A tree risk assessment can feel a bit like reading a medical chart: there are observations, measurements, and a final “what this means for you” summary. If you’ve ever had an arborist walk your property, point up into the canopy, tap a trunk, and jot down notes you can’t quite decode, you’re not alone. The good news is that most professional assessments follow a consistent logic—once you know what to look for, the report becomes much easier to understand.
This guide breaks down how arborists evaluate tree risk, what common terms mean, and how to connect the dots between a defect (like a cavity or dead limbs) and an actual risk decision (like pruning, cabling, monitoring, or removal). Along the way, you’ll also learn what you can check yourself between professional visits—without trying to play arborist or taking unsafe shortcuts.
Since you’re reading this on trudeaumetre.ca, we’ll keep it practical and property-owner friendly. Think of this as a “translator” for the next time you receive a risk assessment or want to ask better questions during an on-site visit.
What a tree risk assessment is really trying to answer
At its core, a tree risk assessment is about probability and consequences. Arborists aren’t trying to predict the future with certainty; they’re estimating how likely a tree (or part of a tree) is to fail, and what it might hit if it does. That’s why two trees with the same defect can get different recommendations—because the targets and site conditions are different.
Most assessments revolve around three big questions: (1) What could fail? (2) How likely is failure within a given time frame? (3) If it fails, what will it impact—people, buildings, vehicles, utilities, or nothing at all? The answers to those questions turn observations into a risk rating and a management plan.
It’s also worth knowing that “risk” is not the same as “health.” A tree can be unhealthy but low risk (for example, a declining tree in the back of a large lot away from anything valuable). And a tree can look vigorous but still be higher risk (for example, a healthy-looking tree with a hidden crack over a driveway).
How arborists structure their observations (and why it matters)
Professional arborists typically observe from the top down and the outside in: canopy, branches, trunk, root flare, and surrounding site. They’re looking for defects, symptoms, and context. Defects are structural issues (cracks, cavities, weak unions). Symptoms are indicators (dieback, fungal fruiting bodies). Context includes soil, grade changes, exposure to wind, and proximity to targets.
Many arborists use established frameworks such as ISA Tree Risk Assessment methods. Even if the report doesn’t name the framework explicitly, you’ll see the same components: tree identification, site notes, defects, likelihood of failure, likelihood of impact, consequences, overall risk rating, and recommended mitigation.
When you read the report, try to keep that structure in mind. If a recommendation feels surprising, trace it back: What defect did they note? What target did they identify? What time frame did they assume? Often the logic is there—just not written in plain language.
Tree ID details: species, size, and why arborists care
Species isn’t trivia—it changes the whole risk picture
Species affects wood strength, decay patterns, typical failure modes, and how well a tree compartmentalizes wounds. Some species tolerate cavities better than others. Some are more prone to brittle failure, while others bend and shed small branches instead of snapping major leaders.
In a report, you might see comments like “species known for weak branch unions” or “decay progresses rapidly in this species.” Those aren’t stereotypes—they’re practical shorthand based on how different trees behave under load and over time.
If you don’t recognize the species name, ask. Knowing whether you’re dealing with a silver maple versus an oak, for example, can help you understand why an arborist is more concerned about a certain defect.
DBH, height, crown spread: size changes the stakes
DBH (diameter at breast height) is one of the most common measurements in a risk assessment. Bigger diameter often means more stored energy if something fails, and it can also mean older trees with more history—old pruning wounds, past storm damage, or long-term decay.
Height and crown spread matter because they influence leverage and target zone. A tall tree leaning toward a home has a different risk profile than the same tree leaning toward an open lawn. Crown spread can also reveal imbalance—heavy lateral growth on one side can increase loading in wind.
Don’t get hung up on exact numbers; focus on what the arborist is implying. If the report highlights size, it’s usually because size amplifies consequences and affects what mitigation is feasible.
The canopy check: what the leaves and twigs reveal
Dieback, thinning, and “sparse foliage” aren’t always immediate danger—but they’re clues
Canopy condition tells a story about energy reserves and stress. Thinning foliage, smaller leaves, or branch tip dieback can indicate root issues, drought stress, soil compaction, or disease. On their own, these symptoms don’t always mean “remove the tree,” but they can signal reduced vigor, which affects how well a tree can respond to defects and wounds.
Arborists often describe canopy condition in percentages (e.g., “20% dieback”). That helps track change over time. A stable 10–15% dieback might be manageable with monitoring, while a rapid jump can indicate a more serious decline.
If you see canopy symptoms noted, look for the “why” in the report—sometimes it’s a site issue that can be improved, like mulching, reducing compaction, or adjusting irrigation patterns.
Deadwood: the most common “easy win” in mitigation
Dead branches are frequent findings because they’re straightforward: deadwood is more likely to fail, and removing it reduces risk quickly. Reports may note deadwood size classes (e.g., “deadwood up to 4 inches diameter”). That detail matters because larger dead limbs can cause more damage.
Deadwood also helps arborists infer other problems. For example, deadwood concentrated in the interior might be normal shading-out, while deadwood at the tips could point to stress. Deadwood in one leader might indicate a localized crack, girdling root, or vascular issue.
If the recommendation is deadwood removal, ask about timing and scope. Some deadwood can wait; some should be addressed sooner—especially over play areas, patios, or driveways.
Branch architecture: where failures often begin
Weak unions and included bark: a small line that can mean a big problem
One of the most important structural notes in a risk assessment is branch union quality. A strong union has a visible branch collar and good wood-to-wood connection. A weak union often has included bark—bark trapped between stems—creating a seam that can split under load.
Reports might mention “codominant stems” (two leaders competing) or “V-shaped crotch.” Those are classic setups for included bark. The risk isn’t always immediate, but as stems thicken, pressure builds along that seam, especially during wind, ice, or heavy foliage seasons.
Mitigation can range from selective reduction pruning to installing support systems (like cabling/bracing) or, in some cases, removing one of the competing stems if it can be done without destabilizing the tree.
End-weight and overextended limbs: leverage is the hidden force
Arborists pay close attention to long, horizontal limbs with lots of foliage at the ends. That “end-weight” increases bending forces at the union and along the limb—especially when wind catches the canopy like a sail.
In assessments, you may see terms like “overextended,” “lever arm,” or “lion-tailed.” Lion-tailing is when inner branches are removed excessively, leaving foliage only at the tips. It looks tidy to some people, but it can actually increase end-weight and reduce the limb’s ability to dampen movement.
Good pruning aims to reduce risk without creating new stress. That’s why arborists often recommend targeted reduction cuts rather than aggressive thinning.
Trunk inspection: cracks, cavities, and what “sound wood” means
Cracks and seams: not all are equal, but all deserve attention
Cracks can be superficial (in the bark) or structural (in the wood). Arborists look for patterns: a single vertical crack might be from past frost or impact, while a crack that opens and closes, or extends through a union, can indicate active movement and higher failure potential.
Reports may describe “shear crack,” “radial crack,” or “seam.” A seam is often associated with old damage or included bark. The key is whether the crack suggests the tree is already failing under normal loads or only under extreme events.
If cracking is mentioned, the report should also reference targets and recommended actions—like pruning to reduce loading, installing support, or scheduling removal if the defect is severe and the target is high-value.
Cavities and decay: the question is how much sound wood remains
A cavity isn’t automatically a death sentence. Arborists evaluate the extent of decay and the thickness of remaining sound wood. You might see notes about “hollow,” “decay column,” or “conk present.” Fungal fruiting bodies (conks) can indicate internal decay even when the outside looks fine.
Some arborists use tools like a mallet (listening for changes in sound), a resistograph, or tomography for deeper analysis. Even without advanced tools, experienced arborists can often estimate risk by combining visible indicators: cavity opening size, location, associated swelling, and the tree’s overall vigor.
If the report mentions decay, look for specifics: where it is (at the base versus higher up), whether it affects a main leader, and whether there are compensating growth patterns (like ribs of wood forming around an old wound).
The base and root flare: where stability is decided
Root collar issues: buried flares and girdling roots
The root flare (where trunk transitions into roots) should be visible on most mature trees. When it’s buried under soil or mulch, moisture can accumulate against the trunk, increasing the risk of decay and encouraging girdling roots—roots that circle and compress the trunk.
In a risk assessment, you might see “root flare buried,” “possible girdling roots,” or “basal swelling.” These notes matter because many tree failures start at the base, and root problems can be hard to detect until they’re advanced.
Sometimes the fix is surprisingly straightforward: careful excavation to expose the flare, correcting mulch practices, and addressing girdling roots when appropriate. Other times, the damage is already structural, and the recommendation shifts toward risk reduction or removal.
Soil, drainage, and construction impacts: the slow-motion risk factors
Trees don’t love sudden change. Grade changes, trenching, new patios, or repeated vehicle traffic can damage roots and compact soil, reducing oxygen and water infiltration. An arborist will often ask about recent construction—even if the tree looked fine last year.
Reports might mention “compaction,” “poor drainage,” or “root zone disturbance.” These conditions can lead to decline, which then increases susceptibility to pests, disease, and structural failure during storms.
If you’ve had work done near a tree, share that history. It helps the arborist interpret symptoms accurately and recommend realistic steps—like aeration, mulching, or adjusting site use to protect remaining roots.
Targets and occupancy: why location changes everything
Risk is a combination of likelihood and consequence, and “targets” are the consequence side of the equation. A target is anything that could be hit: a house, a fence, a power line, a parked car, a neighbor’s shed, or a spot where kids play every day.
Occupancy matters too. A limb over a rarely used corner of the yard may be less urgent than the same limb over a front walkway. Arborists consider how often the target is present—sometimes described as “high,” “medium,” or “low” use areas.
If you’re reading a report and wondering why one tree is flagged more urgently than another, check the target notes. That’s often the deciding factor.
Likelihood of failure vs. likelihood of impact: two different ratings
Failure likelihood: what “probable” and “possible” usually mean
Likelihood of failure is about whether the tree (or a part of it) is likely to break or uproot within a defined time frame. Reports may use categories like “improbable,” “possible,” “probable,” and “imminent.” The time frame might be explicit (e.g., within 1 year) or implied by the inspection cycle.
A “possible” failure might mean the defect is present but not actively worsening, or it requires a significant trigger (like a major storm). A “probable” failure suggests the defect is severe enough that normal weather could cause failure.
When you see these terms, look for the supporting evidence: crack location, decay indicators, root issues, or canopy imbalance. A good report ties the rating to observable facts.
Impact likelihood: even a failing limb isn’t always a high risk
Likelihood of impact asks: if it fails, will it hit something? A tree can be in rough shape but located where nothing is within reach. Conversely, a relatively minor defect can become urgent when it’s directly over a roofline or service drop.
Arborists consider lean direction, prevailing winds, slope, and the “fall zone” based on height and branch length. They also consider whether the target can be moved—like relocating a seating area or adjusting parking habits.
If your report includes both ratings, that’s a good sign. It shows the arborist is separating structural condition from site exposure, which leads to more nuanced recommendations.
Consequences: what “low” vs. “severe” usually reflects
Consequences are about what happens if impact occurs. “Low consequence” might mean damage to shrubs or a fence section. “Severe consequence” could mean injury risk, major structural damage, or critical infrastructure impacts.
Sometimes consequence ratings feel subjective, but they’re usually grounded in common sense: a limb over a bedroom has a different consequence than a limb over a compost pile. Arborists also consider the size and weight of the part likely to fail.
If you want to sanity-check consequence ratings, ask yourself: What would it cost to repair? What’s the safety exposure? Would it disrupt power or access? That’s the lens the arborist is using.
Mitigation options: what the recommendations are aiming to achieve
Pruning for risk reduction: more than “making it look nice”
Risk-reduction pruning is about load management. By shortening certain limbs, reducing end-weight, or removing deadwood, an arborist can lower the forces acting on weak unions and defects. The goal is to keep the tree’s structure within safer limits without stripping it.
Good recommendations are specific: which limbs, what diameter ranges, and what pruning style (crown cleaning, reduction, selective thinning). If a report simply says “trim tree,” it’s fair to ask for clarification so you know what work is actually being proposed.
In many neighborhoods, routine maintenance like pruning and shaping trees is what keeps small issues from turning into urgent problems—especially after storms or rapid growth seasons.
Cabling and bracing: support systems with limits
Support systems can reduce the chance of failure in trees with weak unions or codominant stems. Cabling helps limit how far stems move apart; bracing rods can stabilize a crack or union. These systems are not “set it and forget it,” though—they require inspection, maintenance, and realistic expectations.
Arborists recommend cabling when the tree is otherwise worth retaining and the defect is manageable. They typically won’t cable a tree with severe basal decay or major root instability because the failure mode might be uprooting—something cables can’t fix.
If cabling is suggested, ask about inspection intervals, hardware type, and whether pruning should accompany the support to reduce loading.
Removal and replacement: when risk reduction isn’t enough
Sometimes the safest plan is removal—especially when the likelihood of failure is high and the target is significant. A good report should explain why removal is recommended and whether there are interim steps you can take if removal can’t happen immediately (like restricting access under the canopy).
Removal decisions can also be influenced by practicality. If a tree is too close to structures, or the canopy extends over roofs and wires, specialized equipment may be needed. In those cases, crews may plan tree crane work to lift heavy sections safely and reduce the chance of collateral damage.
If removal is recommended, it’s also worth discussing replacement options. A thoughtful replacement plan can restore shade and habitat while reducing future risk—right tree, right place, right maintenance.
Time frames and monitoring: how often to reassess
Many reports include a reinspection interval: 6 months, 1 year, 2 years, or after major storms. That interval isn’t arbitrary. It’s based on how fast the defect could worsen and how much uncertainty exists. A tree with a stable cavity might be monitored annually, while a tree with active cracking might require a shorter timeline.
Monitoring is not “doing nothing.” It’s a deliberate choice when the risk is acceptable for now, or when mitigation is staged. The report may specify what changes would trigger action: increased dieback, new fungal growth, soil heaving, or crack expansion.
As a homeowner, you can support monitoring by taking seasonal photos from the same spot, noting storm events, and keeping records of any site changes like trenching or new hardscaping.
Red flags you can spot between professional visits
Sudden lean, soil heaving, and new gaps at the base
If a tree suddenly leans more than it did before, or if you see soil lifting on one side of the root plate, treat it as urgent. These can be signs of root failure in progress. Likewise, new gaps between the trunk and soil, or fresh cracking sounds during wind, deserve a quick call to a pro.
After heavy rain or wind, do a quick walk-around. Look for fresh fractures, hanging limbs, or newly exposed roots. Trees can shift subtly before a major failure, and catching that early can prevent damage.
Don’t try to “test” stability by pushing a tree. Visual checks from a safe distance are enough until an arborist can evaluate it properly.
Fungal fruiting bodies: mushrooms are not always harmless
Mushrooms at the base or conks on the trunk can indicate internal decay. Not every fungus means structural failure is imminent, but it’s a sign the tree’s wood is being broken down. The location matters: fungi at the base can be more concerning than fungi on a dead stump nearby.
If you notice new fungal growth, take clear photos and note the season and weather. An arborist can often identify the likely decay type from the fruiting body and its position.
Don’t remove the fungus and assume the problem is solved. The fruiting body is just the visible part; the decay process is inside the wood.
Questions that make an arborist visit more useful
If you have a risk assessment in hand, a few well-aimed questions can turn it from a document into a plan. Ask: “What’s the most likely failure part—limb, leader, whole tree?” and “What’s the primary defect driving the risk rating?” These questions keep the discussion focused on structure, not just appearance.
It’s also helpful to ask about options: “If we prune instead of remove, how much does that reduce risk?” or “What would you monitor over the next year?” A good arborist will explain tradeoffs, not pressure you into the most expensive work.
If you’re comparing quotes, make sure the scope matches the assessment. Two companies can quote very different prices because one is proposing a light prune and the other is proposing major reduction, support systems, or specialized access.
How site-specific experience changes recommendations
Even with standardized methods, local experience matters. Arborists who work in your region understand common storm patterns, soil types, and the species that struggle in local conditions. That context can influence how conservative or proactive a recommendation is.
For example, an area with frequent high winds might warrant more aggressive end-weight reduction on certain trees than a sheltered neighborhood. Similarly, clay soils with poor drainage can change how arborists interpret canopy decline and root health indicators.
If you’re seeking a second opinion, look for someone who knows the local tree population and has seen how similar trees fail (or survive) in similar sites. In some regions, homeowners specifically seek teams like the Shandon tree experts because that kind of neighborhood familiarity can make assessments feel more grounded and actionable.
What “acceptable risk” means for real households
No tree is zero-risk. Even a perfectly healthy tree can drop a limb in a storm. The goal of a risk assessment is to reduce risk to a level you can live with, given your property’s use, your tolerance for uncertainty, and your budget for maintenance.
Some homeowners prioritize preserving mature canopy and are comfortable with ongoing pruning and monitoring. Others prefer a simpler landscape with fewer large trees near structures. Neither approach is “right”—but the risk assessment helps you make that choice with eyes open.
If a recommendation feels too aggressive or too relaxed, talk it through. Ask what assumption is driving it: the target rating, the time frame, or the severity of the defect. Often, adjusting one variable (like moving a target or reducing end-weight) can change the overall risk picture.
Reading the report like a pro: a quick way to connect the dots
When you get a tree risk assessment, start with the summary and recommendations, then work backward. Identify the specific parts at risk (a particular leader, limb, or the whole tree). Next, find the defects listed for those parts. Then look at the target notes—what’s in the impact zone and how often it’s occupied.
If the report includes a risk matrix or overall rating, treat it as a snapshot, not a verdict. The real value is the reasoning: which defects matter, how they interact with site conditions, and what actions reduce risk most efficiently.
Finally, remember that trees change. A good assessment isn’t just about today—it’s a plan for the next few seasons and storms. With a clearer understanding of what arborists look for, you’ll be able to read the report confidently, ask smarter questions, and make decisions that balance safety with the benefits trees bring to a property.
