A tree risk assessment sets out how likely a tree, or part of a tree, is to fail, how likely it is to strike a target if it does, and how serious that would be. It replaces impression with a repeatable method, so the risk of harm is rated, recorded and defensible rather than a matter of opinion.
Why it exists and when we use it
The statistical risk of harm from tree failure is low, but it is often overestimated. A formal assessment gives a landowner, Council or asset manager a rated, documented basis for deciding what, if anything, to do, and when to look again. We assess and communicate the risk; the risk owner decides which controls to put in place. We are a consulting-only firm and do not carry out or sell the tree work we recommend, so the assessment is not written to justify a job.
What it measures
Risk combines the likelihood of failure, the likelihood of impact, which already accounts for how often a target is present, and the consequence if it occurs. Methods differ in how they express those terms: multiplied as probabilities (quantitative), read off a matrix of descriptive categories (qualitative), or scored on ranked scales and combined (comparative). The result is placed against tolerability thresholds following the ALARP principle, broadly acceptable, tolerable, or unacceptable, with the residual risk stated where controls are recommended.
What it is built on
MIS501, the Minimum Industry Standard for tree risk assessment, aligned to ISO 31000:2018, which sets the components every method must address. Four methods are in current use in Australia and New Zealand: QTRA, TRAQ, VALID and AANZAA-Risk/24. We work in the methods we are trained and current in, QTRA and ISA TRAQ, and every report names the method used, the level of assessment, the assessment date and the validity period, and records the inputs so the result can be reproduced.
Want more detail? Get in touch to discuss your project and how we can help.