SRZ calculator
Enter the trunk diameter immediately above the root buttress flare and get the structural root zone radius.
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Calculate
Structural root zone
The SRZ is worked from the trunk diameter immediately above the root buttress flare. It is about what holds the tree up, which is why any encroachment reaching into it is treated as major however small the percentage.
1The tree
Where exactly do I measure?
- Level groundMeasure the diameter 1.4 m above ground level.
- Sloping groundTake ground level on the upslope side, then measure 1.4 m above that.
- Leaning trunkMeasure 1.4 m along the lean, not 1.4 m vertically off the ground.
- Fork above 1.4 mOne trunk at that height, so measure it as a standard tree.
- Fork below 1.4 mEach stem is measured, then combined. Use the multi-stemmed tree type.
- Buttressed treeMeasure just above the buttress, where the trunk stops widening into the roots.
Enter a trunk diameter and the notional root zone appears here.
2Structural root zoneOptional
Enter the buttress diameter and the structural root zone appears here.
3The proposed works
A straight edge — a slab, wall or footing
The encroachment needs a trunk diameter in step 1 first.
Your figures
Enter a trunk diameter and the zones appear here. Nothing on this page is a Tree Protection Zone — that is determined by a project arborist.
- NRZ radius
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- NRZ area
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- From DSH
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- SRZ radius
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- Encroached area
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- Encroachment
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- It does not give you a Tree Protection Zone. The notional root zone is the calculated starting point; the final TPZ is determined by a project arborist after considering the tree, the site and the proposed works.
- It does not tell you whether a development complies, whether a council will approve anything, whether a tree can be removed, or whether works are safe to proceed.
- It does not replace a site inspection. The measurements have to be taken on the tree, and site-specific professional assessment may still be required.
- It does not handle palms, cycads or tree ferns, because the supplied Standard does not support applying the standard method to them.
- It does not calculate a structural root zone for palms, cycads or tree ferns. AS 4970:2025 states that the SRZ calculation does not apply to them.
- It does not tell you where the structural roots actually are. The formula gives an indicative radius; root investigation is what establishes the real extent.
Common questions
About this calculator
Different measurement, different formula, different question. The notional root zone approximates the soil a tree depends on and is calculated from diameter at 1.4 metres. The structural root zone approximates the roots holding the tree up and is calculated from the diameter immediately above the root buttress flare.
The point where the trunk stops widening into the roots and becomes a more or less consistent cylinder. On a heavily buttressed mature tree that can be well above ground level, and the diameter there is considerably larger than at 1.4 metres.
The supplied rules set a minimum: where the measured diameter is below 0.15 metres, the structural root zone radius is 1.5 metres. The calculator says so when that rule has been applied.
Any encroachment intersecting the structural root zone classifies as major regardless of how small the area is, because those roots are what hold the tree up. Whether the works can proceed, and on what terms, is an assessment rather than a calculation.
No. It is a notional area related to tree stability. It is not the notional root zone and it is not the Tree Protection Zone, which is determined by a project arborist.
Contact
Send Adam the address and he will take it from there
For many protected vegetation questions, the property address is enough to start checking. Call, or send the details below.