mature gum, the influence zone is approximately a 10 m to 15 m radius. The exact factor depends on:

  • Species: Eucalyptus and other deep-rooted natives tend toward 1.5×; smaller ornamentals tend toward 1×.
  • Soil reactivity: high-shrink clays (Class H, E) show influence further out than low-shrink soils (Class S).
  • Root barriers: deep concrete or rated polymer root barriers can reduce the effective influence zone.
  • Climate: longer drought cycles amplify tree effect; wetter climates dampen it.

The two ways tree influence damages buildings:

  1. Existing tree, foundation cracks under: a mature tree close to a slab edge drives differential drying under that edge. The slab edge drops; the slab interior stays put. Cosmetic cracks appear at the corners closest to the tree.
  2. Tree removed pre- or mid-construction, foundation heaves later: the soil under where the tree used to be re-wets over years as the root pump stops. The slab edge over that re-wetting area lifts. Same crack pattern but in the opposite direction; commonly seen 3-7 years after construction on lots cleared of mature trees just before build.

The second mode is the more insidious: the damage shows up years after handover when no one connects it back to the cleared tree.

Mitigations the engineer may specify:

  • Stiffer footing design: bigger edge beams or piers to deeper soil, sized for the maximum credible heave.
  • Root barriers: vertical concrete or polymer barriers below the slab edge, intercepting roots.
  • Tree removal scheduling: remove the tree well before build (so initial re-wetting has settled) or after the slab is in (so the dry phase locks in). Both are imperfect; engineering response is the safer bet.
  • Class P site declaration: where the tree influence is too uncertain, classify the site as Class P and design a fully engineered slab.

For builders.

  1. Identify trees on the lot before quoting. Mature trees within 1.5× their height of the building footprint affect the design.
  2. Get the geotech back to look at trees specifically. A standard soil report may not assess tree influence unless asked.
  3. Document tree removals. If trees are cleared during demolition, photograph and note the location and approximate size before they go. The future engineer dealing with a slab crack at year 5 will need that history.

Also known as: tree zone, root influence zone, tree-action distance.

Category: Site preparation / engineering / trees.

See also

Last updated: 2026-05-14. Verified: 2026-05-14.

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