TPZ and SRZ Calculator – AS 4970-2009

Extract from Australian Standard AS4970-2009 (Protection of trees on development sites)

Determining the tree protection zones of the selected trees

Tree protection zone (TPZ)

"The tree protection zone (TPZ) is the principal means of protecting trees on development sites. The TPZ is a combination of the root area and crown area requiring protection. It is an area isolated from construction disturbance, so that the tree remains viable.

The TPZ incorporates the structural root zone (SRZ) (refer to Clause 3.3.5)."

Determining the TPZ:

The radius of the TPZ is calculated for each tree by multiplying its DBH x 12

Where TPZ = DBH x 12

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

Radius is measured from the centre of the stem at ground level.

Structural root zone (SRZ)

"The SRZ is the area required for tree stability. A larger area is required to maintain a viable tree. The SRZ only needs to be calculated when a major encroachment into a TPZ is proposed. Root investigation may provide more information on the extent of these roots."

Determining the SRZ:

SRZ radius = (DRC x 50) 0.42 x 0.64


DRC = trunk diameter, in metres, measured above root crown (DRC = Diameter Above Root Crown)
Radius is measured from the centre of the stem at ground level.

Note: The SRZ for trees with trunk diameters (DRC) less than 0.15 m will be 1.5 m.

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