Free calculator Tree risk matrix: likelihood and consequences
The two matrices of the ANSI A300 Part 9 tree risk assessment method. Pick the likelihood of failure, the likelihood of impact and the consequences to get the risk rating.
The two matrices
Matrix 1 combines the likelihood of failure (rows) with the likelihood of impacting the target (columns).
| Failure \ impact | Very low | Low | Medium | High |
|---|---|---|---|---|
| Improbable | Unlikely | Unlikely | Unlikely | Unlikely |
| Possible | Unlikely | Unlikely | Unlikely | Somewhat likely |
| Probable | Unlikely | Unlikely | Somewhat likely | Likely |
| Imminent | Unlikely | Somewhat likely | Likely | Very likely |
Matrix 2 combines that result (rows) with the consequences of the impact (columns) into the risk rating.
| Likelihood \ consequences | Negligible | Minor | Significant | Severe |
|---|---|---|---|---|
| Unlikely | Low | Low | Low | Low |
| Somewhat likely | Low | Low | Moderate | Moderate |
| Likely | Low | Moderate | High | High |
| Very likely | Low | Moderate | High | Extreme |
How an assessment comes together
- Choose the level: limited visual, basic or advanced.
- Identify the targets and where they sit: within the drip line, within one tree height or within one and a half.
- Note the site factors and the tree's health and condition: crown, trunk and roots.
- For each part that could fail, rate the likelihood of failure and of impact, then the consequences, and read the rating off the matrices.
- Record the time frame, the limitations, the mitigation options and when to reassess.
Questions
What is a tree risk matrix?
Two small tables used in the tree risk assessment method of ANSI A300 Part 9. The first combines the likelihood that a part fails with the likelihood it hits a target. The second combines that result with the consequences of the impact, giving a low, moderate, high or extreme risk rating.
How is a tree's overall rating decided?
Each tree part and failure mode you assess (a branch, the trunk, the roots) gets its own rating against its target. The tree's rating is the highest of them.
What does the likelihood of impact depend on?
Mainly on the target: how often people or property are within reach (rare, occasional, frequent or constant occupancy), whether the target is within the drip line or within one or one and a half tree heights, and whether anything protects it.
What are the levels of tree risk assessment?
Limited visual (level 1), basic (level 2) and advanced (level 3). A basic assessment is a detailed look from the ground with simple tools; an advanced one adds specialized tests or climbing when the basic assessment calls for it.
Does a low rating mean the tree is safe?
No. A rating describes risk over the time frame you assessed, based on what could be seen and tested. Record the time frame and any limitations, and set a reassessment date.
Based on the tree risk assessment method of ANSI A300 Part 9 and the ISA best management practice; the wording here is our own. Not affiliated with ANSI, TCIA or ISA. The rating supports the assessor's judgment; it does not replace it.