Tree Protection Zones Explained (AS 4970, TPZ and SRZ)

If your approval conditions, information request or arborist report mention a TPZ, and you are trying to work out what it actually constrains, start here.

Both terms come from AS 4970-2009 Protection of trees on development sites, the Australian Standard councils across South East Queensland reference when construction happens near retained trees.

Tree Protection Zone (TPZ)

The TPZ is the area around a tree that must be protected for the tree to survive construction. It covers both the roots and the space the tree occupies.

How it is calculated

The TPZ radius is twelve times the trunk diameter, measured at breast height (1.4 metres above ground).

TPZ radius = DBH × 12

A worked example. A tree with a trunk diameter of 600mm:

0.6 m × 12 = 7.2 m radius

That is a protection zone 14.4 metres across — which routinely surprises people, because it extends well beyond the canopy edge on many species.

The standard caps the TPZ radius at 15 metres, and it is not applied below 2 metres.

Why twelve times

It is a proxy. The actual root plate of a tree is impossible to map without excavation, and the relationship between trunk size and root extent is reasonably consistent across species. Twelve times DBH is the standard’s approximation of the root area a tree needs to survive disturbance.

It is deliberately conservative, because the alternative — underestimating and killing a retained tree eighteen months after handover — is the more expensive error.

Structural Root Zone (SRZ)

The SRZ is smaller, and much more serious. It is the area containing the roots that hold the tree upright.

Encroach on a TPZ and you may damage a tree’s health. Encroach on an SRZ and you may cause it to fall over.

The SRZ is calculated from trunk diameter measured at ground level, using a formula set out in the standard. It typically produces a radius of one to three metres for a suburban tree — much tighter than the TPZ, and effectively non-negotiable.

The 10% rule

This is the number that matters commercially.

Under AS 4970, encroachment into more than 10% of the TPZ area is considered a major encroachment and requires justification — supported by assessment demonstrating the tree will remain viable.

A few things people routinely get wrong about it:

It is 10% of the area, not the radius. Because area scales with the square of the radius, 10% of area is a smaller linear intrusion than intuition suggests.

It is not a permission. Minor encroachment under 10% is not automatically fine. It still needs to be located away from the SRZ and assessed in context.

Encroachments are cumulative. A driveway, a service trench and a slab edge each taking “just a bit” add up to a major encroachment collectively.

What actually damages trees on site

Formal encroachment is only part of it. In practice, retained trees die from:

• Soil compaction — vehicles, material storage and site traffic inside the TPZ. This is the most common and most underestimated cause.
• Root severance — trenching for services, almost always installed after the tree protection fencing came down.
• Level changes — fill placed over the root zone suffocates roots; cut removes them.
• Altered drainage — new hardstand redirects water away from, or into, the root zone.
• Chemical spill — washout from concrete, render or fuel inside the protection zone.

A tree can look completely healthy at practical completion and be dead three years later from compaction that happened in week two. This is why councils condition monitoring rather than just fencing.

What this means for your project

Get the assessment before you finalise drawings

Tree constraints identified at concept stage are a design input — you move the slab edge 800mm and the problem disappears.

The same constraint identified after the drawings are done, engineered and priced is a redesign. This is the single most expensive sequencing mistake on small development sites, and it is entirely avoidable.

Expect two documents

They are commonly confused, and councils frequently condition both.

An Arboricultural Impact Assessment determines which trees can be retained, calculates the zones, quantifies encroachments and justifies them.

A Tree Protection Plan is the practical instruction set for the site — where fencing goes, what may happen inside it, ground protection details, and the hold points through the programme.

Expect a project arborist condition

Where works occur near retained trees, approvals commonly name a project arborist who must attend nominated hold points and certify that works were carried out correctly. That certification is what closes out the condition.

The practical summary

• TPZ = DBH × 12. Protects tree health.
• SRZ = smaller zone, calculated at ground level. Protects tree stability. Treat as non-negotiable.
• More than 10% TPZ area encroachment requires justification through assessment.
• Most retained-tree deaths come from compaction and trenching, not from the encroachment that was assessed.
• Assess before the drawings are locked, not after.

If you have an approval with tree conditions on it and are not sure what they require in practice, that is a short conversation rather than a large one.