Understanding Tree Risk Assessments

Every tree carries some level of risk. A perfectly healthy oak could lose a branch in a freak wind gust. A hollow old elm might stand peacefully for another 30 years. The question is never whether a tree could fail — it is whether the risk is acceptable given what surrounds it. That is precisely what a tree risk assessment answers.

What Is a Tree Risk Assessment?

A tree risk assessment is a systematic evaluation of a tree's likelihood of failure and the consequences if it does fail. It is performed by a certified arborist, ideally one who holds the International Society of Arboriculture's Tree Risk Assessment Qualification (TRAQ). The process follows standardized methods developed by the ISA and the American National Standards Institute (ANSI A300 standards).

The assessment evaluates three components that together determine the overall risk rating:

• Likelihood of failure: How likely is the tree — or a part of it — to break or uproot? This considers the tree's structural condition, species characteristics, and site factors.
• Likelihood of impact: If the tree or branch fails, will it hit something valuable? A tree leaning over a playground has a higher likelihood of impact than one leaning over an empty field.
• Consequences of failure: What would the damage be? A branch falling on a house or a person has severe consequences. A branch falling on open lawn has minimal consequences.

These three factors are combined into an overall risk rating — typically Low, Moderate, High, or Extreme — that guides the recommended action.

What the Arborist Examines

During a Level 2 assessment (the most common type for residential properties), the arborist walks around the tree and performs a detailed visual inspection. They evaluate:

• Root zone: Signs of root decay, soil heaving, severed roots, grade changes, or compaction that could compromise the tree's anchorage.
• Trunk: Cracks, cavities, cankers, fungal fruiting bodies, included bark, and lean. The arborist may use a mallet to sound for internal decay.
• Scaffold branches: Co-dominant stems, weak attachments, deadwood, cracks, and signs of past failure.
• Crown: Overall vigour, leaf density, dieback patterns, and any signs of disease or pest damage.
• Site conditions: Exposure to wind, proximity to structures and high-use areas, soil drainage, and recent construction activity that may have damaged roots.

When You Need One

Several situations call for a professional risk assessment:

• Large trees near structures: Any mature tree with a canopy that overhangs your house, garage, or deck should be assessed periodically — every three to five years for healthy trees, more frequently for trees showing signs of decline.
• After storms: A tree that survived a storm may have suffered hidden damage — cracked branches, shifted roots, or trunk fractures — that makes it more likely to fail in the next event.
• Before construction: If you are planning an addition, new driveway, or any excavation near a mature tree, an assessment determines whether the project can proceed without compromising the tree, and what protections are needed.
• Real estate transactions: Buyers sometimes request a tree assessment when a property has large, mature trees near the home. Sellers can proactively order one to address concerns before listing.
• Insurance or legal disputes: If a neighbour's tree is threatening your property, or if an insurer is questioning a claim, a written risk assessment from a qualified arborist provides documented evidence.

What Happens After the Assessment

The arborist provides a written report that includes their findings, the risk rating for each tree assessed, and recommended mitigation measures. Recommendations might include:

• Pruning: Removing deadwood, reducing weight on overextended limbs, or improving branch structure.
• Cabling and bracing: Installing support hardware to reduce the risk of failure in trees with structural defects that are otherwise healthy and worth preserving.
• Monitoring: For trees with moderate risk, the recommendation may be to reassess at a specified interval rather than take immediate action.
• Removal: For trees rated High or Extreme risk with no practical mitigation options, removal is the responsible recommendation.

Risk Assessment vs. a Quick Look

There is a difference between a certified arborist performing a documented risk assessment and someone giving a tree a quick look. The assessment follows a standardized methodology, produces a written report, and is backed by the arborist's professional qualifications. This documentation matters if you ever need to demonstrate due diligence to an insurer, a municipality, or in a legal proceeding.

A verbal opinion from a tree service worker may be well-intentioned, but it does not carry the same weight and may not consider all the factors that a formal assessment addresses.

Peace of Mind

Most trees that are assessed turn out to be perfectly safe or require only minor maintenance. The value of the assessment is not in finding problems — it is in knowing the state of your trees and having a plan. That knowledge lets you enjoy your yard with confidence, knowing that the large elm shading your patio or the spruce beside your driveway has been evaluated by someone who understands exactly what to look for.