Approved Plans and Documents - OPW22/0303

                                                                                  Consult Arborist

7.2 CALCULATING TREE PROTECTION
The descriptions of the Tree Protection Zone (TPZ) and Structural Root Zone (SRZ) are from
the Australian Standards, AS 4970-2009 Protection of trees on development sites. These
descriptions provide the basic prerequisites to calculate the TPZ and SRZ, and the
compensates for when encroachment or a reduction in the size/area of the TPZ is required to
facilitate a development: see Appendix D of the Australian Standard AS 4970-2009.

Many factors need to be taken into consideration when estimating tree root growth and spread.

7.2.1 TREE PROTECTION ZONE (TPZ)
As specified in the Australian Standard, AS 4970-2009, the TPZ is the principal means of
protecting trees on a development site. The TPZ calculates the radius of a circular area
combining the root and crown spread of a tree for protection and isolation from construction
disturbance so the tree can remain in a viable condition for the future. The TPZ incorporates
the SRZ.

DBH = Diameter at Breast Height - trunk diameter measured at 1.4m above ground

TPZ RADIUS = Twelve times the DBH

Example: .5m DBH x 12 = 6m TPZ RADIUS.

A TPZ should not be less than 2m or greater than 15m unless additional crown protection is
required.

7.2.2 STRUCTURAL ROOT ZONE (SRZ)
The SRZ, as specified in AS 4970-2009, is the minimum surface area of protection required
around a tree to maintain that tree’s stability – a larger area than this is required to maintain a
tree in a viable condition for the future. It is only necessary to calculate the SRZ when major
encroachment into a TPZ is proposed.

An indicative SRZ RADIUS can be determined from the lower trunk diameter (above the root
buttress), D, measured in metres and applying the following formula:
                                                      0.42
                                    RSRZ = (D x 50)          x 0.64

The SRZ RADIUS for trees with a trunk diameter less than 0.15m will automatically be 1.5m.


 Please note that these are theoretical calculations and are to be used as a guide. Large tree
 species, particularly Fig trees, will have woody structural and feeder roots that extend to a
 greater distance than the calculations. Trees within urban environments where solid
 structures restrict root growth can also alter a tree’s root spread and direction. An additional
 water/food source would also encourage roots to grow further than expected.




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