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:

1. Likelihood of failure: probability the tree (or part) will fail in the assessment period
2. Likelihood of impact: probability of striking a target if failure occurs
3. Consequences of impact: severity if struck

Risk Rating = Likelihood × Consequences

The assessor evaluates each factor and combines them using a standardized matrix.

Assessment levels

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:

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:

Each defect contributes to failure likelihood.

Step 3: Evaluate likelihood of failure

The TRAQ scale:

Combine all defects to assess overall likelihood. A tree with multiple severe defects = higher likelihood.

Step 4: Evaluate likelihood of impact

The TRAQ scale:

Note the top of this scale is High, not Very High. TRAQ uses four levels here and "Very high" is not one of them; getting the label right matters because the next step is a published matrix keyed to these exact terms.

A house, or a driveway parked in every night, is High. A patio used most evenings is Medium. A back-yard storage shed or a path walked a few times a month is Low. A far corner of the property nobody visits is Very low.

Step 5: Evaluate consequences

The TRAQ scale:

Combine likelihood and consequences for risk rating.

Risk rating matrix

TRAQ combines the three factors in two steps, not one, and knowing that is what lets you reproduce a rating instead of memorising rows.

Step one: combine likelihood of failure with likelihood of impact to get a single likelihood of failure and impact, on its own four-level scale: Unlikely, Somewhat likely, Likely, Very likely.

Step two: cross that combined likelihood against consequences to get the risk rating: Low, Moderate, High, Extreme.

The rows below are worked examples of that two-step process, not a separate matrix:

The rating determines the recommendation.

Step 6: Make a recommendation

Based on the risk rating:

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:

1. Tree identification (species, age estimate)
2. Photographic documentation of defects
3. Detailed visual inspection notes
4. Target identification
5. Risk rating with rationale
6. Recommendation
7. Customer signoff acknowledging the assessment
8. 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 (Resistograph) or sonic tomography, two separate instruments for detecting 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:

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:

• Put the assessment in writing and have them acknowledge it. State the defect, the likely failure, the target, and your recommendation. A signed acknowledgment is not about winning an argument later; it is about making sure the decision was actually informed
• Move the target instead of the tree, where that is possible. Relocate the parking spot, the play structure, the patio furniture, the walking path. Risk is defect plus target, and the target is often the cheaper half to change
• Offer partial mitigation and be honest about what it buys. Crown reduction to cut the lever arm, deadwood removal, supplemental support on a weak union. Say plainly that these reduce risk and do not eliminate it
• Set a monitoring schedule with a date, not an intention. Annual, or after any significant storm, and put it in the system so it actually happens
• Define the tripwires. Tell them exactly what would change the recommendation to removal: new cracking, movement at the root plate, fresh fruiting bodies, a limb failure, a soil heave
• Restrict use during high-risk periods. Keeping people out from under it during storms and high wind is free and effective
• Know your own line. If the tree is so far gone that you would not stand under it, say so and decline the maintenance work rather than take money to prop up a decision you cannot support

The customer owns the tree and the decision. You own the honesty of the assessment and the quality of the record. Document what you recommended, what they chose, and the date.

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