What is a tree protection zone?
A tree protection zone, or TPZ, is the area of roots and canopy that needs protecting so a tree survives building work nearby. Australian Standard AS 4970-2009, Protection of trees on development sites, sets out how to calculate it and what protection is required. Most Melbourne councils refer to AS 4970 in planning permit conditions.
How the TPZ is calculated
The TPZ radius is 12 times the trunk diameter measured at 1.4m above ground, known as DBH. A tree with a 40cm DBH has a TPZ radius of 4.8m. The minimum TPZ is 2m and the maximum is 15m.
The structural root zone
Inside the TPZ sits the structural root zone, or SRZ. This is the area holding the roots that keep the tree standing. It is calculated from the trunk diameter just above the root flare. Work inside the SRZ is generally not acceptable without detailed root investigation, because cutting those roots can make the tree unstable.
Encroachment: how much is allowed
AS 4970 treats an encroachment of up to 10% of the TPZ area, outside the SRZ, as minor. Anything more is major encroachment, and the project arborist has to show the tree will remain viable, often through root investigation using non-destructive digging.
What you cannot do inside a TPZ
- Excavate, trench or change soil levels
- Store materials, park machinery or wash out concrete
- Install services without arborist supervision
- Attach anything to the tree
Protection on site
Protective fencing, usually 1.8m temporary mesh panels, marks the TPZ before work starts. Where access is needed, ground protection such as mulch under rumble boards spreads the load. Signage on the fence is required on many permits.
When do you need an arborist?
If council has flagged trees on or next to your site, the planning application usually needs an arboricultural impact assessment and a tree management plan. We prepare these as part of our arborist reports service, and can act as project arborist through construction.
What about AS 4373?
AS 4373-2007, Pruning of amenity trees, is the companion standard for pruning. Any pruning needed to give clearance to a new building should follow it. It is the standard our teams work to on every pruning job.