Tree Risk Assessment Resource

A practical guide to understanding how arborists evaluate tree risk using likelihood of failure, likelihood of impact, and potential consequences.

How Tree Risk Is Evaluated

Tree risk assessment is a structured process used to evaluate whether a tree or tree part may fail, whether it may impact a target, and what level of harm or damage could result.

1. Likelihood of Failure

How likely the tree or tree part is to fail within the assessment period.

  • Improbable
  • Possible
  • Probable
  • Imminent

2. Likelihood of Impact

How likely the failed tree or tree part is to strike a person, structure, vehicle, or other target.

  • Very Low
  • Low
  • Medium
  • High

3. Consequences

The severity of injury, property damage, or service disruption if failure occurs.

  • Negligible
  • Minor
  • Significant
  • Severe

Weather Context

Normal Weather Weather based on a location’s average temperature, wind speed, and precipitation.
Severe Weather Weather that has the potential to cause damage, such as strong winds, thunderstorms, or freezing rain.
Extreme Weather Historically infrequent weather with the potential to cause devastating damage.

Likelihood of Failure

Improbable Failure is not likely during normal or severe weather within the specified time frame.
Possible Failure is not likely during normal weather, but may occur during severe weather.
Probable Failure may occur during normal weather and is expected during severe weather.
Imminent Failure has begun or is expected soon with minimal additional loading.

Occupancy Rate

Rare The target is not commonly present within the target zone.
Occasional The target is present intermittently.
Frequent The target is present much of the time.
Constant The target is always or nearly always present.

Likelihood of Impact

Very Low The chance of the failed tree impacting the specified target is remote.
Low There is a slight chance that if the tree were to fail, it will impact the target.
Medium The failed tree could impact the target, but it is not expected to do so.
High The failed tree is likely to impact the target.
Very Low
Low
Medium
High

Occupancy Rate & Likelihood of Impact

Likelihood of impact is influenced by how often a target is present, how protected the target is, and whether the target is located in the anticipated direction of fall.

Target Protection
Less Protected More Protected
Target Location
In Direction of Fall Away From Direction of Fall
High
Medium
Low
Very Low
Constant
High
Medium
Low
Very Low
Frequent
Medium
Low
Very Low
Occasional
Low
Very Low
Rare
Very Low

Likelihood of Failure & Impact Matrix

The likelihood of failure is combined with likelihood of impact to estimate whether the failure scenario is unlikely, somewhat likely, likely, or very likely.

Likelihood of Failure Very Low Impact Low Impact Medium Impact High Impact
Imminent Unlikely Somewhat Likely Likely Very Likely
Probable Unlikely Unlikely Somewhat Likely Likely
Possible Unlikely Unlikely Unlikely Somewhat Likely
Improbable Unlikely Unlikely Unlikely Unlikely

Overall Tree Risk Rating

Overall risk is determined by combining likelihood of failure and impact with the expected consequences of failure.

Likelihood of Failure & Impact Negligible Minor Significant Severe
Very Likely Low Moderate High Extreme
Likely Low Moderate High High
Somewhat Likely Low Low Moderate Moderate
Unlikely Low Low Low Low

Consequences of Failure

People

Negligible: No personal injury.

Minor: Minor personal injury not requiring professional medical care.

Significant: Injury requiring professional medical care.

Severe: Serious personal injury or death.

Property

Negligible: Low-value property damage, such as cosmetic damage.

Minor: Low to moderate property damage requiring repair.

Significant: Substantial property damage or extensive structural damage.

Severe: High-value damage, major structural damage, or destruction.

Events & Services

Negligible: Short-term disruption of activities.

Minor: Some disruption, such as a short-term power outage.

Significant: Considerable disruption, such as medium-duration outages.

Severe: Major disruption of important activities, multi-day outages, or wildfire impacts.

Common Risk Mitigation Options

Recommendations depend on the specific tree, target, site conditions, and client objectives.

Monitor

Periodic reassessment may be appropriate when defects are present but risk remains acceptable.

Reduce Risk

Pruning, load reduction, target relocation, access management, or support systems may reduce risk.

Remove

Removal may be recommended when risk is elevated and cannot reasonably be reduced by other means.

Need a Professional Tree Risk Assessment?

A qualified consulting arborist can provide site-specific evaluation, documentation, and recommendations for managing tree-related risk.

Schedule a Consultation