A tree risk assessment answers a simple question: how likely is this tree to cause harm, and is that risk reasonable? Owners, strata committees, schools, businesses and councils use them to make decisions they can defend.
When you might need one
- A large tree overhangs a house, driveway, play area or public path
- A tree has a defect such as a crack, cavity, decay or a changing lean
- An insurer, strata committee or property manager wants an independent opinion
- Council needs evidence to support a pruning or removal application
- You manage a site with many trees and want to plan inspections and works
How QTRA works
Quantified Tree Risk Assessment (QTRA) looks at three things: the target (people or property that could be hit), the size of the part that could fail, and the probability of that part failing within the next year. Combining them gives an annual Risk of Harm, expressed as a probability such as 1 in 100,000.
QTRA's advisory thresholds help owners decide what to do:
- 1 in 1,000 or higher is unacceptable and will not ordinarily be tolerated
- Between 1 in 1,000 and 1 in 10,000 is unacceptable where the risk is imposed on others, though it may be tolerated by agreement
- Between 1 in 10,000 and 1 in 1,000,000 is tolerable if the risk is as low as reasonably practicable, so risk control is weighed against cost and the tree's benefits
- Below 1 in 1,000,000 is broadly acceptable and needs no action other than review
How VALID works
VALID is a tree risk-benefit method. It assesses the likelihood of failure and the consequences, and it also weighs the benefits a tree provides, such as shade, habitat and amenity. That helps owners avoid removing valuable trees when a smaller intervention would make the risk acceptable.
What you get
After the site inspection you receive a written assessment that identifies each tree, describes its condition and defects, states the risk, and recommends what to do and how soon. Recommendations often favour pruning or monitoring over removal when that brings the risk down enough.
Who carries it out
Marcus Nicholson is an AQF Level 5 consulting arborist, a registered QTRA user (#6912) and a VALID Tree Risk-Benefit Assessor. Ask about a tree risk assessment or read more about our arborist reports.