Decisions
How tree risk assessments work
Updated · Guide by TreeRemovalServices.net
Short answer
A tree risk assessment is an arborist's structured evaluation of how likely a tree is to fail, what it would hit, and how bad that would be. Most assessments are visual from the ground; advanced ones add tools like resistance drilling or sonic tomography. The result is a risk rating and recommendations: prune, monitor, cable or remove.
Three levels of assessment
Arborists match the depth of the assessment to the question being asked.
Limited visual
- What it involves
- A quick look, often from a distance, to find obvious defects in many trees
Basic
- What it involves
- A detailed ground-level inspection of one tree and its surroundings
Advanced
- What it involves
- Extra tools: resistance drilling, sonic tomography, root excavation or an aerial inspection
What the arborist looks at
A basic assessment works from the ground up and from the targets in:
- Targets: what's within falling distance and how often people are there.
- Roots and root flare: decay, girdling roots, soil heave, construction damage.
- Trunk: cracks, cavities, lean, fungus.
- Crown: dead limbs, weak forks, overextended limbs, previous failures.
- Site: soil, drainage, wind exposure, recent changes.
Likelihood, consequences, rating
The arborist rates how likely the tree or part is to fail and hit a target, and how serious the consequences would be, then combines them into a risk rating, commonly low, moderate, high or extreme. Recommendations follow from the rating: a high-risk limb over a bedroom might be pruned, while a high-risk tree with root decay might be removed.
When you need one in writing
Several places tie permits to written assessments. In Florida, a single-family homeowner with documentation from an ISA-certified arborist or licensed landscape architect, using ISA risk assessment procedures, can remove a tree posing unacceptable risk without a local permit; see Florida storm damage tree removal. Seattle's hazardous-tree process and many protected-tree ordinances also rely on qualified assessments.
Choosing an assessor
Look for an ISA Certified Arborist, ideally with the ISA Tree Risk Assessment Qualification (TRAQ). An assessor who isn't selling the removal gives the most objective answer. Ask what level of assessment you're getting, what the report will include, and the fee up front.
Common questions
What is TRAQ?
The ISA Tree Risk Assessment Qualification, a credential for arborists trained in the ISA's risk assessment method.
Is a tree company's quote the same as a risk assessment?
No. A quote prices a job; a risk assessment evaluates the tree's likelihood of failure and recommends options, ideally from someone not selling the removal.
Can an arborist see decay inside a tree?
With advanced tools, yes. Resistance drilling and sonic tomography estimate how much sound wood remains.
Editorial note: this guide is general information written for homeowners. It isn't a substitute for an on-site assessment by a qualified arborist, or for legal or insurance advice.