Justifying an encroachment so it survives objection

Under AS 4970:2025 an encroachment into the Notional Root Zone falls into one of three tiers: minor at ten percent of the NRZ area or less, moderate above ten and up to twenty, and major above twenty or any intrusion into the Structural Root Zone at all. If that framing is new, the revision is written up here.

What follows assumes you have the number. This is about what you do with it.

1. What the percentage is not

It is a screening figure derived from a circle drawn around a trunk. It knows the diameter of the tree and nothing else. It does not know the species, the soil, the aspect, the age, the history of the site, or where the roots actually are.

Two encroachments of identical percentage can be completely different propositions. Fifteen percent of the NRZ on the shaded southern side of a mature oak in deep loam, in ground that has never been disturbed, is not the same problem as fifteen percent on the side of a stressed eucalypt where a driveway was cut in twenty years ago and the tree has been compensating ever since.

The tier tells you how much justifying you have to do. It does not do any of the justifying.

This cuts both ways, and the second direction is the one people forget. A minor encroachment is not automatically fine. If the ten percent happens to sit over the only unconstrained root run the tree has left, the classification is comforting and wrong. Your report should say so.

2. Where the encroachment sits matters more than how big it is

The NRZ is a circle because a formula has to draw something. Root systems are not circles. They follow water, air and the path of least resistance, and on a developed site they have usually already been shaped by whatever was built before.

So the questions worth answering in the report are geographic, not arithmetic:

• Which sector is affected? An encroachment on the side already occupied by an existing slab is a very different matter from one on the last remaining open ground.
• What is already there? Existing paving, compaction, services and fill mean the notional zone overstates what the tree actually has. Say that explicitly, with what you observed.
• Is it contiguous or scattered? One continuous incursion cutting off a sector is generally worse for the tree than the same area spread thinly around the edge.
• How deep, and what method? A pier bored by hand under supervision is not a trench cut with an excavator, even where the plan footprint is identical.

None of this shows up in the percentage. All of it belongs in the report.

3. When root investigation earns its cost

The 2025 revision puts more weight on actually looking, and in practice a moderate or major encroachment on a tree worth keeping is where investigation stops being optional.

Non destructive exploration, usually an air spade or careful hand excavation, answers the question the formula cannot: are there significant roots in the affected sector, how big, and at what depth. That evidence changes the conversation entirely.

It is worth doing when:

• The encroachment is moderate or major and the tree is going to be retained.
• The tree is significant enough that its loss would be contentious, which usually means it will be scrutinised.
• You suspect the roots are not where the circle says, because of existing structures, fill or previous works.
• The design could genuinely be adjusted if you found something, which is the important one. Investigation that cannot change anything is money spent on a paragraph.

When you do investigate, report what you found either way. An honest "the affected sector was explored and contained no roots above 25 mm" is powerful. So is finding a major root and saying so, because then you are the arborist who checked rather than the one who assumed.

4. The tree side of the argument

Whether a tree tolerates a given loss depends on the tree, and this is the arboriculture that a percentage cannot reach:

• Species tolerance. Some genera compartmentalise and recover from root loss far better than others, and you know which ones locally. Say which, and why it matters here.
• Age and vigour. A semi mature tree in good health with an extensive undisturbed system has capacity that an over mature, previously stressed tree does not.
• Existing condition. A tree already carrying dieback or previous root severance has less in reserve. If you are relying on the tree tolerating an impact, its current condition is the evidence base.
• Soil and water. Deep, friable, well drained soil supports recovery in a way that a compacted clay profile does not.
• What happens afterwards. Retention is not the finish line. Mulching, irrigation through establishment, and a monitoring period are what turn a survivable impact into a surviving tree, and they are conditions a council can actually impose if you write them.

5. Writing it so it holds

A structure that works, per affected tree:

1. State the figures. NRZ, SRZ, the encroachment area, the percentage, and the tier. Plainly, so nobody has to derive them.
2. Locate it. Which sector, how deep, by what construction method.
3. Describe what is already there. Existing hard surfaces, compaction, services, previous works.
4. Give the evidence. Root investigation findings if you did any, and say clearly if you did not and why.
5. Make the arboricultural case. Species, age, condition, soil, and your judgement on tolerance.
6. Set the conditions. Supervision, method, protection, aftercare, monitoring. Specific enough to be enforced.
7. State the conclusion and own it. "In my opinion the tree can be retained subject to the above" is a professional judgement. Write it as one.

The thing that makes this survive an objection is not confidence, it is traceability. A reader who disagrees with your conclusion can still see exactly what you measured, what you looked at, and how you got there. That is a defensible report. A report that states a conclusion without showing the path is exposed no matter how right it happens to be.

6. Knowing when the answer is no

Sometimes the honest answer is that the tree cannot be retained with the design as proposed.

Saying so early is better for everyone, including the client who is paying you, because a retention argued past what the evidence supports tends to fail later and more expensively. It can fail at the objection, or at the appeal, or three years afterwards when the tree declines and somebody goes back through the file to find who said it would be fine.

The stronger position, and usually the more useful one commercially, is to identify what design change would make retention viable. Moving a footing, changing a construction method, reducing a footprint by a metre. That gives the client a path rather than a refusal, and it is the advice they were actually paying for.