Arborist Risk Assessment


Tree owners and managers have a duty of care to ensure that trees are managed so that they have an acceptable level of risk associated with them. This is especially important in public areas, schools and other high use locations.

Arborists Approach to Calculating Risk from Trees

Assessing risk from trees is always difficult, yet we as professional arborists have a responsibility to do this to the best of our individual expertise. Peter Gray has trained extensively in tree risk assessment with both QTRA’s Quantified Tree Risk Assessment and ISA’s TRAQ.

Can an Arborist Tell when a Tree will Fail?

Predicting tree failure is very uncertain, so instead we seek to manage the risk from tree failure by determining a risk of harm. This risk is calculated as a probability with QTRA and as a value with TRAQ. Thus QTRA has a benchmark of 1/1/000,000 where risks lower than this are perceived to be broadly acceptable. TRAQ gives a “low” value for acceptable risk.

The benefits from risk reduction are obvious. What is less obvious is the cost of carrying out the work to reduce the risk such as the cost of doing the work and the loss of the tree amenity. It is important to take the loss of benefits into account when determining whether tree risk reduction needs to be carried out.