Zone of influence (footing, wall, or tree)
Zone of influence is the horizontal distance around a footing, wall, or tree where loads or moisture changes affect adjacent structures, per AS 2870.
Ask Chalkline about this →A zone of influence is the horizontal distance around a footing, retaining wall, or tree within which the load or moisture effect propagates and can affect adjacent structures or ground. AS 2870:2011 (Residential slabs and footings) uses the term in two distinct contexts.
Footing zone of influence
A footing’s load fans out into the soil below at a typical 2:1 vertical:horizontal angle (sometimes expressed as 30 to 45 degrees from vertical, depending on soil type). Any new excavation, trench, or wall that falls within this fanned-out zone is within the footing’s zone of influence and must be engineered to carry the surcharge load from the original footing without undermining it.
Practical implications:
- A retaining wall closer to a house footing than the footing’s zone of influence must be engineered with the surcharge load included; it cannot be designed for soil pressure alone.
- A service trench dug too close to a footing can collapse the footing or transfer load into the trench unless shored.
- An adjoining property excavation within the zone of influence of your house footing triggers the protection-of-structure obligations on the excavator (Party Wall Act or equivalent state legislation).
For a typical strip footing 600 mm deep, the zone of influence extends roughly 300 mm horizontally past the footing edge at footing level, and widens as you go deeper.
Tree zone of influence
In AS 2870 site classification, a tree’s zone of influence is the horizontal distance from the tree trunk at which its moisture demand (or moisture rejection on a clay site) can change the soil’s shrink-swell behaviour enough to affect a footing. Common rule of thumb:
| Tree class | Zone of influence (horizontal distance from trunk) |
|---|---|
| Single tree, moderate water demand | Roughly 1 x mature canopy height |
| Single tree, high water demand (e.g. eucalypt, willow) | Roughly 1.5 x mature canopy height |
| Group of trees | Up to 1.5 x mature height plus radius of group |
This drives building setback decisions on reactive sites (Class M, H1, H2, E per site classification). A house footing placed within a high-water-demand tree’s zone of influence on Class H1 reactive clay will experience differential settlement as the tree draws moisture in dry seasons, causing wall cracking.
The designer either:
- Sets the footing outside the zone of influence; or
- Designs the footing to a higher class (e.g. M designed as H1) to handle the moisture-driven movement; or
- Specifies tree removal before site classification (note: this triggers a soil rebound period of 1-3 years on clay sites before the design footing can be confirmed).
On-site builder checks
- Footing edge to nearest tree trunk distance recorded on the geotechnical report.
- Retaining wall design near house footings explicitly references the footing’s zone of influence and surcharge in the engineer’s calculations.
- Service trenches stay 500 mm clear of footing edges minimum (project-specific, check the engineer’s spec).
Also known as: zone of influence (ZOI), tree zone of influence.
Category: Footings / AS 2870 / site classification.
Related
See also
Last updated: 2026-05-16. Verified: 2026-05-16.