Treetec Arborists Melbourne Victoria

TPZ / SRZ calculator | Incursion calculations

(calculators align with Australian Standard AS 4970-2009)

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Tree calculator incursion or encroachment into Tree Protection Zone (TPZ)
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DBH calculator

Use the calculator below to determine the Diameter at Breast Height (DBH) of a multi-stemmed tree.

Use the red boxes to input the measurements, always in metres e.g. 65 cm calliper - input: ' 0.65 '

The result is displayed in the green box. If there are more than 5 stems enter the total from the first 5 back into a red box and continue.


Tree Protection Zones - TPZ / SRZ calculator

Enter the relevant figures (in metres) in the red boxes below to calculate the dimensions of a

Tree Protection Zone (TPZ) or Structural Root Zone ( SRZ).

The result is a measurement of the radius (R) of a circle taken from the centre of the trunk.


Incursion into the TPZ

For complex situations there is a great tool here: TPZ Encroachment Calculator

Enter the TPZ radius and the applicable distances in the red boxes to calculate the degree of incursion into a TPZ. All figures should be entered in metres.

The first image is for situations where an incursion does not cut entirely across a TPZ.

Australian Standard AS4970-2009 Protection of Trees on Development Sites

These calculators are based on the Australian Standard AS4970-2009, commonly used by arborists. They can be used by anybody for working out the calculated Tree Protection Zone (TPZ) or Structural Root Zone (SRZ) of a given tree, or degree of TPZ incursion (encroachment) for a given set of plans. This information is normally provided by a consulting arborist in an arboricultural report (tree report) and helps guide planning departments and architects etc on how far from a given tree works should be kept. Within AS 4970-2009 it states that an incursion of any more than 10% of the calculated TPZ is considered a 'major' encroachment and certain conditions should apply; it may trigger the requirement for root mapping, design revisions or low impact construction methods to help protect the tree, often there will be a requirement for all planned works within a TPZ to be supervised by an arborist.

Arborist reports

It is recommended that a consulting arborist be engaged very early in the planning process to help determine which trees, on or near a site, are of higher retention value. Once high value trees are identified subsequent planning can incorporate tree protection, this will save delays and re-working of designs. If a tree is identified in the arborist report as being of high retention value, however it needs to be removed, then that removal application and permit process can commence early, saving on delays and further costs.

Designing around trees should be done in consultation with an arborist. Where it is not possible to restrict all works from within a calculated TPZ there are tree sensitive construction methods and materials that can facilitate some TPZ encroachment whilst still protecting the longer term viability of the tree.

AS 4970-2009 considers both above and below ground tree components, whilst it is imperative that roots are protected it is also important that any pruning of the canopy is appropriate and undertaken by a suitably qualified arborist in line with the Australian Standard on the Pruning of Amenity Trees (AS 4373-2007). Tree lopping causes unnecessary and permanent damage.

How close to a tree can you build?

Building a house or shed or pool near a tree is problematic, will it damage the pool? and/or will it damage the tree?

There are a whole range of factors that come into play when planning buildings or works near trees:

  • Distance - how far from the tree is the building / pool / structure?
  • How valuable is the tree - what is the amenity, ecological or heritage value? What status will Council place on a given tree?
  • Extent of construction - how large is the structure, how much excavation is involved?
  • How tall can you build - will the building shade the tree or funnel wind?
  • Size of tree - Juvenile, large old tree, semi-mature vigorous (expanding) tree
  • Mature size of tree - how large will the tree be at maturity given the species and growing conditions?
  • Species of tree - will it have a large root system, is the species resilient, will the roots damage the new structures?
  • Tree type and characteristics - is the tree deciduous, will it attract or provide habitat for certain fauna, will it drop debris?
  • Soil type - how deep are the roots, how stable is the soil profile?
  • who's tree is it - are you damaging a neighbour's or Council tree?
  • Local tree laws - am I legally allowed to cut, lop or remove the tree?
  • Tree Protection Zone - What is the calculated TPZ and how does it impact my building plans?

The short answer is that for your average, garden variety tree (see what we did there :)), in a standard suburban setting, Council will normally permit works so long as pruning is minimal and the area of TPZ lost (excavated, trenched, covered) is less than 10% of the total calculated TPZ area for that tree.

Given the variables listed above it is strongly recommended that an arborist be engaged early in the planning process to give guidance in these areas. The arborist report you commission should be objective, if you pay an arborist to write what you want to hear such as "The tree is high risk, low value and needs to be removed, and therefore our new pool should be approved" then you are probably making a mistake. Council arborists are normally very good, they know immediately if a report is biased and you will be instructed to have a new inspection and new arborist report submitted, this will increase your timeframes and costs. Your arborist should provide an objective and constructive report detailing realistic tree values and practicable tree protection measures or design amendments.

Do I need to know the Structural Root Zones?

A reporting arborist will happily collect any data the client requests, the more data collected and included in the arborists report, the more it costs. For tree reports that are used for planning applications often SRZ will be included however there is very rarely a need for this.

TPZ is essential - it informs both tree protection requirements and likely impacts however SRZ is rarely required, and in fact it gets only a small mention in AS 4970-2009. This is because if a development is anywhere near the SRZ, on a larger tree, then the encroachment into the TPZ is classed as 'major' and therefore the tree is often considered lost - it simply doesn't matter what the encroachment into the SRZ is - the tree is already lost.
Then on very small trees where a 10% encroachment into the TPZ does actually reach the SRZ the arborist will still make assessments based on the TPZ, tree species, age, size and value rather than the SRZ.
There are exceptions: for example when constructing decking near a tree, in this case the deck can be supported on piers, the soil is then protected from compaction and so long as there is no cover, the rain gets through, therefore decking close to a tree can be a good thing. However it's imperative that the piers do not damage structural roots and for this reason the SRZ should be calculated and included in the arborists planning report.

SRZ is a more useful tool when assessing a tree after root damage has occurred inadvertently, this may occur where a neighbour is constructing near the fence line and digs or trenches without considering the nearby tree. The SRZ calculation provides the arborist (and lawyer) a tool by which they can say "They have excavated within the calculated SRZ and therefore the tree is likely to fall over, it's now high risk because the trench is inside the Structural Root Zone (SRZ)"

Why does it cost more to include SRZ in an arborists report? - TPZ is based on a measurement usually at 1.4m whilst SRZ is calculated from a measurement just above buttress, each with a different calculation applied. Therefore collecting SRZ information adds time in the field for every tree and time in the office preparing tables and the report.
Additionally SRZ could be considered an issue because it adds just one more component to the arborists report when ideally arborist reports should be as simple and useful as possible, not full of self important padding.
Occasionally a Council will ask for Structural Root Zone to be included, clearly Coucil requirements need to be met however this requirement usually comes about through Council trying to cover all eventualities i.e. all report types, not because they actually want or need SRZ information for that specific planning report.

Standards Australia. 2009, AS 4970-2009 JProtection of Trees on Development Sites.

Standards Australia. 2007, AS 4373-2007 Pruning of Amenity Trees.

Lawrence, T., Norwquay, P. & Liffman, K.1993, Practical Tree Management: An Arborists Handbook, Inkata Press, Melbourne.

Matheny, N. and Clark, J. R. 1998, Trees and Development: A Technical Guide to Preservation of Trees During Land Development, International Society of Arboriculture, Champaign, USA.

Kenyon P. 2004, Senior lecturer, Institute of land and food resources, Burnley campus, Melbourne Victoria,

Arborist Melbourne

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