Tree Protection Zone (TPZ) Encroachment Calculator


How to use the Tree Protection Zone (TPZ) encroachment calculator

Drag the radius red dot (R) to set the size of the green TPZ circle, then drag the 'buildings' over the TPZ to calculate the percentage of encroachment. The two numbers displayed within each building are the percentage of encroachment into the TPZ and the area encroached. You can change the shape of each building by dragging the red dots.

Drag and extend the blue ruler to set or measure distances.

If you're working with a small TPZ you can zoom in on your web browser screen by holding Ctrl and pressing + or scrolling your mouse, once zoomed in you can more precisely set the TPZ size and building locations.  

Structural Root Zone Calculator (SRZ)

The Structural Root Zone or Critical Root Zone (CRZ) is an area (radius) around a tree trunk that must be protected to ensure stability of the tree in the ground. If larger roots within this area are damaged it is highly likely the tree's structure will be compromised, possibly causing whole tree failure.  The SRZ primarily relates to structure as opposed to tree health, to protect tree health The TPZ should be adhered to.

In the Australian Standard 4970-2009 the SRZ is calculated based on a measurement of trunk calliper taken just above the root buttress, this ProofDocs calculator assumes SRZ calliper as: the measured Diameter at Breast Height (DBH) plus 10%.

Building footprints

When both buildings are used, and they overlap, then that area of overlap that sits within the TPZ is calculated and displayed.

A calculation of overlapping buildings is useful when a proposed development sits partially or fully within the footprint of a pre-existing structure on site. There are many factors that contribute to the impacts of development on trees such as building methods, tree species, soils, water catchment area and tree age however if an existing structure near a tree, predated that tree, then it is likely the distribution or density of roots below that structure will be less than the roots in nearby areas such as open lawns and garden beds. In a situation where a proposed footprint overlaps that of a pre-existing building, a major encroachment may not significantly damage the tree.

Tree Protection Zone and encroachment calculations provide an excellent guide for working near trees however root distribution and density is rarely uniform and therefore non destructive root mapping may provide valuable insights around likely impacts. The best approach if at all possible is to keep development well away from trees - outside the TPZ. The less incursion and less disruption to normal tree function (above and below ground) the happier trees will be and the more likely development plans will be passed at council.

Reference: Standards Australia (2009), AS 4970-2009 Protection of trees on development sites