How to read an arborist report

An arborist report is easier to read than it looks. Start with the summary: it tells you how many trees are affected and which ones stay. Then go through the inventory table for each tree's size, root zone and condition, and read the protection measures you'll have to follow on site. Finish with replacement trees, securities and the limitations.

What is an arborist report, and when do you need one?

An arborist report is a professional assessment of trees on or near a property, written by a certified arborist. Municipalities usually ask for one when you want to remove or prune a protected tree, or when building, demolition or servicing work could harm protected trees, including city boulevard trees and your neighbour's trees.

Rules differ between municipalities, so check your local guide (for example, Saanich tree permits or Langford tree permits). Development reports often go by longer names, such as "construction impact assessment" or tree management plan, but they're put together the same way.

What should the introduction and summary tell you?

The opening pages should tell you who asked for the report, why, when the arborist visited, and which drawings they reviewed. The executive summary then gives the headline numbers: trees assessed, trees protected by the bylaw, trees to be removed, trees to be kept, and replacement trees required.

Two things to check here. First, the drawing list: the arborist's conclusions only hold for the plans they saw, so if your architect has changed the site plan since then, the report may need a revision. Second, the counts. Retained plus removed should equal the total, and the summary and table should agree.

How do you read the tree inventory table?

The inventory table is the core of the report. Each row is one tree, and each column records one measurement or judgement about it. Read across a row to understand a single tree. Read down a column (retention status, say) to see what's happening across the whole site.

Example only: the three trees below are made up to show how a typical table works. They aren't from any real site.

In this example the arborist used a multiplier of 12 to calculate root zones: DBH in cm × 12 ÷ 100 = radius in metres.

Tag number

Arborists usually nail a small numbered metal tag to the trunk so everyone on site can identify each tree. Trees on a neighbour's land or the city boulevard may not be tagged. You'll see a code such as "NT" (no tag) or an "off-site" prefix.

DBH (diameter at breast height)

DBH is the trunk diameter measured 1.4 m above the ground. Most tree bylaws use it to decide whether a tree is protected. If a tree has several stems, the report should list each one. Watch for symbols meaning "approximate" or "measured over ivy".

TPZ, CRZ or PRZ (the root protection zone)

This is the radius around the trunk where roots need protecting during construction. Different municipalities and arborists call it the tree protection zone (TPZ), critical root zone (CRZ) or protected root zone (PRZ). It's usually worked out from DBH and how well the species copes with disturbance. This is the number that decides where fencing goes and where digging needs supervision. A good report explains how it was calculated.

Health and structure ratings

Most reports rate health (how vigorous the tree is) and structure (how soundly it's built) separately, often as Good, Fair or Poor. A tree can be healthy but structurally weak, for example where it was topped years ago. The remarks column gives the reasons: decay, co-dominant stems, dieback, lean or past pruning for hydro lines. If a tree near your home was flagged for risk, a separate tree risk assessment can take a closer look.

Retention status

This is the verdict on each tree. "Retain" means it can be kept if the protection measures are followed. "Remove" means it can't, and the text should say why. Many reports also use a conditional status, such as "Retain*" or "TBD", for trees whose survival depends on how carefully the work is done. If one of yours is conditional, read its paragraph in the impact section closely.

What does the impact assessment section explain?

This section goes tree by tree: how the proposed work affects roots and canopy, and what has to change to keep the tree. Expect measured distances from the trunk to foundations, driveways, services and sidewalks, plus the method and supervision required for each conflict.

What to look for:

• Specific triggers, with wording like "project arborist to supervise excavation for the driveway within the root zone of tree 102". Each one is a moment when your contractor has to call the arborist.
• Design changes. Sometimes keeping a tree means shifting a wall, rerouting a service line or using a different paving detail, and your designer needs to know early.
• Pruning estimates: how many limbs, what size, and roughly what share of the canopy will come off. Good reports give all three.

What protection measures will you have to follow?

Protection measures are the site rules that keep retained trees alive through construction. They're often drawn up as a tree protection plan. The usual ones:

Fence height, sign wording and spacing are set by each municipality. Your municipality's tree permit guide links its fencing standard.

How do replacement trees and securities work?

When a protected tree is removed, most bylaws require replacement planting, usually at a set ratio of new trees for each one removed. Many municipalities also hold a refundable security deposit until the replacements are planted, or until retained trees have come through construction. A post-construction letter from the arborist is often what gets that money released.

Check the number of replacements required, where they'll go (on the plan or the landscape plan), and who decides the compensation for city trees. Amounts vary by municipality. For example, the City of Langford lists a refundable deposit per required replacement tree on its tree removal page. Check the current figure with your municipality before you budget.

Why does the limitations section matter?

Every professional report ends with limitations. They set out what the arborist did and didn't do (usually a visual assessment, without drilling or digging unless stated), that trees change over time, and that no one can guarantee a tree will never fail. Don't skip it as fine print. It tells you how far you can rely on the findings.

Look for the assessment level, the date the findings apply to, and any instruction to have retained trees rechecked after storms or major grading.

A quick checklist for homeowners and builders

• Confirm the drawings listed match your current plans.
• Find every tree marked conditional, and read its section.
• Mark the root zones and fence lines on your site plan.
• List every "call the arborist before…" trigger for your site supervisor.
• Budget for replacement trees, any security deposit and arborist site visits.
• Ask questions before you sign off. Sorting something out on paper is far cheaper than fixing it in the field.

Need a report written, or a second opinion on one you've received? Oakstead's ISA Certified Arborist writes permit-ready reports for properties in Greater Victoria, the Saanich Peninsula and the Cowichan Valley. We assess and advise; any hands-on tree work goes to vetted local contractors. See our arborist report services or request a consultation.