Notional Root Zone (NRZ)
Trees are a vital aspect of our natural environments and play integral roles in the aesthetic appeal of our parks and greenspaces which is why we have a responsibility to ensure that no harm befalls them when urban development and construction is undertaken nearby.
Notional Root Zones (NRZs) are designated areas that surround trees, protecting them from disturbances and damage during construction, development or otherwise any kind of planned activities that pose a risk to the tree in question. The NRZ acts as a buffer to shield the tree from receiving damage to its roots, trunk or canopy and can vary wildly depending on both the size, type and shape of the tree.
Notional Root Zones (NRZ’s) were previously referred to as Tree Protection Zones (TPZ’s) on this page but have been updated to represent the recent changes in terminology used by Australian Standard 4970:2025.
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On Site Risk Mitigation.
The NRZ further helps minimize risks such as soil compaction, trenching or other changes in the drainage capabilities of the soil which may cause a wide array of health problems for the tree, such as increase susceptibility to disease and pathogens as well as a reduction in the supply of nutrients, water and/or air. Furthermore, NRZ’s also greatly assist in protecting both the canopy and the structural roots of the trees themselves.
How Is The NRZ Calculated.
Typically, the NRZ for a specific tree is calculated using either the tree’s drip line, which is the outer edge of its canopy, or a formula based on the diameter of the tree’s trunk at breast height (DBH). The common standard of practice is to create a buffer zone that extends 1.5 times the DBH or the drip line, whichever is greater, which will then become the TPZ for that specific tree.
Structural Root Zone (SRZ)
The Structural Root Zone (SRZ) is a fundamental part of a TPZ and is the area of roots around the trees that are considered vital for the tree’s ongoing structural integrity. While a much larger area is required to maintain a viable tree, the structural root zone is considered critically important for the structure of the tree to remain upright and sturdy.
The SRZ is only needed in calculations when there is a major encroachment into the TPZ of any tree (> 10% of the TPZ). The radius for the SRZ can be calculated by taking the diameter of the root buttress and then utilizing the following formula:
                 (D x 50)0.42 x 0.64
(D = The diameter of the root buttress)
Supervision and Auditing in Tree Protection Zones
In order to ensure that damages are not experienced when development is conducted in or around VPO’s, arborists may be required to supervise any sites that conduct any form of excavation or operate heavy machinery that have VPO’s on or immediately surrounding them. This is to ensure that no damage is taken during work due to negligence or accidents.
Auditing takes places once all development is completed, and any temporary equipment and structures are removed from the property. The audit checks for any damage that has been dealt to vegetation within VPO’s and, if said damage exists, includes a remediation plan to help the damaged vegetation recover.
Need Help With Your Trees, Garden or Lawn?
Heritage Consultancy has over 20+ years of experience within the arboriculture industry across South-East Queensland, with a proven track record of excellence and diligence.