Tree Risk Assessment (ISA TRAQ Framework)
Why this matters
Customers frequently ask "is my tree going to fall?" - and the answer requires systematic risk assessment, not casual visual inspection. The International Society of Arboriculture (ISA) developed the Tree Risk Assessment Qualification (TRAQ) framework as the professional standard for this assessment. Arborists who hold the TRAQ credential perform documented risk assessments that customers, insurers, and courts recognize. This reference covers the framework for technicians and customer education.
The TRAQ framework
Tree risk is the combination of three factors:
- Likelihood of failure: probability the tree (or part) will fail in the assessment period
- Likelihood of impact: probability of striking a target if failure occurs
- Consequences of impact: severity if struck
Risk Rating = Likelihood × Consequences
The assessor evaluates each factor and combines them using a standardized matrix.
Assessment levels
| Level | Detail |
|---|---|
| Level 1 | Limited visual inspection (drive-by, walk-around) |
| Level 2 | Basic assessment (visual + simple tools) |
| Level 3 | Advanced (instruments, lab tests, detailed analysis) |
Most residential tree service performs Level 2 assessments. Level 3 is for high-value trees or pre-litigation situations.
Step 1: Identify target areas
The "target" is what might be struck if the tree fails:
| Target type | Examples |
|---|---|
| Permanent occupied structures | House, garage, occupied building |
| Permanent unoccupied structures | Shed, fence, storage |
| Occupied vehicles | Driveways, streets near the tree |
| Occupied recreation areas | Pool deck, patio, play area |
| Occupied pathways | Sidewalks, walking paths |
| Other trees / landscape | Less critical |
The higher-value targets drive the risk priority. A tree over a house has higher target value than a tree over an open field.
Step 2: Identify defects
Visual indicators of tree defects:
| Defect | What to look for |
|---|---|
| Decay | Visible rot, mushrooms, conks, hollow sound when tapped |
| Codominant stems | Two or more main trunks of similar size; risk of split |
| Included bark | Bark trapped between two stems; common splitting point |
| Cracks | Vertical cracks in trunk or major branches |
| Cavities | Visible holes or hollows in the trunk |
| Lean | Significant lean, especially recent or progressive |
| Root damage | Construction damage, soil disturbance |
| Root rot | Fungal growth, mushrooms at the base |
| Dead branches | Significant deadwood, especially over targets |
| Cracks in scaffold branches | Major limb attachment points |
| Storm damage | Recent breakage, hanging branches |
Each defect contributes to failure likelihood.
Step 3: Evaluate likelihood of failure
The TRAQ scale:
| Likelihood | Definition |
|---|---|
| Imminent | Failure expected within the assessment period (next year typical) |
| Probable | Failure may occur during normal weather |
| Possible | Failure may occur during extreme weather |
| Improbable | Failure highly unlikely during the assessment period |
Combine all defects to assess overall likelihood. A tree with multiple severe defects = higher likelihood.
Step 4: Evaluate likelihood of impact
The TRAQ scale:
| Likelihood | Definition |
|---|---|
| Very High | Target is essentially always present |
| High | Target is typically present (occupied area) |
| Medium | Target is sometimes present (occasional use) |
| Low | Target is rarely present |
A house is "very high" target presence; a back-yard storage shed is "high"; a wooded area path is "medium."
Step 5: Evaluate consequences
The TRAQ scale:
| Consequences | Definition |
|---|---|
| Severe | Significant property damage or personal injury / death |
| Significant | Moderate property damage or possible injury |
| Minor | Limited damage; unlikely injury |
| Negligible | Little or no consequence |
Combine likelihood and consequences for risk rating.
Risk rating matrix
The TRAQ combines the three factors:
| Likelihood of failure | Likelihood of impact | Consequences | Risk rating |
|---|---|---|---|
| Imminent | Very high | Severe | Extreme |
| Imminent | High | Significant | High |
| Probable | High | Severe | High |
| Probable | Medium | Significant | Moderate |
| Possible | High | Severe | Moderate |
| Possible | Medium | Minor | Low |
| Improbable | Any | Any | Low |
The rating determines the recommendation.
Step 6: Make a recommendation
Based on the risk rating:
| Rating | Recommendation |
|---|---|
| Extreme | Immediate action (remove or restrict access) |
| High | Action within weeks (remove, brace, or restrict access) |
| Moderate | Action within months (monitor closely; treat if mitigable) |
| Low | Routine monitoring; treat at convenience |
The customer's input matters in the recommendation:
- They may accept higher risk in exchange for keeping a tree
- They may prefer removal even at lower risk
- The recommendation is a professional opinion; the customer's choice
Documentation
For each risk assessment:
- Tree identification (species, age estimate)
- Photographic documentation of defects
- Detailed visual inspection notes
- Target identification
- Risk rating with rationale
- Recommendation
- Customer signoff acknowledging the assessment
- Date and assessor credentials
This documentation is critical for:
- Liability protection
- Insurance claims if failure occurs
- Customer's records
- Court proceedings (some cases)
When to refer to Level 3
For high-value trees:
- Significant residential value (large tree near house)
- Historic or specimen trees
- Trees with multiple uncertain defects
- Pre-litigation or insurance dispute
Level 3 may include:
- Resistance drilling (sonic tomography to detect internal decay)
- Pulling tests for stability
- Soil moisture analysis
- Detailed root crown excavation
- Microscopic examination of decay samples
Specialty consulting arborists perform Level 3 work.
Mitigation options
For trees with identified defects:
| Defect | Mitigation |
|---|---|
| Codominant stems | Cable or brace; thin crown |
| Major decay | Often removal; thinning may extend life |
| Lean (recent) | Removal or major bracing |
| Lean (long-standing without progression) | Monitor |
| Storm damage | Repair if salvageable; remove if structurally compromised |
| Dead branches over targets | Remove the branches |
| Root damage | Soil treatment, watering; severe damage = removal |
| Tree-of-heritage on the property | Investment in maintenance + monitoring |
Mitigation extends the tree's life but doesn't eliminate risk. Continued monitoring is required.
When the customer wants to keep a high-risk tree
If the customer values the tree more than the risk:
References
- ISA Tree Risk Assessment Qualification (TRAQ) training materials
- ANSI A300 (Part 9) - Tree Risk Assessment
- ISA Best Management Practices: Tree Risk Assessment
- Manuall internal: Declining Tree Diagnosis, Chainsaw Safety, Tree Pruning Service SOP